Yield rankings for soybean cultivars grown in narrow and wide rows with late planting dates

Cultivar selection is a major problem for late-planted soybean [Glycine max (L.) Merr.] in the southeastern USA. Cultivar yield rankings differ between optimal and late planting dates. Thus, recommendations for late plantings are based on statewide cultivar trials at late planting dates. Because such trials are large and expensive, high-yielding cultivars in late plantings could be more rapidly and efficiently selected through identifying morphological and developmental parameters associated with cultivar yield performance. Furthermore, consistency of yield rankings between narrow- and wide-row culture within a late planting is not known. Our objectives were to (i) determine if row spacing significantly affects cultivar yield rankings at a late planting date and (ii) identify yield components, growth dynamic factors, and phenological characteristics associated with cultivar yield differences at a late planting date. Twelve commercial soybean cultivars were planted in 91- and 50-cm row spacings during 1992 and 1993 on a Commerce silt loam (fine-silty, mixed, non-acid, thermic Aeric Fluvaquents) near Baton Rouge, LA (30 degrees N lat). No significant cultivar x row spacing interaction for yield occurred. Yield rankings for the 12 cultivars vvere similar between row spacings (r(2) = 0.95, P < 0.0001). Cultivar yield differences were attributed largely to differences in pod number (r(2) = 0.35, P < 0.0001), rather than seed per pod (r(2) = -0.07, P < 0.0001) or seed size (NS). Among all parameters, total dry matter at R5 vc as most highly correlated with yield (r(2) = 0.55 to 0.62, P < 0.0001) across different row spacings or within the same row spacing. In conclusion, total dry matter at R5 and other parameters associated with yield were identified, parameters that can be used to develop regression equations for predicting cultivar yield performance in late plantings.