Response of maize and soybean to variability in stand uniformity

The literature indicates variable results in response to uneven stands. We tested the hypothesis that the effect of uneven stands on grain yield varies depending on the vegetative and reproductive plasticity of the species and cultivars. Treatments consisted of a uniform control, nonuniform plant spacing between plants, and uneven seedling emergence. For maize (Zea mays L.), average yield per plant (Yp) decreased 0.68 g for every unit increase in percentage coefficient of variation (CV) for vegetative biomass per plant (Vp). Contrarily, soybean [Glycine mar (L.) Merr.] Yp was not affected by increments in plant size variation. A model based on the relationship between Yp and Vp and on mean and CV for Vp was used to estimate the effect of nonuniformity on grain yield in maize and soybean cultivars. Based on these models and assuming constant average Vp with a normal frequency distribution throughout different stand uniformity treatments, increases in CV for Vp resulted in less grain yield in maize but not in soybean. Reductions in vegetative biomass produced further yield drops according to the specific relationship between Yp and Vp. The effect of unevenness in plant sizes on maize grain yield would depend on the characteristics of the hybrid. A stable hybrid is characterized by low decreases in Vp in response to heterogeneity, low threshold Vp for prolificacy and for grain yield, and a low curvature in the Yp/Vp relationship.