GROWTH DYNAMICS OF DETERMINATE SOYBEAN IN NARROW AND WIDE ROWS AT LATE PLANTING DATES

Previous studies indicated that the increased light interception available in narrow-row plantings generated greater total (dry-weight) biomass at growth-stage R5 (B(R5)) in soybean [Glycine max (L.) Merr.]. This increased biomass accumulation resulting from narrow rows is especially important in increasing seed yields at late planting dates compared with optimal. The first objective of this study was to determine the relative importance of crop growth rate (CGR) and relative growth rate (RGR) in causing greater B(R5) in narrow rows, compared with wide, at late planting dates. Since CGR is determined by net assimilation rate (P(n))xL (leaf area index) and RGR is determined by L(r) (leaf area ratio), another objective was to determine which of these secondary factor(s) were responsible for changes in CGR and RGR in narrow rows compared with wide. Field-studies were conducted at late planting dates (in July) during 1987 and 1988 with two commercial soybean cultivars, at 100-cm (wide) rows at normal plant density, 50-cm (narrow) rows at normal plant density, and 50-cm rows at high plant densdity (50H). The test was conducted at Baton Rouge, Louisiana on a Mhoon silty clay (fine-silty, mixed, nonacid, thermic, Typic Fluvaquents) soil. Relative growth rate made no contribution to greater biomass accumulation in narrow rows. Increased CGR in narrow rows resulted more from increased L than changes in P(n). This increased L resulted in greater light interception (Q) and hence greater CGR. Duration of L (L(d) in narrow rows compared with wide was related to increased leaf number rather than area per leaf (A1). Genetic/cultural manipulations which increase light interception during emergence-to-R5 would be expected to enhance CGR, increase B(R5), and hence increase seed yield at late plantings.