In its 8 000-year history, wheat continues to be the major food grain crop consumed by humans. World wheat production, now averaging nearly 600 million tonnes annually, is currently maintaining pace with population growth. Production gain for the last half century has been about 1 percent per year, due to the technological advances of more productive cultivars and adoption of improved cultural practices. Future projection is that the annual gain will need to reach near 2.5 percent by the year 2025 to keep abreast of population growth. Although land area devoted to wheat worldwide is more than for any other crop, further area expansion is limited. The majority of wheat is produced in the temperate climates of the world. Some area increase has occurred in recent years by moving wheat into non-traditional areas formerly thought unacceptable for production. Although moving wheat culture into non-traditional growing areas offers some promise for area expansion, production measures are more critical due to greater abiotic and/or biotic stresses. Future production increases must come largely from greater output per unit area, which will require more intensive research for further improving cultivars and enhancing cultural technology.
Wheat research in the twentieth century has been monumental in providing a basic understanding of this food crop in terms of origin, diversity, botany, culture, genetics and cytogenetics, physiology, disease and insect pests, food quality aspects and how to improve its productivity. Two American Society of Agronomy (ASA) monographs, the 1967 and 1987 editions of Wheat and Wheat Improvement, provided comprehensive compilations of facts and information about wheat. Since publication of the latter edition, much new information has accumulated in several disciplines. This publication contains a large number of references published after the second edition of the ASA monograph.
Several chapters in this volume suggest a trend in the amalgamation or synthesis of research towards goal accomplishment. International wheat breeding based on emphasizing a broad genetic base, shuttle breeding and multilocation testing has been led by the International Maize and Wheat Improvement Center (CIMMYT) for the past three decades. It has been successfully used to increase yield potential and to improve resistance to both biotic and abiotic stresses under widely varying environments. In semi-arid areas, this approach aims at constructing a genetic system in which plant responsiveness provides a bonus whenever environmental situations improve due to higher rainfall. Breeding for drought tolerance also uses landraces or similar low-yielding materials with drought tolerance for breeding parents.
Understanding the physiological basis of yield and growth can potentially complement traditional breeding in three main ways: i) by identifying traits that may serve as indirect selection criteria for yield; ii) by developing selection methodologies that increase the efficiency of parental and progeny selection; and iii) by providing insights into the physiological and genetic basis for raising yield potential. Physiological selection traits for drought tolerance have been incorporated into a number of wheat breeding programmes, including higher transpiration efficiency, greater early vigour and reduced tillering.
Another science reaching maturity is the utilization of intergeneric and interspecific crosses, particularly interspecific, to broaden the germplasm base in breeding lines. Interspecific crosses have been used with limited success for several decades, but only recently have large numbers of breeding populations involving a widened range of A- and D-genome donors from wheat-related diploid species been made available to breeders. Value assessment of the broadened diversity is underway in several research programmes.
Apart from studies that examine the introduction of novel genetic material, developments in wheat molecular genetics have been relatively slow due to the plants ploidy level, the size and complexity of its genome, the very high percentage of repetitive sequences and the low level of polymorphism. The major contribution of molecular genetics in wheat has been the identification of markers linked to important genetic traits. Molecular genetics is also likely to aid conventional breeding in changing the quality of wheat grain by developing it for novel industrial uses and improving its nutritional structure in ways that will benefit consumers (increasing its content of available iron, zinc, vitamin A and certain amino acids).
Hybrid wheat continues to offer promise as a means to increase wheat yields. Work on the male-sterile and restorer mechanism, although somewhat diminished recently, continues on several fronts. Improved chemical hybridizing agents (CHAs) are undergoing evaluation for production of hybrids on a large scale. Although high-parent heterosis is reported to have exceeded 40 percent in some cases, the question remains as to whether wheat hybrids will become an economic reality.
The understanding of the genetics of leaf and stem rust resistance is rapidly improving, which should enhance breeders efforts in developing more durable disease-resistant cultivars. As new cropping systems and agronomic practices, such as zero-tillage and exotic rotations, gain prominence, diseases caused by Fusarium, Helminthosporium and the Septoria become more economically important and will necessarily consume more research resources. Recent research has shown that soil-borne diseases exact a much heavier toll in wheat productivity than previously thought and will deserve greater attention in the future. Exploitation of synthetic wheats derived from interspecific and intergeneric crosses appears to be an avenue for finding useful sources of genetic resistances to these diseases.
In general, wheat pest control needs to be addressed from an ecologically based integrated pest management (IPM) viewpoint and in conjunction with established wheat breeding programmes. Control of insect pests of wheat is often secondary to the more pressing problems of diseases and abiotic constraints, such as heat, drought and salinity, and thus the development of cultivars possessing resistance to insects has for the most part lagged behind the development of high-yielding, disease-resistant cultivars. Cultural control techniques are more common since they usually require little new farmer input.
They may range from altering seeding rate or date as with Hessian fly to planting depth, fertilizer regimes and early harvesting as in the case of suni bug.
The cultivation of an improved cultivar possessing the needed disease and insect resistances does not necessarily result in higher sustainable yields if inappropriate management is used. In concert with breeding efforts is the need to invest more research resources in developing better crop, tillage and water management. It is important to rebuild or maintain a production base that allows maximum sustainable yield and optimum use of inputs whether they are fertilizer (emphasis on nitrogen in most irrigated situations), water, land or time. Currently, there is information available on practices that can potentially improve the efficiency of nitrogen fertilizer use in irrigated wheat areas in developing countries. Some of this information, such as better synchronization of nitrogen supply and demand, could be transferred readily to different regions of the world. Other information will require local calibration. Farmers participation is essential for the adaptation of technologies to local conditions.
In recent years, production declines have occurred in some major irrigated areas where wheat is important. In some areas of South Asia, for instance, the intensive rice-wheat systems are beginning to show signs of decline associated with loss of soil quality and increased plant health problems. An understanding of why productivity appears to be declining even though farmers are increasing levels of inputs used is crucial to reversing the trend or to preventing it where it may occur. A narrow rotation can exacerbate production declines, especially when continuous cereal crop rotations are followed as compared to more complex rotation systems, especially those including legume or Brassica sp. crops.
The wheat area under conservation tillage is increasing significantly. In many cases, the apparent force pushing farmers to reduce tillage is cost reduction. However, in some situations such as in Brazil, the initial driving force was to reduce erosion. It has become clear that once production systems using conservation tillage with proper crop residue management are implemented, many other beneficial changes can occur, such as increased soil organic carbon, which will eventually translate into improved water-use efficiency. On the negative side, and as mentioned before, disease problems, such as Fusarium, may become more important under such production systems. A crucial issue is how to develop strategies to make these technologies available to the multitude of small farmers in the developing countries.
This volume presents comprehensive coverage of wheat seed production technologies to be used by developing countries. Apart from good agronomic management of the crop, seed production differs from grain production in land requirement, isolation, roguing and prevention of contamination, and must meet specific quality standards prescribed by the national seed regulations. However, the future for high-quality wheat seed appears to be mixed. Wheat is a high-volume, low-profit seed crop and has been produced primarily by highly subsidized government seed programmes. With privatization and restructuring, many of these programmes are being eliminated. The private sector can, however, ill afford to focus on wheat seed, due to its characteristic low-profitability. Furthermore, in many countries there has been no ongoing effort to promote the use of improved seed by wheat farmers, and no significant breeding developments have recently taken place. It is, therefore, expected that in the future the large majority of resource-poor, small-scale farmers in many developing countries will have to rely on seed saved from the previous harvest.
Future wheat improvement must emphasize grain yield enhancement and yield stability within interdisciplinary, integrated approaches and in conjunction with farmers. Issues of environmental sustainability must become an integral part of the crop improvement research agenda. This challenge requires: concerted, complementary efforts to gather a critical mass of scientists and achieve essential operational sizes; sound hypotheses and strategies translated into research objectives; free exchange of germplasm and information; and dynamic cooperation among the global community of scientists. Each one of these requirements must be met to accomplish the common mission of meeting the food needs of the world population and alleviating poverty in developing countries. The aim of this book is to provide research and development communities with updated summaries on the various disciplines and sub-disciplines of wheat and wheat research.
Plant Production and Protection