| Agricultural scientists involved in extending modern crop varieties in developing countries should respect indigenous practices which maintain varietal diversity within cereal monocultures. In situ diversity of germ plasm preserves evolutionary options, enables the "reversibility" of varietal change at the local level, and reduces disease and insect problems.; Fieldwork among lowland Thai, Hmong, and Karen farming communities in northern Thailand demonstrates that there was active retention of rice varietal diversity in irrigated and upland agroecosystems, whereas in the rainfed area lowland Thai farmers discarded all traditional rice varieties, apparently due to the incompatibility of these varieties with chemical fertilizer, which was widely used on rainfed soils.; Some farmers actively exchange rice varieties along kinship lines between villages and districts due to their perception that yields decline when the same variety is grown successively for more than three years. Genetic erosion of varieties has been high, but so is the influx of new varieties to irrigated and upland areas. Of 89 lowland Thai varieties collected in the approximate study area in 1950-61, only 15 were collected under the same names in 1982-83. However, about 100 lowland Thai varieties were collected in 1982-83 in the approximate study area. Of 166 rice farmers sampled in a random survey, only one planted a modern rice variety (MV) in 1979, compared to 28 per cent planting MVs in 1984. Adoption of MVs often was not a unidirectional process at the farm level. Early adopters of MVs displayed "contrarian" or "diversity maintaining" behavior by (a) shifting to another outside variety before the first became popular, and (b) also using out-of-favor traditional varieties.; In 1983, irrigated, upland, and rainfed farms had average rice yields of 3.6, 2.9, and 1.9 MT/ha, respectively. Traditional and modern varieties gave comparable yields under rainfed conditions.; Isozyme analysis of rice variety accessions indicated that many lowland Thai traditional varieties that were morphologically diverse were genetically similar, whereas modern varieties brought new isozyme groups to the region. Adoption of MVs has led to the genetic simplification of lowland rice agroecosystems in the Philippines and South Korea, but partial adoption of MVs in northern Thailand has increased genetic diversity of lowland rice agroecosystems. |
