Factors Associated with Reduced Yields of Delayed Plantings of Soybean¹

Field experiments were conducted for four years (1981 to 1984) at Lexington, KY, USA (38° N latitude) to determine the causes of reduced yields associated with delayed plantings of soybean ( Glycine max [L.] Merrill). Five cultivars were planted in mid‐May and early July in row spacings ranging from 18 to 89 cm and the plots were irrigated to minimize moisture stress. Yields were significantly reduced by delayed planting because of reductions in the number of seed m −2 and mass seed −1 . The yield reduction under irrigation indicates that moisture stress is not a major cause of reduced yields from delayed plantings. The reductions in seed number were associated with lower insolation interception during flowering and pod set and with smaller vegetative mass (g m −2 ) at growth stage R5 (beginning seed fill). The data suggest that a vegetative mass at R5 greater than that required to intercept 90 % or more of incident insolation is required to maximize seed number. The length of the seed filling period was not affected by planting date. Mass seed −1 was positively related to mean air temperature during seed filling, which suggests that the smaller seed from delayed plantings are a result of the lower temperatures associated with the later maturity.