CHANGES IN YIELD AND SEED GROWTH TRAITS IN SOYBEAN CULTIVARS RELEASED IN THE SOUTHERN USA FROM 1945 TO 1983

The successive release of new cultivars over time implies that crop yield, and perhaps the physiological traits which influence it, have been improved or altered. For soybean [Glycine mar (L.) Merr] cul tivars released in the southern USA, there is little information on the impact of cultivar improvement on physiological traits. The objectives of our studies were to (i) estimate the genetic change in seed yield relative to year of cultivar release for a set of cultivars released over nearly a 40 yr period, and (ii) assess what concomitant changes occurred in the above-ground biomass at the beginning of seed growth, dates of reproductive stage R5 (beginning seed fill) and R7 (physiological maturity), final harvest index, dry matter allocation during seed growth, and seed filling duration. Eighteen determinate soybean cultivars of Maturity Groups (MG) VI through VIII released in USA from 1945 to 1983 were field tested in eight environments, four in Florida and four in Argentina. Annual rates of genetic gain in seed yield were significant and positive in only two U.S. environments (1.59 and 1.87 g m(-2) yr(-1)). The newest cultivars exhibited superior yields over the oldest cultivars only within MG VII and VIII, when the analysis included all four USA environments. In Argentina, no increase in yield was observed in relation to year of cultivar release, even in an environment where the mean seed yield exceeded 4000kg ha(-1), which was 73% higher than the best USA trial. In fact, some of the oldest released cultivars were found to have the highest yields in the Argentina environments. None of the physiological traits measured at the USA locations were found to change appreciably with year of cultivar release. In fact, seed yield within and across environments was not found to be consistently associated with any of the measured seed growth traits. Seemingly, a number of alternatives for combining seed growth traits have been used in the succession of released soybean cultivars.