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Soybean Yields in Relation to Days Between Specific Developmental Stages¹

Environment, production practices, and genetic differences are known to influence the number of days between developmental stages of soybeans [Glycine max (L.) Merr.], but the yield response to differences in days between various stages is poorly understood. Twice weekly observations were made on 10 central Iowa replicated field trials over a 3-year period to determine the date that each plot reached several specific stages of plant development. These dates and final seed yields were utilized to investigate whether number of days between specific stages of development were associated with final seed yield. All sites were mapped as Webster silt loams (a member of the fiie-loamy, mixed noncalcareous, mesic family of typic haplaquolls). Variations among the 119 cultivars represented in the study accounted for more of the differences in days between developmental stages than did sites or yearn. There was no association between seed yield and number of days between any two vegetative stages of development. The length of the seed development period [days from stage R4 (pods 2 cm long at top four nodes) to R7 (pods yellowing)] was more highly correlated with seed yield than was the length of any other developmental period. Plots which initiated flowering later and matured later tended to be the highest yielding plots. Longer periods of pod development [days from stage RZ (flowers at top node) to R4] tended to be associated with lower yields. The three locations which also included fertility variables showed that differences in the P and K fertility levels utilized did not influence the number of days between any two developmental stages. These findings provide breeders with another potential selection criterion and production researchers with a better understanding of how environmental and production practice differences influence soybean yields.

Key Words: Glycine max (L.) Merr. • Cultivar • Vegetative stages • Reproductive stages

² Associate professor of Crop Science, North Carolina State Univ. (formerly assistant professor of Agronomy, Iowa State Univ.), and professors of Agronomy, Iowa State Univ., respectively.

Received for publication December 15, 1978.

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