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Effect of Early and Late Flowering on Agronomic Traits of Soybean at Different Planting Dates¹

Double-cropping soybean [Glycine max (L.) Merr.] after a winter small grain crop often forces planting to be delayed beyond the date when maximum yield, plant height, and vegetative mass can be attained. The objective of this study was to evaluate soybean plant introduction (PI) lines selected for differing flowering dates, but with a constant maturity, for differences in vegetative size and yield reduction due to delayed planting. Two groups, each consisting of 12 PI lines, were compared in the field at Lexington, KY for 3 yr (1982–1984) in early (15 May) and late (21 June) planting on a Maury silt loam (fine, mixed, mesic Typic Paleudalf) soil. The early flowering group (group EF) flowered 10 days and matured 1 day earlier than the late flowering group (group LF). Group LF was taller with more nodes at Rl, R4, and R7 in both planting dates. The flowering and pod initiation (R1–R4) and seed filling (R4–R7) periods were shorter in group LF than in group EF. Yields averaged over all 3 yr were 1510, 1520, 1370, and 1380 kg ha^–1 for group EF early planting, group LF early planting, group EF late planting, and group LF late planting, respectively. Both groups showed a 9% decrease in yield due to delayed planting. The planting date effect was highly significant, while the group effect and the planting date x group interaction were nonsignificant for yield. In 1983 and 1984, group LF had smaller yield reductions in delayed plantings than group EF (6 and 17% reductions for each group, respectively). Although the planting date x flowering group interaction for yield was highly significant in 1983 and 1984, group LF did not yield significantly more in late plantings. In 1982, the two groups did not differ in yield in late plantings. Thus, in this sample of soybean Pis, delayed flowering increased height and vegetative size but did not provide a yield benefit in late plantings.

Key Words: Double cropping • Glycine max (L.) Merr. • Growth stages

² Assistant professor and research technician, Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091.

Received for publication November 22, 1985.

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