Modern cereal varietal adoption patterns

Modern cereal varieties, developed by scientific breeding programs, began to spread through many of the countries now considered "industrialized" in the late 19th century. The green revolution accelerated this process and extended it into much of the developing world. The adoption of modern cereal varieties has been most widespread in land-scarce environments and/or in areas well connected to domestic and international markets, where the intensification of agriculture first began. Even in these areas, the profitability of modern variety adoption has been conditioned by the potential productivity of the land under cultivation. For instance, while modern rice and wheat varieties spread rapidly through the irrigated environments, their adoption has been slower in the less favorable environments—the drought-prone and high-temperature environments for wheat and the drought- and flood-prone environments for rice. Maize has an even spottier record in terms of farmer adoption of modern varieties and hybrids. For all three cereals, traditional landraces continue to be cultivated in the less favorable production environments throughout the developing world (Pingali and Heisey, 2001).

Evenson and Gollin (2003) provide information on the extent of adoption and impact of modern variety use for all the major food crops. The adoption of modern varieties (for 11 major food crops averaged across all crops) increased rapidly during the two decades of the green revolution, and even more rapidly in the following decades, from 9% in 1970 to 29% in 1980, 46% in 1990 and 63% by 1998. Moreover, in many areas and in many crops, first-generation modern varieties have been replaced by second- and third-generation modern varieties (Evenson and Gollin, 2003).

According to Smale (1997), the adoption of modern cereal varieties has been characterized first by a concentration on a few varieties followed by diversification as more varieties became available. In the 1920s, for example, a single variety accounted for more than 60% of the wheat crop in the northern and central parts of Italy. Single cultivars became similarly dominant in many countries in Europe and North America, as mechanization created a need for uniform plant types and uniform grain quality. As the process of modernization proceeded and the offerings of scientific breeding programs expanded, the pattern of concentration declined in many European and North American countries (Lupton, 1992; and Dalrymple, 1988, cited in Smale, 1997). Similarly, in the early years of the green revolution, the dominant cultivar occupied over 80% of the wheat area in the Indian Punjab, but this share fell below 50% by 1985. By 1990, the top five bread wheat cultivars covered approximately 36% of the global wheat area planted to modern varieties (Smale, 1997).

4.1.1 Implications of modern varietal distribution for crop genetic diversity

Whether the changes in crop varietal adoption described above have resulted in a narrowing of genetic diversity remains largely unresolved due to conceptual and practical difficulties3 (Pingali and Smale, 2001). Scientists

3 Crop genetic diversity broadly defined refers to the genetic variation embodied in seed and expressed when challenged by natural and human selection pressure. In applied genetics, diversity refers to the variance among alternative forms of a gene (alleles) at individual gene positions on a chromosome (loci), among several loci, among individual plants in a population, or among populations (Brown et al., 1990). Diversity can be measured by accessions of seed held in gene banks, lines or populations utilized in crop-breeding programs, or varieties cultivated by farmers (cultivars). But crop genetic diversity cannot be disagree about what constitutes genetic narrowing or when such narrowing may have occurred. Several dimensions of diversity must be considered in this regard, including both the spatial and temporal variations between landraces and modern elite cultivars and the variation within modern cultivars. Hawkes (1983, cited in Smale, 1997) argued that the genetic diversity of landraces and modern varieties is incomparable by definition because landraces, which are mixtures of genotypes, "could not even be called varieties," and he called the range of genetically different varieties available to breeders the "other kind of diversity" (pp. 100-101). Smale (1997) argued that the range of genetic material available to breeders is not directly correlated with the number of varieties in use because a single modern variety may contain a more diverse range of genetic material than numerous landraces.

Scientists also disagree about what constitutes genetic narrowing within modern varieties. For example, Hawkes (1983) cites the introduction of the Rhtl and Rht2 dwarfing genes into wheat breeding lines as an example of how diversity has been broadened by scientific plant breeders, while Porceddu et al. (1988) argue that the spread of semidwarf wheat varieties during the green revolution led to a narrowing of the genetic base for that crop (Pingali and Smale, 2001).

These points imply that comparing counts of landraces and modern varieties or changes in the number of modern varieties over time may not provide a meaningful index of genetic narrowing. They also imply that even if reliable samples of the landraces originally cultivated in an area could be obtained, analyses comparing their genetic diversity might provide only part of the answer regarding genetic narrowing. Although the landrace in the farmers' field is a heterogeneous population of plants, it is derived from generations of selection by local farmers and is, therefore, likely to be local in adaptation. In contrast, the plants of a modern variety are uniform but the diverse germplasm in their genetic background may enable them to adapt more widely. The diversity in a modern variety may not be expressed until challenged by the environment. On the other hand, the landrace may carry an allele that occurs rarely among modern varieties and is a potentially valuable source of genetic material not only for the farmer who grows it today but also for future generations of producers and consumers. (Pingali and Smale, 2001).

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