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Effect of Planting Date on Natural Selection in Soybean Populations¹

Early-maturing segregates are frequently eliminated in soybean (Glycine max (L.) Merr.) bulk populations. Six simulated bulks were made of five cultivars in maturity groups I through V, the two cultivars in each bulk differed by at least two maturity groups. These bulks were planted at several dates in 4 years to determine whether late planting would increase the relative reproduction of the early cultivars in the bulks. Their relative reproductivity increased twice, decreased once, and did not change once. The early cultivars in all bulks were at a reproductive disadvantage compared to the relationship in pure stands. The reproductive disadvantage tended to increase with differences in the time of maturity in the simulated bulks, although plant size probably was more important than maturity per se. Even though the early cultivars in the bulks did not consistently gain in reproductive advantage with late planting, late planting should help maintain them in bulk populations because of improved seed quality.

Key Words: Competitive ability • Maturity • Plan height • Weathering • Glycine max (L.) Merr.

² Research agronomist, USDA, SEA/FR, Columbia, MO 65201.

Received for publication October 31, 1977.