The term 'genetic diversity' is in common parlance. However, for genetic diversity to be useful in plant breeding to serve farmers and consumers, it must encompass genetic variability that is not present in the materials with which breeders are currently working. It is necessary to have new sources of germplasm for present and future uses since environmental conditions, disease pressure, technologies and demands from the farmer and consumer are constantly changing. New sources of germplasm must have yield potential with other useful traits so that breeders can be encouraged to use sources of new genetic diversity.
Despite their best efforts, by the late 1920s, farm breeders in the US had not been able to raise average maize yields above 1880 kg ha-1 (30 bushels per acre). Plants remained very susceptible to heat and drought stress, and ravages by pests and diseases continued to be disastrous. As a result of research and practical experience gained by breeders from the early part of the 20th century, yield gains for maize increased to 3136 kg ha-1 during 1930-1960, due to the use of double-cross hybrids. The rate of yield gain due to genetic change alone tripled again after 1960, due to the utilization of single-cross hybrids. These yield gains are due solely to plant breeders achieving increased managerial control over germplasm-producing hybrids. Effective plant breeding requires vastly more effort to improve crop yields than mass selection and seed saving which are the strategies of informal breeders.
Modern corn agriculture in the US uses hybrids from a cross of two inbred lines, which gives the current average yield of about 8152 kg ha~1 (130 bushels per acre). The improvement in yield is due to years of selection by breeders to improve agronomic characteristics such as: reducing plant height, selection of the plants to stay alive until maturity, improvements in stalk and root lodging, selection for upright habit, tolerance to European corn borer, and greater stress tolerance that allows consistent yields at high plant densities.
The history of plant breeding is one of a continual increase in the range and capabilities of techniques that have come from basic and applied research in genetics, physiology, statistics, molecular biology, etc. Plant breeding exemplifies the continuing adaptation by humans to meet the increasing food needs of growing populations. And like the first agricultural revolution some years ago, which demanded more effort per unit area to provide more food, successful and sustainable plant breeding requires yet more effort to increase food production.
The bases for successful and sustainable progress in plant breeding are: (i) the ability to find useful genetics affecting traits of agricultural importance; and (ii) the ability to recombine genetics favourably affecting agronomic traits into new varieties that are overall better adapted to the target environment of the farmer and the demands of the consumer or processor.
Key resources that are necessary to achieve these objectives are:
• available sources of useful genetic diversity;
• adequate knowledge of these genetic resources to facilitate sourcing of useful germplasm;
• adequate availability of technologies and skills to enable sourcing, manipulation and effective transference of germplasm from exotic sources into adapted and improved varieties;
• adequate knowledge of customer needs and target environment;
• adequate funding to support the relatively long-term programmes of research and product development that are required in plant breeding (10-15 years) and in effective sourcing of exotic genetic diversity (15-25 years).
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