MECHANISMS RESPONSIBLE FOR SOYBEAN YIELD RESPONSE TO EQUIDISTANT PLANTING PATTERNS

Increased soybean [Glycine mar (L.) Merrill] yields in narrow rows can occur without an increase in light interception during reproductive growth. Experiments were conducted for 3 yr in the held to investigate mechanisms responsible for this response. Two Maturity Group IV cultivars (Pennyrile, indeterminate growth habit, and Ripley; determinate growth habit) were grown in 0.76-m rows or an equidistant planting pattern (each plant approximate to 0.2 m from all neighboring plants) at a population of 27 plants m(-2). Seeds were planted on 5 June 1990, 22 May 1991 and 15 May 1992. The experiment included irrigated and nonirrigated treatments; however, rainfall was above normal every year, and there was no effect of irrigation on yield. Light interception at growth stage R2 was higher (14%) in the equidistant pattern for both cultivars but there was no difference at growth stage R5. Yield of Ripley in the equidistant pattern nas significantly higher (9-19%) than the rows every year, while Pennyrile showed a significant increase only in 1990. The increase in yield occurred because of an increase in seeds per square meter; seeds per square meter was significantly Linearly related to fertile nodes (nodes with at least one normal pod) per square meter for Ripley (r(2) = 0.42), but not Pennyrile. Ripley showed an increase in fertile nodes per square meter in the equidistant pattern but Pennyrile did not. The data suggest that the increase in seeds per square meter occurred as a result of a more efficient utilization of assimilate associated with more nodes per square meter.