Initial Plant Size Affects Response to Thinning in Soybean

Soybean [Glycine max (L.) Merr.] can produce compensatory yield following plant stand reduction, but the effect of plant size within a developmental stage on this ability is unknown. We seeded two cultivars at 15, 30, 45, and 60 plants m−2, and identified large and small plants within each density at V3 (third trifoliate), V6 (sixth trifoliate), R2 (full bloom), and R4 (full pod) developmental stages. Subplots were either thinned to a density of 5.3 plants m−2 or left at intact densities. The ability for large or small plants to produce compensatory growth and yield was evaluated by the differences between subsequent growth and yield produced in thinned vs. intact stands. We found small plants had equivalent harvest index to large plants across all plant densities and developmental stages, suggesting small plants were as efficient as large plants for compensating seed yield relative to biomass when stands were not thinned. However, when stands were thinned, compensatory growth and yield were different between large and small plants. For example, across all plant densities thinned at V3, large and small plants produced 51 and 37 g plant−1, respectively, and compensatory yield contributed 30 and 24 g plant−1, respectively. Thus, small plants produced 80% as much compensatory yield as large plants when thinned at V3. However, by the same comparison, the contribution of small plants declined to 57% when thinning was delayed to R4 development, suggesting small plants lost more compensatory ability over time. We conclude initial soybean plant size impacts compensatory yield plant−1.