Seed Yield of Soybean Bulk Populations Grown for 10 to 18 Years in Two Environments

The bulk method of plant breeding has been used extensively by breeders working with variouself- and cross-pollinated plant species, but the value of this breeding technique as a means of improving gene frequency for seed yield is not well-documented in soybean [Glycine max (L.) Merr.]. The objective of this research was to determine the effect of two diverse locations, where bulk populations had been grown for 10 to 18 yr, on seed yield. Seed of these bulk populations were harvested in 1981 and evaluated for seed yield in 1982 and 1983 at the two locations (Rohwer and Marianna, AR), where the bulks were developed and at a neutral location (Stuttgart, AR). The soil series at the three locations are: Rohwer—Perry clay (very-fine, montmorillonitic, nonacid, thermic Vertic Haplaquepts); Marianna—Loring silty loam (fine-silty, mixed, thermic Typic Fragiodalfs); and Stuttgart—Crowley silty loam (fine, montmoriilonitic, thermic Typic Albaqualfs). Three of the four bulks developed at Rohwer produced significantly higher seed yields at this location than the same bulks developed at Marianna. The bulk population that did not show a yield increase was derived from parents that are resistant to Phytophthora rot (caused by Phytophthora megasperma Drechs. p. glycinea Kuan and Erwin), whereas the other bulks contained segregates that were resistant and susceptible to this disease. Seed yields at Marianna and Stuttgart were not significantly different for the two seed sources of the bulk populations, although there was a trend for the bulk populations developed at Rohwer to produce slightly higher yields at Stuttgarthan those developed at Marianna. Damage from Phytophthora rot was evident each year the bulks were grown at Rohwer, whereas damage from this disease was not observed at Marianna during the development of the bulk populations. Greenhouse tests also indicated there was a significantly greater number of plants resistant to Phytophthora rot in the Rohwer seed source than in the seed source of the bulks developed at Marianna. These results indicate that the bulk method of breeding for soybean is moreffective where there is constant year-to-year selection pressure from a disease or other stress factors.