DYNAMICS OF SOYBEAN GROWTH IN VARIABLE PLANTING PATTERNS

Reported soybean [Glycine max (L.) Merr.) yield responses to variable plant populations have been inconsistent. The varied responses are tied to the capacity of the plant stand to attain a minimal level of dry matter prior to reproductive growth. This study examined soybean growth expressed as functions of both plant and ground area to identify associations with yielding ability. Treatments consisted of high, medium, and low populations (approximately 21, 13, and 3 plants per square meter) at 0.96-m row widths and a medium population (approximately 13 plants m-2) at 0.43-m row widths for 2 yr. The values of total dry matter (TDM), stem DM, leaf DM, leaf area, height, and main stem node number were determined on both a whole plant and ground area basis. In addition, relative growth rate (RGR), relative leaf area growth rate (RLAGR) and net assimilation rate (NAR) were examined. Treatments with the greatest interplant space had the largest RGR, RIAGR, and NAR early in growth, thus resulting in compensation for reduced plant population. Decreases in plant DM at 91 DAP in response to increasing plant population were greater in 1988 than in 1989, and were related to poorer growing conditions in the latter year. The related TDM values were larger in 1988, with all treatments exhibiting a TDM above 500 g m-2 by early podfill (R5). In contrast, the TDM at 91 DAP of the wide row, low and medium population treatments was 300 and 426 g m-2, respectively in 1989. Seed yield differences were found in 1989 but not 1988. Differences among years and treatments concerning growth rates, plant DM, TDM, and seed yield emphasize the impact of limiting environmental factors on plant compensation in response to increased space.