A CRITERION FOR ACCEPTANCE OF NARROW-ROW CULTURE IN SOYBEAN

Soybean [Glycine max (L.) Merr.] yield response to narrow compared with wide rows in the southeastern USA has usually been greater for late than for optimal plantings. Identification of growth responses explaining this pattern could help develop criteria for acceptance of narrow-row culture. Narrow-row yield responses result from increased pod number created by greater light interception and crop growth rate between first flowering and seed initiation (R1 to R5). Lack of yield response to greater light interception in narrow rows in optimal plantings may be related to the association between pod number and total dry matter at harvest maturity (R8). Our objectives were to determine if total dry matter at R8 [TDM(R8)] could be used as a criterion for acceptance of narrow-row culture and to identify the morphological relation between pod number and TDM(R8). Field studies conducted in 1987 through 1990 used optimal and late planting dates, two soybean cultivars, and several row; spacing-plant density combinations. Results indicate a quadratic response of pod number to TDM(R8). Narrow-row yield responses occurred when light interception (R1 to R5) increased crop growth rate (R1 to R5) to less than or equal to 15 g m(-2) d(-1) and increased TDM(R8) to less than or equal to 800 g m(-2). The ratio of pod number to TDM(R8) declined as TDM(R8) approached 800 g m(-2), because of a reduced ratio of branch nodes to branch dry matter, In conclusion, narrow-row yield responses ran be expected if TDM I(RS) in wide rows is <800 g m(-2).