Yield Components Associated with Soybean Yield Reductions at Nonoptimal Planting Dates1

Abstract Expansion of the optimal planting dates for soybean [Glycine max (L.) Merr.) would allow for more flexibility in management to fit into weather and cropping patterns. A field study was conducted to determine yield components associated with soybean seed yield reductions at nonoptimal planting dates in the southeastern USA. This information provides insight into how seed yield reductions occur at nonoptimal planting dates. Optimal planting period in the area is between early May and early June. Cultivars Forrest, Tracy‐M, Centennial, Davis, Ransom, Bragg, and experimental genotype D77‐12480 were planted at early April (nonoptimal), mid‐May (optimal), and mid‐June (nonoptimal) planting dates in 1981 and 1982 at Baton Rouge, LA (30° N Lat) on an Olivier silt loam (fine‐silty, mixed, thermic Aquic Fragiudalf) soil. At maturity, plant samples were separated into main and branch stem components. Main and branch stem length, branch number, node number, fertile node number, pod number, pods per fertile node, seed per pod, seed size, and seed yield were determined. In all genotypes, seed yield reduction at the mid‐June planting date was related to a decline in branch seed yield associated with a reduction in number of branches and branch nodes. The decreased number of fertile branch nodes resulted in fewer branch pods and seed. Davis and D77‐12480 showed no seed yield reduction at the early April planting date. Reduced seed yield of the other genotypes at the early April planting date was related to a reduction in branch nodes and proportion of the branch nodes that were fertile. The decreased number of fertile branch nodes resulted in fewer branch pods and seed. In conclusion, seed yield reduction at nonoptimal planting dates was associated with reductions in branch rather than main stem yield components.