Global Resources: Food:

Abstract. Global food security and a healthy environment are major concerns throughout the world early in the new millennium moment as the world population reached six billion. It is hypothesized that in order to achieve increased yields, the primary requirement is not research into new methods, but the increased application of techniques and practices that are already available and proven to be feasible. What have gone through severe trials both temporal and spatial and recognized by governmental officials, farmers, and scientists to be successful and non-alternative in curing food production to meet the population’s demands and environmental problems both in developed and developing countries are: (1) application of mineral fertilizers, (2) crop variety improvement (mainly exploitation of heterosis), (3) protected cultivation (mainly plastic-film mulch), (4) rice cultivation technique of dry nursery and low density transplant, (5) water-saving agriculture, (6) conservation tillage (mainly no-tillage), and (7) sustainable crop production. There exists much room for each measure to increase crop yield in agricultural practices. Each measure not only has its own enhanced effect on crop yield but also strongly interacts with others to produce positively cumulative effect more than the sum of the individuals. It is concluded that the global food security of the current population and sustainable development would be achieved completely if the growers and policy-makers could identify the limiting factors of the local crop production and environmental health and effectively integrate the relevant measures above-mentioned to solve problems.

One of the most impressive achievements in the later half of the 20^th century is the increase in food production in step with the increase in human population. Despite local famine and poverty episodes, the food supply has so far surpassed the increase in population. It was the increase in agricultural productivity that made this possible. However, this was to a great extent achieved at the expense of resources and environment. Soil desertification and global warming are the most obvious consequences. As a result, human is confronted with the irreversible trends of continuously increasing population, shrinking resources of cropland and water, raising consumption standards, and aggravating globalization of consequence from individuals unwise action. Increasing food supply in the future to meet human demands will be fulfilled by means of enhanced productivity per unit arable land (Evans, 1998).

The human race has entered the knowledge-economic times. A unique characteristic of modern agriculture is the greater risk both from uncertain market and frequently natural disaster compared with agriculture ever before. According to the viewpoint of sustainable development, the resources are limitable, but the advances in science and technology are non-limitable (Evans, 1998; Klepper, 1998). The fundamental way out for sustainable agriculture depends on the advances in science and technology. The currently additional population to the 1950’s 2520 million has been partly supported by the contributions of science and technology, as quite a lot of cropland has been occupied by increasing urbanization and industrialization. Advances in science and technology currently account for 40% of the annual growth in Chinese agricultural production (Normile and Lei, 1999), to which the successful alleviation of the Chinese poverty is mainly attributed just next to the policy in the last two decades. Technology improvements have led to an amazing increment in total cereal production from average 0.7 Mg ha^-1 in 1960/61 to about 2.1 at the present in India, which has saved land for production of other crops and for preservation of nature (Krauss, 1998). In order to achieve sufficient food supply, the primary requirement is not research into new methods, but the increased application of techniques and practices that are already available and approved to be feasible.

Furthermore, scientists and policy-makers must seek truth from the local fact, i.e., must well follow the ‘law of the minimum’ illustrated by barrel staves of varying lengths representing growth-controlling factors. While industrialized nations are concerned with overproduction, many developing countries are striving to meet their basic needs. The immediate problem is to double the yield in per unit of existing arable land of peasant (subsistence farmer) agriculture in under-developed countries. At the same time, the mitigation of soil erosion should be achieved by applying elementary agronomy. Soil erosion is largely resulting from cultivation of marginal and vulnerable land and over grazing rangeland driven by hunger and poverty (IFPRI, 1995). The cumulative evidence shows that there is much deeper potential to achieve this increase of peasant productivity than to raise production of farmers in highly developed and capitalized countries (Conway, 1998; FAO, 1998a; Hazell and Lutz, 1998; UNDP, 1998; Serageldin, 1999). The annual growth rate in world agricultural production was 2.3% in the period 1970-1990 and is expected to be 1.8% during 1988/90-2010. For developing countries, the numbers are 3.3 and 2.6%, respectively, in the same periods (Alexandratos, 1995). Although the local conditions may be greatly different, the theory and principles are universal. Therefore, it is worthy to identify the breakthroughs of crop production in science and technology already proven to be successful and non-alternative in supporting population.  Thus, more and more countries would effectively apply and integrate them to solve the common problems of the global food security and environmental health. These of techniques and practices should have gone through severe trials both temporal and spatial and recognized by governmental officials, farmers, and scientists (Conway and Sechler, 2000) to be successful both in developed and developing countries. Certainly, the authors did not imply that other practices including plant protection are not important, because they can be alternated by wise integrated management, such as crop rotation. First, this paper is designed to aim at creating a better understanding and awareness of these breakthroughs in crop production for a wide range of readers both of scientists and policy-makers. These breakthroughs should be as follows:

1) Application of mineral fertilizers,

2) Crop variety improvement (mainly exploitation of heterosis ),

3) Protected cultivation (Mainly plastic-film mulch),

4) Rice cultivation with dry nursery and sparse transplanting,

5) Water-saving agriculture, and

6) Conservation tillage (mainly no-till)

7) sustainable crop production

At present, two outstanding shifts in breaking the barriers of crop yield potential are: (i) more strengthening sources than sinks (Sayre et al., 1997; Singh et al., 1998), by incorporating morphological improvement rather than counting on heterosis alone in rice breeding to raise yield (Reynolds et al., 1996; Normile, 1999), by improving the ideotype to lessen interplant competition (Reynolds et al., 1994 and 1999), and by applying rice cultivation techniques of dry nursery and low density transplant, and, (2) more highlighting stress tolerance than performance in favorable condition in crop yield improvement (CIMMYT, 1996; Villareal et al., 1995 and 1997), e. g., the improvement efficiency of newer maize hybrids in capture and use of resources is frequently only evident under stress (Tollenaar and Wu, 1999), and, the advantage of increasing yields of rice in dry nursery and low density transplant is more obvious under adverse climatic condition compared with traditional flooding nursery and dense transplant.   

The essence of crop production is to capture and store solar energy by photosynthesis of green plants, although it is at the beginning of most food chains and is an essential human activity. Once retrospection of agricultural history, it is easy to find that all the measures enhancing crop productivity improve the ability of plants to capture and use of solar radiation, either directly or indirectly, or, either temporally or spatially. Globally, where hunger and poverty are pervasive and environment is unfriendly is just where solar radiation is not well utilized due to some uncertainty. However, the regions where peoples are suffering from poverty and hungry are usually the regions where solar radiation may be the most abundant. Successful practices both of crop variety improvement and cultivation technique have demonstrated that the greatly increasing in cereal yield is mainly by enhancing biomass yield rather than raising harvest index (Jiang et al., 1988; Akita, 1989; Song et al., 1990; Tollenaar, 1991; Amano et al., 1993; Ishii, 1993; Loomis, 1993: Bennett et al., 1994; Yamauchi, 1994; Yeo et al., 1994; Nissanka et al., 1997; Conocono et al., 1998; Austin, 1999; Normile, 1999; Loomis and Amthor, 1996, 1999; Tollenaar and Wu, 1999; Peng et al., 1999, 2000). In addition, as the fossil fuel is limited and non-renewable, the impending industrial revolution may be exploring new and renewable energy source. Solar radiation is the largest energy source human has ever known so far. It was harnessing solar radiation by burning fuel-wood and crop straw that maintained thousands of years of ancient Chinese civilization. As a matter of fact, the objective of modern agriculture is just to maximizing the utilization of solar radiation. In recent years, the link between the combustion of fossil fuel and climate change has become stronger. More and more countries are promoting the development of alternative energy, primarily to meet ever-increasing environmental protection legislation. The energy crisis and the link between fossil fuel usage and environmental issues may eventually result in the equivalent of mandated fossil fuel conservation (Richard and Szuromi, 1999; Turner, 1999). Agricultural production is to help promote the fast-growing sector of replenishable energy obtained from biomass (Klass, 1998; Turner, 1999). The second important objective of this paper was to substantiate the point that the breakthrough of crop production lies in the improvement of green plant ability in capture and use of solar radiation. Only harvesting more solar energy can increase the input both of matter and energy into agriculture production system. 

Successful practices of crop variety improvement and cultivation techniques of sustainable agriculture are the most rational and economical in the use of resources. The resources of crop production can be divided into the renewable and the non-renewable. Sustainable agriculture should utilize the former as much as possible so as to minimize the use of the later. Fossil fuel that can be stored is non-renewable resource; solar radiation, rainfall, and manpower, which cannot be stored, are renewable resources. It should be particularly stressed that cheap and abundant manpower in developing countries is a very important resource, which should be more fully absorbed and digested in per unit land within a perennial year and would otherwise be wasted. In addition, developing countries should make more full use of time by raising cropping index so as to compensate space compared with developed countries. The third objective of this paper was to demonstrate that successful crop production practices are rational in the deployment and use of resource components.

The potential of consecutively running human (generation by generation) to understand the nature is historically non-limitable while that of a particular person is very limited. Also, a weakness of human nature is that each person has a bias to over-emphasize the importance of his own work. The fourth objective of this paper was to help agricultural scientists realize the importance to deal well with the three relationships by reviewing the development of these breakthroughs. The three relationships should be the ones between the predecessor’s knowledge and the new ideas, between macroscopic knowledge and microscopic knowledge, and between the current routine work and the potential accumulation for a great future. Generally, successful scientists may have been able to correctly solve the three contradictions. This is consistent with the implication of great expectations of the presidential address by Klepper (1998). He pointed out that there are factors operating synergistically to allow improvements in crop production even with the technology of today. These are (1) improved communications, (2) vertical and horizontal integration of science, and (3) co-existence of gene banks and new genetic transfer technologies.

The ultimate objective of this paper is to convince scientists and policy-makers that it is really these breakthroughs that are supporting the current population and that the global food security and environmental health could be achieved by extending and integrating the breakthroughs already known. They are separately reviewed in detail as follows in this report.

Mineral fertilizers are an important resource for human food production. No one suspects the fact that global famine, refugees, and consequent social unset would emerge without fertilizers. Particularly, nitrogen (N) is the largest single nutrient input required for crop growth among them. It is essential for sustained food and fiber production (Mengel, 1990). In essence, N fertilizer is renewable and can be recycled as N exists in air and the energy of N-fertilizer synthesis only accounts for less than 1.2 % of the world’s fossil fuel consumption (Kongshaug, 1998). World food production depends on mineral fertilizers. These are now indispensable to ensure sufficient food production and preventing declines in soil productivity resulting from nutrient depletion. The rapid increase in global population and the consequent rise in consumption have rendered fertilizers an integral part of the food supply chain. There is no alternative to mineral fertilizers in modern agriculture to succeed in feeding the world.

Crop Yield Increases by Application of Mineral Fertilizers

Since the mid-20^th century, world average cereal yields have increased from about 1.1 to 2.9 Mg ha^-1 in 1996-1998, and have kept pace with the increase in human and livestock populations (Kaarstad, 1997). This double was partly due to the increased use of N fertilizer. It implies that substantially more N now circulates through the various compartments of the global N cycle, which are related to agriculture. Based on the estimation of FAO, fertilizer application ranks the first and irrigation the second among the factors increasing crop yields. Fertilizer accounted for 55% of the increase in food production in developing countries in 1965-1976. Of the management practices, fertilizer ranked the first by accounting for 51.4% of the increase in Chinese maize yield in 1985-1994, variety improvement the second by 35.5%, and the remainder by only 13.1% (Wu et al., 1998). Moreover, the contribution of fertilization to the increase of Chinese food production increases with the advance of time, for example, it increased from about 37% in 1981 to about 50% in 1990 (Lin, 1996). The newly improved high-yield crop varieties are more dependent on fertilizer than traditional ones. Therefore, fertilizer subsidies, mainly for N fertilizer, have been common to stimulate agricultural production in many developing countries. It has increased yields, but also encouraged unbalanced fertilizer application resulting in soil mining of nutrients that are not subsidized (e. g., P and K).

Mechanisms of Yield-Increasing Effect of Mineral Fertilizers

Crop plant functions as both photosynthesis factory and harvesting products. This process requires inputs of matter and energy in order to continuously expand and circulate the production. At present, fertilizer input can earn high profit margin in crop production as it can help green plant capture and store more solar radiation by promoting plant growth. The energy required for the production of fertilizer has a high ratio of output/input. Based on the energy required for the production of 1 kg fertilizer N-73, P-14, and K-8 MJ, respectively, Lewis and Tatchell (1979) estimated that the ratios of fertilizer application are 3.3 for only wheat grain harvest and > 6.6 for harvest of both grain and straw in UK. However, the corresponding ratios of totally agricultural input are 2.2 and > 4.4, respectively. Their results show that the application of mineral fertilizers improved the out/input ratio of crops. Their finding is consistent with the results of Pimentel et al. (1973) obtained for maize production in USA. A particularly low energy input is required for the cultivation of legumes as these crops require very little N fertilizer with an out/input ratio about 35 for alfalfa (Gasser, 1977). In addition, crop production is one of the few production processes with a positive energy balance. Thus, the energy acquisition by plants is called as sun energy harvesting (Hall, 1977). Clearly, it is economical and environmentally friendly to apply fertilizer to crop production, in terms both of agriculture production cycle to support population and geologic matter and energy cycle to explore alternative and renewable biomass energy. The fact that some countries in need resulted from disaster of flood and drought prefer international materials aid of fertilizer rather than food of equivalent cost exemplifies the return of expansion.    

The evidence currently available shows that the achievement of fertilizer application is mainly by improving the ability of green plants to capture and use of all kinds of resources and to resist the adversities. The resources include light, heat, air, water, land, and time. The increase in leaf area index per se resulted from application of N fertilizer correspondingly increases the use efficiencies of solar radiation, effectively cumulative temperature, CO₂ in air, water, and land. It is well known that rational application of fertilizers markedly improves crop resistance and tolerance of drought, salt and alkali, pest insects and disease, hardiness, and weeds. As soil erosion and water shortage is globally exacerbated, this paper is focused on the effect of fertilizer application on alleviation of soil degradation and improvement of water use efficiency. In addition, it is worth mentioning that the advances in the major role of N accumulation on the maximum value of harvest index have been excellently reviewed and interpreted by Sinclair (1998), which also greatly improves our understanding of N fertilizer importance.

Soil Erosion Mitigated by Mineral Fertilizers                       

The recently exacerbated soil erosion is mainly brought about by human activities of economic growth and eradicating poverty and hunger. Thus, fertilizers can mitigate the problem by: (ⅰ) increasing crop growth so as to cover the soil by vegetation resulting in the reduction of eroding action of raindrops and wind (Wood et al., 1993; Starling et al., 1998), (ⅱ) increasing biomass of not only the crop but also roots binding soil particles and crop residues covering the soil after harvest (Tanaka, 1995), (ⅲ) helping re-establish plant cover where erosion has removed the topsoil and nutrients (Olson, 1977; Tanaka and Aase, 1989; Massee, 1990), (ⅳ) increasing yields thus resulting in the reduction of need to cultivate fragile soils and to over-graze the range land (Hannaway and Shuler, 1993; Raun et al.,1999) , and (ⅴ) contributing to maintenance of soil organic matter (Rasmussen and Rohde, 1988; Paustian et al., 1992; Varvel, 1994; Vanotti et al., 1995; Campbell et al., 1997; Bowman and Halvorson, 1998; Herrick and wander, 1998; Halvorson et al., 1999).

Soil Water Use Efficiency Improved by Mineral Fertilizers

Moisture often is the most limiting factor for crop yield in rain fed and dryland area, and it is a major challenge to adjust nutrient applications to the moisture regime. Chemical fertilizer and irrigation besides triumphs of plant breeding are two other, equally important reasons for the success of the Green Revolution in 1960s (Mann, 1999). Positive interaction between moisture and nutrient significantly increases with the increase of nutrient level when and where low moisture is supplemented. The combination of fertilizer with irrigation also improves harvest index (Li and Li, 1994). Applying N fertilizer in dry land raises maize plant transpiration intensity, bleeding sap amount, amino acid concentration in sap, and leaf sugar concentration, and, reduces leaf water potential, and consequently improves grain yield (Li et al., 1994).  Both application of fertilizer and irrigation are the universal and feasible practices to enhance yield, but irrigation is usually limited by water availability whereas fertilizer is temporally and spatially available. Therefore, judicious fertilizer application is one of the most important factors increasing and stabilizing yields of dryland crops. A fertilized crop yields 100-150% more than an unfertilized crop when only 20-25% additional moisture is provided (Sing, 1994; Carvalho and Basch, 1996). An important approach to increasing crop production without additional costs of resources and environment is to exploit the positive interactions between factors that affect yield (Cooke, 1975). However, the easiest to adjust and control within a short time span available is fertilizer among all the factors.

Side-Effects of Mineral Fertilizers

Any breakthrough in science and technology would bring side effect to human if unsuitably handled in the long run of history. There is no exception to application of fertilizers. Fertilizers application does locally results in some issues such as nitrate leaching, eutrophication and consumption of non-renewable resources. However, this is not the inevitable output of fertilizers per se. Rational timing, rating, and methods of fertilizer application can control or even completely avoid the problems (Mengel, 1990; Burt et al., 1993; Bacon, 1995; CFA, 1995; Ferguson et al., 1996; Ressler et al., 1998; FAO, 1998b; MAFF, 1998a, b, and c). The most successful N management strategies on deep loess soils in terms of the seasonal water balance will probably be those that minimize the amount of residual nitrate in the soil profile at the end of the growing season (Mengel, 1990; Adams et al., 1994; Vanotti et al., 1995: Karlen et al., 1998; Matson et al., 1998). Soil moisture status controls gaseous N emission from nitrification and denitrification (Bouwman, 1998). Management practices to improve N use efficiency and minimize losses mainly include avoiding excessive or untimely N inputs and keeping the fields green under winter or cover crops to avoid fallow periods. Phosphorus is largely lost by surface runoff and wind erosion of colloidal particles adsorbing P. Therefore, all management practices of reducing erosion also result in reduced P losses, e. g., conservation and no-till systems (Baker et al., 1996; Mengel, 1997).

Mineral Fertilizers Necessary in Crop Production

Organic agriculture of excluding water-soluble N and P has developed in some western developed countries (Looker, 1997; Neenan, 1997). This phenomenon is mainly caused by environmental concerns of inorganic agriculture (USDA, 1980) and partly by over-production of food and increase in the proportion of rich people being able to afford to buy expensive agricultural products. However, it can only exist in very small proportion as a special form of diverse agriculture. It cannot support the current population at all. One hectare of land was able to support less than one person before fertilizer application in Germany in 1800, but it can support 5 persons in modern times (Mengel, 1990). The yield of food production would return to that of 1900’s in middle Europe in 5-15 years without fertilizers. Ancient Chinese wheat yield increased only by 6.9 kg ha^-1 per year depending on organic agriculture within a period from 221 BC to 1911 AD while the modern’s by 79.7 kg ha^-1 per year on mineral fertilizers from 1970 to 1984 (Liu, 1989). Cereal yields per unit area in1994 were 4500 kg ha^-1 for China, 1763 for India, and 5572 for US, respectively, which synchronizes with the amount of fertilizer application of 165 kg ha^-1 for China, 70 for India, and 154 for US, respectively (Cao, 1996). Yields in China for most crops are higher than in the rest of Asia, and China’s average yields are also above the world average for many crops. This is because China alone uses the same amount of fertilizer as the rest of Asia combined, and is now the world’s largest consumer of mineral fertilizers, as well as the largest producer of N fertilizers (FAO, 1998a).

It should be emphasized that there is much room to improve crop production with fertilization in developing countries. The room consists of unbalanced distribution of fertilizers between regions and low use efficiency in fertilized regions, which represents an opportunity and challenge for increasing food production. For example, if an additional 225 kg ha^-1 nutrient is applied to the 65.33 million ha of medium- and low-yield land and balanced fertilization technique applied to the 28.00 million ha of high-yield land, China would increase 205.00 million and 25.00 million Mg food, respectively. This could support 460.00 million of additional population (Cao, 1998).        

Green Revolution and Genetically Improved Crops

In the 1950s and 60s, agricultural scientists at IRRI and CIMMYT (the International Maize and Wheat Improvement Center) developed the package of improved crop varieties and agricultural management techniques collectively known as the Green Revolution. It helped world grain harvests more than double since 1960. The triumphs of plant breeding are one of the most important reasons for its success. For example, in maize, 60 to 90% of the yield increase in USA can be attributed to cultivar improvement (Duvick, 1984). Yield gains in the USA from genetic improvements during the twentieth century have been shown for other major crops such as soybean [Glycine max (L.) Merr.] (Specht and Williams, 1984), wheat (Cox et al., 1988; Schmidt, 1984; Waddington et al., 1986), cotton (Gossypium hirsutum L.) (Meredith and Bridge, 1984), barley (Hordeum vulgare L.) (Wych and Rasmusson, 1983), sorghum [Sorghum bicolor (L.) Moench] (Miller and Kebede, 1984), potato (Solunum tuberosum subsp. tuberosum) (Douches et al., 1986), and bromegrass (Bromus inermis Leyss.) (Casler et al., 2000). Similar work has done in Canada (Voldeng et al., 1997; Morrison et al., 1999) and China (Xin et al., 1995; Zhang et al., 1998; Wu et al., 1998). China has undergone 3 to 5 times of variety turnover for food and cash crops in the last half century and each time of varietal replacement leads to 10 to 30% of yield increase and a better improvement of crop quality and resistance to disease and pests. Significant breakthrough in the field of dwarf breeding, radiation breeding, and exploitation of heterosis has rendered China one of the advanced countries in the field of crop breeding and its application (Xin et al., 1995). Maize yield increase as high as 35.5% in whole China from1985 to 1994 was accounted for by variety improvement in contrast to 51.4% by fertilizers (Wu et al., 1998). However, variety improvement accounted for 68.98% maize yield increase in Henan of China, a staple food province, from 1963 to 1993. It is estimated that only the contribution of hybrid maize in China can substantially attribute the feeding of approximately 100 million persons per year in excess of those who could otherwise have been well fed during the period 1952 to 1965 (Chase, 1999).

Heterosisi as a Major Factor

Among the practices of variety improvement, heterosis, or hybrid vigor, has been a major factor in the increased production of several important plant species during this century. The momentous contributions of hybrid crop varieties is expected to play a key role in meeting humanity’s expanding food and feed demands and in natural resource conservation through its use in a range of crops-including maize, rice, wheat, sorghum, millets, cotton, vegetables, and oil seeds in the future. It is by exploiting heterosis that maize yield has been dramatically increased. The background of U. S. hybrid corn suggests that the increasing rate of corn yields was 1.0 kg ha^-1 per year during the period of civil war to 1930s derived from open pollinated cultivars, 63.1 during 1930s to 1960s from double cross cultivars, and 110.4 during 1970s to 1998 from single cross cultivars, respectively (Troyer, 1999). Hybrid corn and recurrent selection led to steady increases in yield. The U.S. average corn yield has surpassed 7839 kg ha^-1 in most years of 1994 to 1998 (Troyer, 1999). The greatest breakthrough in the last three decades is successfully exploiting heterosis of rice. The cumulative increased-yield has reached 350 million Mg derived from introduction of hybrid rice in China since 1976 and the increased food can support an additional 86 million persons per year in China (Zhu, 2001). Similarly, by Rutger’s calculation, the increased output feeds an additional 100 million Chinese every year (Normile, 1999). Hybrid rice has a mean yield advantage of about 15% over the best inbred cultivars (Yuan et al., 1994). In 1978, IRRI began to develop hybrids for the tropical lowlands (Khush, 1995). In the later 1990s, some hybrid rice cultivars developed at IRRI have shown a yield advantage of about 15% compared to the best inbred cultivars when grown in farmers’ fields. Recently, commercialization of hybrid rice has been initiated in India, Vietnam, and the Philippines (Virmani, 1996). About 50 000 and 80 000 ha were planted with hybrids in 1996 in India and Vietnam, respectively. There is a trend to expand hybrid rice planting in India and Vietnam.

The history of agriculture and crop science has proven that once there is a breakthrough in crop breeding a big jump in agriculture production will follow. For example, the successful development and extension of hybrid corn and sorghum, semi-dwarf rice and wheat have helped to increase the yield of these crops by a big margin worldwide. China’s rice breeding had made two breakthroughs: the first was the success of breeding semi-dwarf varieties in the early 1960’s, which out-yielded the traditional rice by 20%-30%. The second was the successful development of hybrid rice in the mid-1970’s, leading to another 20% increase in yield compared to the improved semi-dwarf rice varieties. Currently china is brewing the third breakthrough in rice breeding, i.e., utilization of inter-subspecific heterosis. Great progress has been made in this research program and many inter-subspecific hybrid rices with very heterosis, the yield of which is 15%-20% more than the existing intervarietal hybrid rice, have been developed. More advanced in rice breeding is the utilization of distant heterosis through biotechnolgy. Based on careful evaluation in the experimental field with the help of molecular markers, two important quantitative trait loci (QTLs) have been discovered from a wild rice in 1995 (Xiao et al., 1996). The two genes are located on chromosome 1 and chromosome 2, respectively, each having an effect of increasing grain yield by 20% compared to the control hybrid. Yuan’s group started the new strategy to exploit heterosis of distant hybrids in1996. It is expected that skillful combining of conventional breeding method with the molecular biotechnology would lead to another breakthrough in hybrid rice breeding (Chen et al., 2000). Recently, the super-high yield breeding practice to incorporate heterosis with morphological improvements has been carried out. Since 1996, Yuan’s group has selectively bred potential parents for long, narrow, and very erect top leaves. This configuration captures sunlight more effectively. In 1997, one of such crosses yielded an average of over 13 Mg ha^-1- well above the 10.5 Mg for existing hybrids grown under ideal conditions (Normile, 1999).

Hybrid Rice

Rice is the largest food source for the poor. It is the staple food of Asia, providing 50 to 80% of daily calorie intake. It is also the single most important source of employment and income for rural people (Hossain, 1998). Although hybrid rice has not been planted as extensively as hybrid maize, it is useful to retrospect and summarize the history of hybrid rice development in order to improve our understanding of scientific research. The success is a typical example of multi-disciplines integration, international communication, and combination of conventional knowledge and modern biotechnology. Hybrid rice breeding can be divided into three developmental phases, i.e., three line system, two line system, and one line system in terms of breeding methodology, and into inter-varietal heterosis, inter-subspecific heterosis, and distant heterosis in terms of enhancing heterosis level. Scientists have overcome one hurdle after another step by step. First is obtaining male sterile line and maintaining line to form a set of three lines, i.e., male sterile line, maintaining line, and restoring line. After the failure in their efforts to find a maintaining line within the cultivated varieties in the period 1965-68, Yuan, L. P. (1973), the initiator and organizer of hybrid rice breeding in China, had the idea to select cytoplasm-nucleus male sterile line by crossing wild rice with cultivated rice to broaden the parents’ relationship. Then Yuan’s assistants Li and Fen found a wild abortive rice in China’s Hainan in 1970 and Yuan achieved the formation of a set of three lines by means of a notional joint effort of crossing and selection (Yuan, 1973). Second is advantageous combination of three lines. Yuan held that an advantageous combination depends on the restorer and that the restorers of wild abortive male sterile line are related to the geography distribution of rice species. As they found that the rice varieties originating from the tropical have a higher restoration proportion, they concentrated their efforts of search on the tropical countries and finally formed a group of strongly advantageous combination of three lines in 1974 (Yuan, 1977). Third is the concern of hybrid seed production cost. Conventional belief is that rice is self-pollinated, has fewer pollens, short span of blooming period, small pillars, consequently has lower seed yield when cross-pollinated. However, Yuan (1977) believed that, as rice is open-husk pollinated and the pollens are light, it can sprinkle all pollens almost at the same time when anther is shattered, which renders it possible for pollens to disperse to a distance of 50 meters with the aid of human and to produce enough hybrid seeds if the time of parents’ blooming synchronize. Fourth is the discovery of the photosensitive recessive male-sterile rice by Shi (1985), which laid the basis of two line system. The wide compatibility varieties found by Ikehashi (1984) and then their ‘genes’ introduction solved the problem of low F₁ fertility of inter-subspecific crosses. Yuan (1990) solved the problem of low and unstable multiplication yield of thermo-sensitive genic male sterile line with low sterility inducing-temperature by means of identification of real hybrid with running cold water irrigation and decapitating and regenerating real sterile plants. By means of introducing some genes of C₃ plant –maize, did Yuan try to solve the problem of poor grain plumpness in inter-subspecific hybrid rice (Yin, 1994). Fifth is the strategy of one line, the concept of which is exploiting somaclonal reproduction to fix heterosis. This is a very hard research task that is a long-term target.

Hybrid Maize

Yield improvement in hybrid maize is attributable to greater stress tolerance and adaptation (Evans and Fischer, 1999; Tollenaar and Wu, 1999; Troyer, 1999). It is the result of more efficient capture and use of resources, and the improved efficiency in resource capture and use of newer hybrids is frequently only evident under stress. Improved resource capture has resulted from increased interception of seasonal incident radiation and greater uptake of nutrient and water. The improved resource capture is associated with increased leaf longevity, a more active root system, and a higher ratio of assimilate supply by the leaf canopy (Source) and assimilate demand by the grain (sink) during the grain-filling period. Genetic improvement of maize has accompanied by a decrease in plant-to-plant variability and the increased stress tolerance is associated with lower plant-to-plant variability and the increased plant-to-plant variability results in lower stress tolerance. Tollenaar (1991) pointed out that maize yield improvement was largely attributable to increased seasonal dry matter accumulation (primarily during the grain filling period), since harvest index has essentially remained relatively constant. The accumulated body of evidence suggests that improved tolerance of environmental stress has contributed significantly to grain yield improvement of maize in Ontario (Tollenaar et al., 1994). Differences in rate of dry matter accumulation between old and new Ontario maize hybrids appear to be greatest under marginal conditions (Tollenaar, 1991). The results of Nissanka et al. (1997) showed that the differences in the photosynthesis and transpiration of old and new maize hybrids were small when they were grown under well-watered conditions. However, the disparity between the two hybrids increased markedly when they were exposed to moisture deficit stress. Based on their results, it is apparent that the new hybrid was more tolerant to water stress and recovered faster upon rehydration than the old hybrid. Also, their results suggested that canopy respiration was lower for the new hybrid compared with the old under unstressed control conditions. Their data confirmed the contention that the new maize hybrid performs better than the old hybrid under stressful environmental conditions. Troyer (1999) concluded that old cultivar and inbred line background sources, few in number, indicate adaptiveness is more important than diversity to increase yield.

Mechanisms of Yield-Increasing Effect of Variety Improvement

Comparisons between semidwarf and traditional rice cultivars attribute improvement in yield potential to the increase in harvest index rather than to biomass production (Takeda et al., 1983; Evans et al., 1984; Peng et al., 2000). When comparisons were made among the improved semidwarf cultivars, however, high yield was achieved by increasing biomass production (Jiang et al., 1988; Akita, 1989; Amano et al., 1993). Hybrid rices have about 15% higher yield than inbreds mainly because of an increase in biomass production rather than increased harvest index (Song et al., 1990; Yamauchi, 1994; Normile, 1999). This suggests that further improvement in rice yield potential might come from increased biomass production rather than increased harvest index. Total dry matter at harvest of the hybrids at IRRI and PhilRice was 10 to 17% higher than that of inbred cultivar IR72 (Peng et al., 1999). The greater leaf area index and lower leaf N concentration of the hybrid might contribute to the observed heterosis in biomass production and grain yield, which may be the physiological basis for heterosis (Sinclair and Horie, 1989). Therefore, ideal morphology plus hybridization would further enhance rice yield and crop improvement has not yet reached the yield ceilings of rice. The theoretical potential yield has been estimated at 15.9 Mg ha^-1 in the tropical irrigated lowlands based on the total amount of incident solar radiation during the growing season (Yoshida, 1981). On the basis of this estimate, there appears to be a large gap between the yield potential of the best available rice cultivars and the maximum theoretical yield. The very opportunity and challenge are whether this gap can be narrowed by genetic crop improvement. The improvement in yield potential of other crops has been associated with increases in biomass yield in wheat (Waddington et al., 1986: Singh et al., 1998; Austin, 1999; Reynolds et al., 1999), maize (Tollenaar, 1989), oat (Payne et al., 1986), and soybean (Cregan and Yaklich, 1986). McEwan and Cross (1979), Wych and Rasmusson (1983), and Wych and Stuthman (1983) stated that the improvement in grain yield has been related to both dry matter accumulation and harvest index in wheat, barley, and oat.

   What has been achieved in the hybrid rice and hybrid maize is predicting that greater yield potential remains a valid and by no means exhausted goal for plant breeding programs.

White Revolution

Protected cultivation mainly represented by plastic-film mulch and greenhouse has greatly improved vegetable and small fruit markets worldwide since 1950s and crop production in China since 1980s. Plastic-film mulch and greenhouse fundamentally reduced the seasonal difference within a perennial year in amounts and prices of vegetable and small fruit in markets. In the last three decades, plastic-film mulch cultivation has gradually become a great breakthrough in agricultural production, which almost has the same importance as that of fertilization and crop improvement, cannot be replaced by other means, and thus has been given a good name of White Revolution. In addition to agronomy production, plastic-film greenhouse has been used to raise swine, cow, sheep, and chicken in high-cold regions in winter growing season. No person would suspect the momentous role of plastic film in combating poverty and hunger in China.

Plastic Mulch

Researches on vegetable crops have demonstrated that mulches provide several benefits to crop production through soil and water conservation, improved soil physical and chemical activity (Cooper, 1973; Tumulhairwe and Gumbs, 1983; Tindall et al., 1991). In a review on soil temperature conditions and plant growth, Cooper (1973) concluded that mulches consistently improved root development in several plant species. Cooper stated that the improved root development was mainly due to increased soil temperature. The incident radiation absorbed by mulches can be readily transmitted to the soil surface. Thus, mulches applied on or near the soil surface cause a consistent increase in soil temperature. Tumulhairwe and Gumbs (1983) reported that mulches improve soil water conservation. In cabbage (Brasica oleracea var. Capitata L.), the use of mulches significantly improved cabbage yield in dry seasons. The use of irrigation and mulches are attractive in the establishment phase of raspberry (Rubus idaeus L.) for reducing the risk of drought stress and modifying root-zone temperature while simultaneously minimizing cultivation and herbicide applications (Trinka and Pritts, 1992). Plastic film and straw mulch are inexpensive, readily available, can be mechanically applied, do not leave potentially harmful residues in the soil, do not have a deleterious effect on soil structure, can conserve soil moisture, and modify soil temperature by as much as 5℃ (Trinka and pritts, 1992). Percival et al. (1998) reported that plastic film mulch increased raspberry root and shoot mass, total flower number, and total berries harvested. Maximum leaf net photosynthetic rates were observed under cool air temperatures and root-zone temperature of 25℃. Field photsynthetic measurements indicated that there was no seasonal decline in photosynthesis. However, use of plastic film mulch in an established hedgerow planting was not beneficial due to the root system being largely shaded by the canopy, and also at a greater depth from the mulch treatments. A study by Tindall et al. (1991) demonstrated that different types of mulches may not have the same effect on soil physical properties. Plastic mulches may have less impact on soil bulk density, aggregate stability, and water infiltration than crop straw mulches.

Plastic mulches over trickle irrigation systems are widely used in raised-bed culture of strawberry (Fragaria×ananassa Duch) to conserve water by blocking rapid evaporation from the soil surface, control weeds with less herbicides, and keep fruit clean by preventing soil from splashing onto the fruit in the production of tomato and other food crop (Bhella, 1988; Lamont, 1993). Applying water through trickle-irrigation tubes located below the plastic mulch can provide enough water for optimal growth and avoid nutrient leaching by excessive rainfall. Black is the most widely used color of plastic mulch (Blackhurst, 1962; Bhella, 1988; Lamont, 1993). It is well documented that black polyethylene mulched strawberry produced higher yields of high quality fruit than unmulched controls (Wittwer and Castilla, 1995).

Recently developed colored mulch technology adds a growth regulatory effect of reflected wavelength combinations in the visible and far-red parts of the electromagnetic spectrum (Kasperbauer, 1992). The approach combines the benefits of black plastic mulch with additional growth regulatory benefits of reflected morphogenic light to improve yield and quality of field-grown plant products. Kasperbauer and Hunt (1998) conclude that increased tomato (Lycopersicon esculentum Miller) yield over the new red plastic mulch was caused by reflection of far-red to the growing plants and its subsequent phytochrome-mediated regulation of photosynthate allocation to developing fruit. However, the role of photomorphogenic light as a regulator of photosynthate allocation in growing plants under field conditions is a relatively recent discovery (Kasperbauer, 1971; Kasperbauer et al., 1984; Kasperbauer and Karlen, 1986). In the first field studies with tomato, yield differed over plastic mulches that were painted with different colors of exterior enamel (Decoteau et al., 1989). Subsequently, a large number of field experiments were conducted with a range of paint colors. In most of the experiments, tomato yields were higher over red-painted mulch than over black plastic mulch. Ultimately, Kasperbauer (2000) corroborates that strawberries were larger over the new red plastic mulch because reflected far-red and red light affected phytochrome-mediated allocation of photosynthate, and more was directed to developing fruit.         

Greenhouse Production

Greenhouse production of commercial crops can to a greater extent control plant development so that plants can be grown to commercial size with date specifications. This is achieved by supplementing light duration (Lin and Jolliffe, 1996; Demers et al., 1998), and temperature duration (Faust and Heins, 1993 and 1998) in winter, by alternating dark and light (Wang, 1998), by adjusting soil moisture (Xu et al., 1994) and nutrient (Bunnag et al., 1996), and by supplying and maintaining CO₂ levels (Jiao and Grodzinski, 1998). Thus, more and more new greenhouses have been built worldwide for growing and marketing vegetables, ornamentals, and small fruits. Even in the Tibet Autonomous Region of China, vegetables have been successfully grown in greenhouses of altitudes of more than 4000 meters above sea level.

Yield Increases by Protected Cultivation

As press for food supply has increased resulting from recent population explosion, the acreage of cereal crops mulched with white plastic films has very fast expanded in China after the great success of cash crops with it. Numerous field experiments and average production practices show a yield increase in wheat and dryland rice by more than 20% and maize more than 30% at least, even 2 to several-fold yield increase in conjunction with supplemental irrigation with harvested rainfall in some semi-arid regions. Due to extending non-frost duration by early-planting spring crops about 10 d earlier and late-planting winter crops 10 d later, respectively, as well as the resulting earliness of crops’ maturity by about 10d, plastic film mulch cultivation has the major contribution to effectively increased multi-cropping index in China. This is an important cause why China can support the increasing population while the arable land is rapidly decreasing with the increasing urbanization and industrialization. Besides its increasing soil temperature and moisture, plastic film mulch cultivation has rendered it possible for conservation tillage system mainly represented by no-till to be practiced in China. Plastic film mulch effectively reduces soil wind and water erosion and soil salification and alkalization by covering soil surfaces. Moreover, plastic film mulch can raise N and water use efficiency. Wang et al. (1998) reported that unmulched maize could reach the maximum yield of 4240.5 kg ha^-1 with 120 kg N ha^-1 while mulched could reach the maximum of 6232.5 kg ha^-1 with only 90 kg N. Their experiments also quantitatively differentiated the effect of N and plastic film mulch. They found that plastic mulch increased maize yield by only 24.6% without N; N increased yield by 126.5% with mulch while by 97.9% without mulch. Their results strongly confirmed that the role of N fertilizer is much far more than that of plastic mulched. 1 kg N can produce 23 kg maize grain without mulch while 27 kg with mulch; 1 mm soil water can produce 25.95 kg grain ha^-1 with mulch while 10.65 kg without mulch. In addition, plastic mulch can increase water storage by 25.2 mm in 0-20 cm soil for the following crop compared with unmulch. Fan et al. (1996) observed that plastic-mulched winter wheat in dry growing year increased yield by 32.9% for no fertilizer, 62.5% for medium-rate fertilizer, 99.5% for high-rate fertilizer, and 105.5% for super-high rate fertilizer, respectively, compared with unmulched. Mulch increased average yield by 23.1% for medium-rate fertilizer, 52.6% for high-rate fertilizer, and 55.4% for super-high rate fertilizer, respectively, in normal climatic year.  It intended to help winter wheat absorb deep soil water, and one time of irrigation with harvested rainfall during jointing period increased yield by 57.1% under mulched conditions. Niu et al. (1998) showed that compared with unmulched plants, the mulched spring wheat plants accumulated greater (26%) dry matter at anthesis, mobilized a large percentage of dry matter to grains after anthesis, and produced 36% more grain yield. Application of plastic mulches to spring wheat effectively increased dry matter production and mobilization from vegetative organs to grains. Clearly, plastic mulch can effectively enhance the ability of crop to capture solar energy, particularly in dry growing-season.

Enlarged Cropping Possibility by Protected Cultivation  

Another important role of plastic mulch is to greatly extend some tropical crops to higher latitude and to raise their-planting altitude for its increased temperature. In the northern U. S. cotton belt, earliness of maturity has long been recognized as essential to cotton crop adaptation (Hoskinson, 1964; Richmond and Ray, 1966). Managing the crop for earliness increases the likelihood that harvest can be completed before a killing frost arrests boll development. Plastic film mulch may be one of agronomic practices to optimize cotton yields and earliness in regions with short growing seasons. Giardina et al. (2000) pointed out that soil heating had a much larger influence on soil P and N availability than inputs of ash by the practice of slash-and-burn to clear forest land for agriculture at dry forest site. Plastic mulch can to a certain extent adjust the local cropping system and thus enhance the local economic development.

Plastic film mulch cultivation has much more room for supplying food to support population, as many peasants can’t afford to buy film and many countries have not yet used this efficient technique. If more farmers utilize this technique, more fragile and marginal land can be saved for forest and pasture to conserve soil and water.

Yield Increasing Effect of Dry Nursery of Paddy Rice

Rice has responded extremely well to intensification. It is the first staple food crop in China and the second in the whole world. About 90% of the world rice production is in the tropical areas of South and South-East Asia, where are the most populated areas of the overall world. The main problems encountered in intensive rice-growing regions are the low ability of rice to resist adversity, particularly the low temperature and coldness. In much of the southern USA, rice is produced in a dry-seeded, delayed-flood cultural system in which the flood is not applied until the four- to five-leaf growth stage. It is a simplified rice-growing technique of dry nursery and low-density transplant. The key of it is to foster strong seedlings by controlling soil moisture at low temperature in seed-bed and to form a uniform canopy layer of maturity stage and plant height by making the fullest use of the early tillered panicles with only one or two primary translated-seedlings. This set of technique was developed in Japan in 1930s and adopted by China in 1980s. It has become a great breakthrough in rice-growing techniques and rapidly expanded in China; it has been a key program to be extended to combat hunger and poverty in China. This set of technique has produced great economic and social benefits in China. For example, it was by means of the introduction of this set of techniques that Helongjiang province of China ended the practice of feeding human with sorghum (Sorghum) grain as main food. The technique had accounted for 57.5% of rice acreage in Fangchen County in Helongjiang province in 1983 and averagely increased yield by 12% against the conventional intensified wet-nursery cultivation and doubled the yield compared with field-seeded (non-transplanting) cultivation. In 1985, rice yield had averagely reached 7597.5 kg ha^-1 by the new technique in this county in comparison with the 4950.0 kg ha^-1 of the conventional intensified cultivation. Until 1990, the new techniques had been adopted by more than 10 provinces of China. The technique increased yield by 10-50% averagely and by 100% the maximum, and created the highest record of 13 800 kg ha^-1 in Xingjiang province. This set of technique has increased yield by 9-73% in Shaanxi province, 20-77.4% in Hubei province, more than 20% in Shanxi province, and 58.2% in Sichuan province, respectively. The yield increment of the technique in Hebei province is 2250-3000 kg ha^-1. The wonderful role of the technique in increasing yield renders peasants and scientists surprised in Gansu province. In 1991, the yield was increased by average 29.6% with the absolute increment being 2280.0 kg ha^-1, several peasants produced a mean yield of 12 136.5 kg ha^-1 with the highest record of 12 825.0 kg ha^-1, and the response in increasing yield of different cultivars ranged from 27.9 to 42.0% indicating the significant interaction between the techniques and rice cultivars. In 1992 when there emerged low temperatures and coldness in spring and autumn, conventional rice of wet-nursery and dense-transplanting had a seriously decreased yield or even failure whereas dry-nursery and low density transplanting rice had a mean yield of 9915 kg ha^-1 with the highest record of 13 950kg ha^-1. No peasants would convince the great potential of increasing yield if they did not really practice the new technique.

Advantages of Dry Nursery

The advantageous yield of rice of dry-nursery and low density transplant is attributable to its higher tillered plant yield at low primary transplanted population. Such a cultivation technique can result in uniformity both of primary plant and tiller plant in capture of resources of light, temperature, and nutrient. The cause is that the rice seedling of the new technique has a strong root system, high nitrate content in seedling plant, and higher dry matter per seedling, ultimately resulting in vigorous and uniform early tiller plant and strong ability to resist adversity. For example, the dry nursery weight of strong seedling of 3.5 leaves is 45 mg per seedling while the conventional only 29.3mg (Agricultural Bureau of Gansu Province, 1994, personal communication). Another advantage of the new technique is to save water, fertilizer, and seed. In contrast, conventional rice-growing technique of high-temperature flooded-seedbed and densely transplanting produce spindle seedling with low nitrate content, weak root system, and less dry matter accumulation, resulting in weak ability to tiller and to resist adversity. Such spindle seedlings after transplanted are slow and late to revive, and its tiller plant is subject to shading of primary plant and results in lower dry matter partitioning and later maturity compared with primary plants. Such a population structure has low light use efficiency both temporal and spatial and low harvest indexes. The advantageous yield of the new rice growing technique is due primarily to improved tolerance of abiotic and biotic stresses, coupled with the maintenance of the ability to maximize yield per tiller plant under non-stress growing conditions.

The results of Duvick (1997) suggested that the ability of increased grain-yield of successful maize hybrids was attributable to the same characteristics as rice grown with the new technique. Specht et al. (1999) observed that changes in some traits of soybean new varieties compared with old varieties were obvious (improved lodging), but more subtle in others including greater stress tolerance). In terms of photosynthate supplied to sinks across a wide range of environments, recent cultivars seem to be superior to obsolete ones. As the seedling of dry-nursery is developed under sparse seeding, limited moisture, and low temperature, its vertical growth is slow enough to form thick, dwarf, and solid stem with a flourishing root system. The seedling accumulates much higher NO₃-N and starch and a stronger ability to produce root than conventional flooded seedling. The number and weight of roots increase by 25% and 72%. As the seedling has a stronger reducing ability and lower rate of respiration, it keeps strong and healthy when the flooded seedling has been exhausted to root rot. The dry seedling has a stronger ability to produce tillers, because its growth point has received low temperature treatment and thus the time of forming a leaf is longer, which results in more opportunities to produce tillers and lays the basis to establish early tillers after transplanted. The dry seedling can produce 7-12 tillers in the mid-June after transplanted in the beginning of May under normal conditions, but the flooded seedling can produce 3-5 tillers even in the end of June under the same conditions. After transplanted, the dry seedling is controlled by limited N fertilizer, shallow water irrigation, and drying land in order to keep strong plant and to resist diseases. Since the technique emphasizes tiller panicles by low-density transplanting, individual plant develops well and has a high fertility. Each transplanting pit of dry seedling can form 20-25 panicles at the same level-layer whereas the flooded one only 10-12 panicles at two rough layers. The dry seedling is transplanted earlier and has a stronger vegetative growth in all life, particularly in the early stage of reproductive growth, so that more solar energy is captured compared with conventional cultivation. Similar phenomenon is observed in soybean. Soybean plants grown with no-tillage often appear smaller than those grown with conventional tillage, yet they produce similar grain yield. Yusuf et al. (1999) confirmed the hypothesis that the early-season growth depression is offset by compensatory growth and changes in plant development. Deeper understanding of how soybean yield compensation occurs across plant populations (Board, 2000) has aided our research aimed at reducing optimal plant population.

Advantages of Sparse Transplant

Peng et al. (1999) believed that the poor rice grain filling of new plant type lines is probably due to a limited number of large vascular bundles for assimilate transport. However, Ladha et al. (1998) demonstrated that the poor rice grain filling is attributable to early leaf senescence. The objective of sparse seeding rice in dry seed-bed is to increase the number of vascular bundles. The results of Fang et al (1992) showed that rice of drought seedling and sparse plant in  increasing yield could be due to strong stem, long spike and high fertile, and stabilize high yield potential. By studying morphology function of individual development and population structure, high yield fundament on rice of drought seedling and sparse plant has been shown as follows: (ⅰ) foundations on which strong stem and big spike were obtained were heavy dry weight per plant, longer upper leaves, thicker basal inter-node and more vascular bundles, and (ⅱ) thicker leaves, longer and erect upper three leaves, canopy layer of high photosynthetic efficiency, thick neck of spike and large section vascular bundles were foundations on which high fertility was obtained. 

Hardening Effect of Dry Nursery

As a matter of fact, besides dry-nursery rice, it is a well-known fact that drought hardening in earlier growing stage can alleviate the susceptibility of crops to drought stress. Culter and Rains (1977) observed that the susceptibility of cotton to water stress in the elongation of shoot top and leaves was obviously mitigated. Culter et al. (1980a, b, c) also found that dry hardiness reduced the susceptibility of rice to moisture stress. Similar results were obtained in grain sorghum (Stewart et al., 1983). Guo and Shan (1994) reported that drought hardening in seedling or jointing stage of millet (Setaria) remarkably improved water use efficiency in late stage by means of increasing the leaf photosynthetic ability, but the water consumption did not decrease because the increase in assimilation exceeded the increase in transpiration. Under the condition of serious drought hardening in earlier growing stage, the grain yield and water use efficiency of crop could be significantly increased after adding a little amount of water. Much water was saved through medium drought hardening while the grain yield did not significantly decrease so that water use efficiency increased remarkably. Winter hardiness acclimation resulted in an increase in freezing tolerance of alfalfa (Castonguay et al., 1993, and 1997). Similar findings were obtained in bermudagrass (Anderson et al., 1993: Samala et al., 1998) and winter rye (Secale cereal L.) (Uemura and Steponkus, 1994).                  

There is great potential to improve crop yield by means of adversity hardening in earlier growing stage so as to raise resource utilization efficiency. This is a sustainable strategy, because it does not need additional resource input but depends on science progress. The principle is true for many crop species in stress environment. 

Irrigation Increasing Yield but Causing Problems

Water-saving agriculture includes improved water management of conventional irrigation and rain-fed agriculture and rainfall- harvested irrigation. In addition to the great achievements of plant breeding and chemical fertilizer application, irrigation was another reason with the same importance as fertilizer for the success of the 1950-60s Green Revolution. Between 1961 and 1996, according to FAO statistics, the world total of irrigated land almost doubled, from 139 million ha to 263 Mha, which accounted for one third of the increased food supply. However, prospects for expanding irrigation are only somewhat better. Irrigation, too, has caused environmental damage, depositing toxic salts on poorly drained agricultural land. And because irrigation is already used in most of the areas where it is practical, future water projects will be increasingly expensive. Worst of all, many countries including northern China, eastern India, and much of Africa, will be so short of water that they will be forced to reduce conventional irrigation rather expand it. So these countries focus on both improving water management of conventional irrigation and rainwater and developing and extending rainfall-harvested irrigation system to raise water-use efficiency.

Improvements in Irrigation Techniques

Technological innovation of irrigation can advance water-use efficiency, even in poor countries. The water-saving irrigation agriculture of Israel has is a good example for the whole world and indicates a great potential to improve water management. Water-saving agriculture has been practiced with the following approaches: (ⅰ) mulch cultivation to reduce soil water evaporation, which includes plastic film mulch, straw mulch, gravel and sand mulch, and vegetation mulch (cover crop), (ⅱ) fertilizer application to raise water-use efficiency by means of positive interaction of fertilizer and water, (ⅲ) conservation tillage system (mainly no-till and ridging-cultivation) to reduce runoff and evaporation, (ⅳ) improved irrigation methods by altering conventional flooded-irrigation to drip-irrigation, sprinkle-irrigation, furrow irrigation, limited irrigation, and ridge-furrow alternative irrigation, (ⅴ) growing drought-tolerant or –resistant crop species and deep-root crop species to use subsoil water, (ⅵ) both in-situ and ex-situ rainfall-harvested irrigation by redistributing rainfall temporally and spatially.

Limited Irrigation with Soil Mulched      

In China, drip-irrigation with plastic dotted-pipes embedded into soil in rows of crops mulched with plastic film greatly raised yield resulted from improved water-use efficiency. In addition, plastic mulch per se can substantially increase crop yield as a result of water saving. Liang et al. (1999) reported that plastic mulch increased yield of upland rice to 6372-13 500 kg ha^-1 from 4530-5400 kg of unmulched upland rice and the irrigation requirement of the former was much less than that of the latter. Winter wheat with perennial plastic mulch collected and conserved rainwater in rainy seasons by reducing evaporation in the whole growth seasons and ultimately resulted in yield increase by 57.1% with an irrigation at jointing stage and saved about 12 days non-frost growing season enough to multiple-crop other crop (Wang et al., 1997; Fan et al., 1997 and 1999). Sauer et al. (1996) reported that corn residue showed discernable effects on soil moisture and thermal regimes and the various residue covers reduced evaporation from 34 to 50%, with the range of values reflecting differences in residue layer thickness and placement. Allen et al. (1995) observed that the very long-term residual effect from vertically mixing A and B soil horizons increased irrigation water infiltration after primary tillage. Unger (1994) also showed that tillage practices that retain residues of winter wheat and grain sorghum on the soil surface (no-tillage) are effective for producing these crops in rotation under limited-irrigation conditions in the southern Great Plains. Retaining residues on the surface tends to increase water conservation and reduces the potential for soil erosion. Gulick et al. (1994) demonstrated that winter and perennial cover crops increase summer soil permeability to irrigation water even on intractable central California soils. Striking improvement in furrow water penetration was achieved during the first study year on a fine sandy loam vineyard soil. Differences in cumulative infiltration doubled the second year and were two and one-half times higher in continuous cover than in the bare soil herbicide treated as needed. Gee et al. (1992) pointed out that percolation is high where the soil surfaces are composed of sands or gravels. Kemper et al. (1994) substantiated that gravel mulches 5 cm thick resulted in accumulation of 80 to 85% of the annual precipitation. The earliest gravel mulches was recorded in 1968 (Corey and Kemper, 1968). Pebble mulches have been practiced for hundreds of years in the northwestern China and have been believed to be the best compared with the other mulch methods known because of depressing insect pests and diseases as well as conserving water and temperature if the manpower is available and cheap.                   

Crop production in arid regions is particularly sensitive to deficiencies of soil moisture and plant nutrients, especially N. Pronounced water×N interactions on crop yield have been documented for many crops. This has been mentioned in the section of plastic film cultivation. Recently, Thompson and Doerge (1996a and b) reported a pronounced water×N interaction in fresh and marketable yields and net return of leaf lettuce (Lactuca sativa L. cv. Waldmann’s Green) when multiple irrigation treatments were used.  

Water Saving in No-Till Systems

No-till system increases soil storage of precipitation especially when the traditional winter- fallow cropping system in the USA western Great Plains is converted to more intensive no-till cropping sequences (Greb et al., 1979). It is possible to store as much as 49% of the precipitation received in the soil during fallowing on no-till, compared with 30 to 33% for a conventional wheat-fallow cropping system (Greb, 1983; Nielsen and Anderson, 1993). The observations of Porter et al. (1996) indicate that, under no-till dryland conditions with an intensive cropping sequence, applied fertilizer N is conserved in organic forms through the biological processes of plant assimilation of NO₃, residue deposition and slow decomposition at the soil surface, and incorporation of the residue N into microbial biomass. The minimum-till and no-till systems are effective steps in efficiently saving more precipitation for crop production (Aase and Schaefer, 1996; Black and Bauer, 1990; Peterson et al., 1996; Tanaka, 1985; Tanaka and Anderson, 1997). The results of Halvorson et al. (2000) indicate that farmers in the northern USA Great Plains can successfully produce spring wheat in a spring wheat–fallow systems using minimum-till and no-till systems, but yields may be slightly reduced when compared with conventional-till systems some years. An Israel report by Bonfil et al. (1999) indicates that crop yield and water use efficiency can be increased in arid zones with annual precipitation of less than 200 mm, through use of a wheat-fallow rotation system that is managed by no-till. In general, No-till management increases water infiltration, reduces evaporation, and keeps the soil in the fallow fields moist (Jones and Popham, 1997; Norwood, 1994; Waddell and Weil, 1996). Furthermore, water absorption by crops is improved by enhanced root development (Kirkegaard et al., 1995; Merrill et al., 1996). Under dryland conditions, not tilling the soil but mulching the soil with straw is the simplest way to reduce soil water evaporation. In regions receiving annual precipitation of more than 250 mm, application of no-till and straw mulch management raises both yields and water use efficiency and also benefits the soil structure (Aase and Pikul, 1995; Jones and Popham, 1997; Lopez-Bellido et al., 1996; Norwood and Currie, 1996). Tillage practices leaving crop residues on the soil surface can reduce or eliminate surface crusting, increasing infiltration, and reduce surface runoff and soil loss while increasing crop yield (Cassel et al., 1995).

Water Saving by Intensified Cropping

Cropping intensification improves the efficient use of precipitation. Farahani et al. (1998) showed that the gains in efficient use of precipitation with intensification resulted from (ⅰ) reducing the frequency of the inefficient fallow preceding wheat, and (ⅱ) using water for transpiration that would otherwise be lost during fallow through soil evaporation, runoff, and deep percolation. In the rain fed Mediterranean environment, water use efficiency can be substantially improved by adopting deficit supplemental irrigation to satisfy up to 2/3 of irrigation requirements, along with early sowing and appropriate levels of N (Oweis et al., 2000). Polyacrylamide applied in the first irrigation at low rates effectively reduced runoff and erosion (Aase et al., 1998). Rather than watering crops by flooding whole fields, for example, farmers in parts of northern India are employing cheap, movable plastic pipes dotted with pinholes to drip-irrigate their fields. New, low-cost, high-efficiency sprinklers are also under development. Bargar et al. (1999) concluded that water infiltrated in furrows and primarily moved laterally to row positions, minimizing downward water movement under the row. These results explain greater solute movement under furrows than under rows.

Water Saving By Improving Crop Varieties

Crop selection and rotation are major factors in efficient water management for dryland grain production. In the research of Jones and Popham (1997), sorghum was more than twice as efficient as wheat in using available water for grain production. Sorghum produced much greater biomass and grain per unit of water and had a harvest index 33% greater than wheat. Cropping systems exerted a fourfold influence on average annual grain production and the efficiency with which precipitation was used for grain production, with continuous sorghum being the most efficient and wheat-fallow the least efficient dryland grain production system. Norwood (1999) observed that corn and soybean were similar in their depletion of soil water, as were sorghum and sunflower. Below a depth of 1.2 m, sorghum and sunflower removed the most water. Sunflower removed the most water from the last 0.3 m of the profile and probably removed deeper water. Sorghum and sunflower removed an average of 19 mm more water from the 1.8-m soil profile than did corn and soybean.

Cost-Down By Limited Irrigations

Considerable research on limited, rather than full, irrigation for the crops in some regions characterized with the decline in the aquifer, coupled with increasing pumping costs. In Nebraska, Hergert et al. (1993) reported corn yields of 5.6, 10.1, and 11.8 Mg ha^-1 for dryland, limited irrigation, and full irrigation, respectively, and marginal returns of 31 kg ha^-1 mm^-1 for limited irrigation and 11 kg ha^-1 mm^-1 for full irrigation. Stone et al. (1987, 1993) found that the traditional preplant irrigation of corn did not result in additional yield over that resulting from an in-season irrigation in western Kansas. Also in western Kansas, Hooker (1985) reported that two timely irrigations for grain sorghum [Sorghum bicolor (L.) Moench] did not reduce yields markedly compared to three irrigations. In the Texas Panhandle, Stewart et al. (1983) devised a graded furrow system, which included full irrigation at the upper half of the field, tailwater for the next one quarter, and dryland for the remaining one quarter of the field. This system increased grain sorghum yield by an average of 0.15 Mg ha^-1 for each 10 mm of irrigation, compared to 0.09 Mg ha^-1 for conventional irrigation. Norwood (2000) concluded that corn will yield adequately with one or more irrigations; thus, limited irrigation combined with proper fertility and plant population is a viable alternative to dryland in areas of declining groundwater.  

Improvements in Water Use Efficiency

Alternate-furrow irrigation has been proposed as a method to increase irrigation water use efficiency, increase capture and storage of rainfall during the irrigation season, and decrease deep percolation of water. Benjamin et al. (1998) showed that placing fertilizer in the non-irrigated furrow of an alternate-irrigation systems or placing fertilizer in the row with either alternate-or every-furrow irrigation has the potential to decrease fertilizer leaching without reducing crop production. Greater use of this technique of fertilizer management could significantly reduce groundwater pollution by applied fertilizers and also has the potential to increase N use efficiency by maintaining the fertilizers in the rooting zone of the crop. Irrigating alternate furrows instead of every furrow in a field increases the chance for rainfall storage and increase water use efficiency (Fischbach and Mulliner, 1974; Musick and Dusek, 1974; Crabtree et al., 1985). Small yield losses were recorded for sugar beet, sorghum, and potato by Musick and Dusek (1974) and for soybean by Crabtree et al. (1985) for the alternate-furrow irrigation system when compared with every-furrow irrigation, but irrigation water use decreased by 30 to 50%. Fischbach and Mulliner (1974) reported that corn yields with alternate-furrow irrigation were not lower than with every-furrow irrigation even though irrigation water application was 30% less in alternate-furrow irrigation. Evaporation from soil constitutes a large proportion of evapotransporation of row crop fields in West Africa and North China. Tillage after rain events with a traditional, shallow-cultivating hoe that pulverizes and darkens the soil surface, conserves soil water and increases crop yields, which has been successfully practiced for thousands of years in China. Payne (1999) showed that tillage after rain events increased soil water storage in the upper 1.4 m by up to 32 mm, and increased pearl millet grain yield by 68% in 1991 and 70% in 1992. Fertilizer addition increased yield by 21% in 1991 and 116% in 1992. Tillage reduced evaporation in 1992 from 417 to 372 mm and increased water-use efficiency from 0.99 to 1.94 kg ha^-1 mm^-1. Fertilizer application increased water-use efficiency from 0.95 to 1.94 kg ha^-1 mm^-1. His results also confirmed that fertilizer effect is greater than water conservation even in very dry West Africa. The major factor limiting crop production in the Texas Rolling Plains is water, and better methods of conserving soil water in dryland wheat production are needed. Chain diking is a novel method of forming basins on flat-tilled land to impound and conserve water received from precipitation. Wiedemann and Clark (1996) found that diking reduced runoff an average of 40%, which amounted to 1.0 ha-cm over three crop years, compared with tillage without diking. Diking increased winter wheat yields 11% (277kg ha^-1) in 1989, when cropping-season rainfall was 22% below average.     

Alley cropping system can cycle water and nutrient from the subsoil to surface horizons by means of inter-cropping perennial deep-root crop species to reduce the risk of uncertainty in rainfall between growing seasons in India and North China.  

Rainwater Harvesting Practices

Practice of rainwater harvesting was extensive and developed rapidly in Israel, India, and in the Loess Plateau of China, and formed a special agricultural model. In China, building water cellar has become a common practice organized by government. Gansu province of China carried out “121 rainwater harvesting project” meaning that one family establish 100 m² rainwater catchment with concrete in courtyard and rooftops, build two water cellars and irrigated about one mu (0.067 ha) courtyard cash crop. This plan has made obvious benefit and shown great promise in alleviating the drinking water and food problem of dryland. Rainfall harvesting system consists of in-situ and ex-situ. In-situ practice means that rainfall harvesting zone and crop planting zone are in the same field. The popular form used is the ridge-furrow tillage with the ridge being covered by plastic film and served as rainwater harvesting zone for concentrating rainfall and furrow as crop planting zone. By this treatment, soil moisture storage in planting zone increases by 57.4-272.7 mm in 0-200 cm soil layer, corn yield 20-332% and water use efficiency 40-99%, in regions of mean annual rainfall of 250-300 mm (Li et al., 2000). The traditional rainfall harvesting system is ex-situ that can be defined as artificial methods for collecting and storing precipitation until it can be used for watering livestock, small-scale subsistence farming and domestic use. These systems include a catchment area, usually prepared to improve runoff efficiency, a storage facility for the harvested water, unless the water is to be immediately concentrated in the soil profile, and a water distribution scheme to use harvested water efficiently.

The reasonable allocation of water resources including rivers and reservoir is important to earn the best profit. For example, minimizing water consumption in the upper valley and maximizing it in the lower of Yellow River greatly increased water use efficiency, as the upper valley has a higher altitude, lower temperature, and thus lower valuable crop species. It is a wise strategy to improve the whole nation economic development.

Clearly, the potential is deep to improve the current water management for crop production. If all the farmers in the west and all the peasants in developing countries used water resources as economic as mentioned above, more water would be saved to expand crop production.      

Conservation tillage

Yield Reduction by Soil Erosion 

Although conservation tillage systems, especially no-tillage, do not increase crop yield as much as those above-mentioned in short-term, they ensure the sustainability of crop production in long-term due to their protection of the most important agricultural resource, the soil. Worldwide, erosion is probably the most serious threat to agricultural sustainability. It is especially severe in parts of Asia, Africa and Latin America. Analyses of the available data by Lal (1995) indicate that yield reductions due to past erosion may range from 2 to 40%, with a mean of 8.2% for the Africa continent and 6.2% for sub-Saharan Africa. If accelerated erosion continues unabated, yield reductions by the year 2020 may be 16.5% for the continent and 14.5% for sub-Saharan Africa. Annual reduction in total production for 1989 due to accelerated erosion in the continent was estimated at 8.2 million Mg for cereals, 9.2 million Mg for roots and tubers, and 0.6 million Mg for pulses. The reduction in total production in 1989 for sub-Saharan Africa was estimated at 3.6 million Mg for cereals, 6.5 million Mg for roots and tubers, and 0.36 million Mg for pulses.

Soil erosion in the United States has been a matter of public concern since the 1930s. Conditions were improved by the 1960s, although no one knew just how much. During the 1930s when wind erosion was really a crisis, huge dust clouds from the Dust Bowl darkened the skies of the eastern United States and moved out over the Atlantic Ocean in the upper westerly winds. Wind erosion is a serious problem in many parts of the world. Consequences of wind erosion are many: valuable components of the soil are removed, plants are damaged, air is polluted, ditches are filled, roads are covered, mechanical and electrical components are damaged, respiratory ailments are exacerbated, more frequent cleaning of homes and other buildings is required, and so forth. Nutrient loss due to erosion is one of the major causes of soil fertility depletion of Kenyan soils (Gachene et al., 1997). Also in China, topsoil loss by wind in the North and flooded sediment input of rivers in the South indicate that erosion is the most important contributor of soil depletion although there has not exactly direct determination results of soil loss amount by erosion yet. The fact is that it is not the crop that does or does not account for soil depleting but the crop management. Loss of fertility is not a question of which crop is grown but how it is grown. Certainly some crops will be particularly prone to erosion because of the way they are grown, like clean-tilled row crops. Other closely growing crops like grass will usually suffer less erosion. But these general trends can be quite overruled by the effect of management. Grass, if overgrazed and under-fertilized, can allow so much erosion that instead of being a soil builder the fertility actually declines as has been seen in the Inner Mengu Rangeland of China recent years and in the western part of the USA Southern Great Plains as the center of the Dust Bowl in the 1930s. Maize can, under suitable management, both slow down erosion and raise the fertility as is the case of the current no-tilled maize in USA. Fortunately in USA, much wind erosion of the past few decades appears to be mainly local redistribution-some areas lose, others gain. Wind erosion can be minimized or eliminated. Conservation tillage, particularly no-till, has played a major role in the effective control of USA soil erosion. No-tillage, ridge tillage, and reduced tillage practices can all be considered conservation tillage practices as long as adequate crop residue levels (30% of ground covered by crop residue) are maintained on the soil surface.

No-Tillage, a Biological Tillage

In essence, both no-tillage and conventional tillage are all a tillage process. Only the former is a biological tillage, mainly through root penetration, with mulch in the soil surface, whereas the latter is a mechanical tillage without mulch. As no-tillage with crop residue cover can be exerted continuously, its effect of tillage can be maintained and accumulated, but the effect of conventional tillage without cover cannot be maintained as a result of compaction of raindrops and water evaporation. Therefore, what the no-tillage is maintaining and accumulating is just what the conventional tillage is creating. Clearly, the exhibition of the advantage of no-tillage needs temporal process and continuous practice such as virginal land. Conservation tillage system renders it possible to synchronize cropland utilization and protection in the same way as virginal land. Dick et al. (1991) reported that more than 25 year continuous application of no-tillage to Ohio soils indicates that significantly lower yields for no-tillage, as compared to conventional tillage, were observed for monocultured corn and for soybean in rotation during the first 18 year on a very poorly drained Mollic Ochraqualf soil. The yield differences observed for corn could be largely eliminated by crop rotation and for soybean by the use of phytophera resistant/tolerant soybean cultivars. On a well-drained Typic Fragiudalf soil, crop yields were always higher with no-tillage than with conventional tillage. After a 18 year period, yield trends indicated the negative impact of no-till on the very poorly drained was greatly decreased and the yield advances associated with no-till on the well-drained soil became even more pronounced. The change in yield trends did not appear to be associated with change in weather patterns. The long-term NT (no-tillage) sites also revealed that organic matter, nutrients, and soil enzymes accumulated at the soil surface but decreased deeper (>20 cm) in the soil profile. Surface water runoff was found to be greatly decreased from the long-term NT watershed site (≈9% slop) with only 12 mm of runoff measured between 1979 and 1985. Reduced tillage has increased dramatically over the past several years and is expected to continue to increase in the future. Continuous no-till may become a popular tillage system with growers to facilitate compliance with government programs to control soil erosion. Concerns about corn grain yield reduction with no-till following winter wheat in rotation on clay soils has been a major factor in slowing the rate of adoption of no-till systems in Ontario. Opoku et al. (1997) recommended zone-till or tandem disk in the fall for corn production on clay soils following wheat. Kapusta et al. (1996) revealed that there was no difference in corn yield among tillage systems after 20 years of research when NPK was broadcast. Corn yield was equal in conventional till, alternate till, reduced till, and no-till with fertilizer applied broadcast on an imperfectly drained soil. Continuous no-till with an imperfectly drained soil does not reduce corn yield. No-till can reduce tillage operations by as many as six to eight operations (Nyakatawa et al., 2000), which reduce machinery, fuel, and labor costs and increases machinery life and profits (Keeling et al., 1989). The most important of all, no-till can reduce soil erosion while maintaining or increasing soil productivity (Stevens et al., 1992; Triplett et al., 1996). No-till and mulch-till can lead to the build up of soil degradation by erosion (Edwards et al., 1988; Mills et al., 1988). Conventional tillage increases the risk of soil erosion and contributes to contamination of water resources by phosphates and pesticide leachates. Some area of South China has achieved good results of soil and water conservation and high-yield without irrigation by integrating mulch with contour-ridge culture in upland and by no-tilling ecosystem characteristic of successive culture of ridge-furrow with planting on the ridge, successive no-tillage, successive capillary water circulation, and successive multiple crop rotation (Hou, 1987; Hou and Chang, 1988). Conservation tillage system of USA is dependent on herbicide and pesticide to control weed and pest and disease whereas that of China on ecological control resulted from available and cheap manpower.

Advantages of Conservation Tillage         

The conservation tillage systems resulted in greater economic returns, compared with a conventional tillage system, due to both greater corn yields in dry years and lower production costs in all years (Smart and Bradford, 1999). Conservation tillage improved cotton germination, emergence, dry matter, and lint yield, especially in drier year (Nyakatawa and Reddy, 2000; Nyakatawa et al., 2000). Ridge tillage for row crop production is continuing to increase slowly in popularity across the USA. Benefits of ridge tillage include reduced soil erosion losses (SWCS, 1990) and reduced costs for machinery, labor, and herbicides, while maintaining competitive yield (Reeder, 1990). Ridge tillage allows early planting of corn and soybean on poorly drained soils in the Midwest (Eckert, 1987). No-till reduces runoff, but increases infiltration and macropore flow, compared with conventional tillage. Because it controls traffic and creates a unique surface microtopography, ridge tillage may alter water entry and distribution in the soil profile. Waddell and Weil (1996) observed that a dry period after corn harvest produced an upward water potential gradient in ridge tillage from –18 kPa in the furrow to –35 kPa on the ridge-tops, but a downward gradient in no-till from –10 kPa near the surface to –25 kPa at 80 cm deep. Their results suggest that ridge tillage may have potential as a tool for controlling the patterns of water flow during the fall and winter recharge period. Dou et al. (1995) observed that no-tillage reduced the amount of NO^-₃ accumulated in the 0- to 120-cm soil profile to one-half of the NO^-₃ levels in the corresponding conventional tillage treatments, regardless of N source. Jaynes and Swan (1999) confirmed that fertilizer injection within the ridge shows promise for reducing leaching and potentially increasing nutrient availability to plants.

Soil tillage is believed to influence soil organic matter decomposition. The short-term impacts of tillage on soil surface CO₂ emissions would vary following spring, summer, and fall moldboard plowing. Rochette and Angers (1999) reported that fall incorporation of fresh maize residues increased CO₂ losses by 173 kg C ha^-1 co