ABOVEGROUND DRY-WEIGHT AND YIELD RESPONSES OF IRRIGATED FIELD CORN TO DEFOLIATION AND ROOT PRUNING STRESSES

Insecticide-modified levels of western corn rootworm, Diabrotica virgifera virgifera LeConte, larval feeding, manually applied preautotrophic (seedling stage) defoliation, and autotrophic (four-leaf stage) defoliation were evaluated for their independent and combined effects on aboveground dry matter production and grain yield of field corn, Zea mays L. Defoliations were imposed to simulate cutworm feeding damage, late frosts, or hail. Total reductions in leaf dry weight attributable to treatments became less evident as plants compensated over time. However, leaf groups undergoing initiation or expansion when stresses were imposed produced significantly less dry weight after each treatment was applied and were not able to compensate fully by season's end. Stalk dry weights showed less change than leaves after treatment. However, once detectable, reductions in stalk dry weight remained significant throughout the growing season. Decreases of 3-12% in corn yield were attributable to root pruning stress from larval feeding. Grain yield decreased 3-9% in response to seedling defoliation and 13-22% following four-leaf stage defoliation. Yield declines resulted from reduction of total kernels per ear (both kernels per row and kernel rows) and not from kernel weight, which suggests that treatments effectively stressed plants before or during kernel initiation rather than during kernel fill. Significant treatment interactions were evident in responses of some vegetative and dry weight yield parameters. In each instance, less reduction was caused by combinations of treatments than was expected if effects of single treatments were added.