Sweet corn is planted over a long season to temporally extend the perishable supply of ears for fresh and processing markets. Most growers' fields have weeds persisting to harvest (hereafter called residual weeds), and evidence suggests the crop's ability to endure competitive stress from residual weeds (i.e., crop tolerance) is not constant over the planting season. Field studies were conducted to characterize changes in the residual weed community over the long planting season and determine the extent to which planting date influences crop tolerance to weed interference in growth and yield traits. Total weed density at harvest was similar across five planting dates from mid-April to early-July; however, some changes in composition of species common to the midwestern United States were observed. Production of viable weed seed within the relatively short growth period of individual sweet corn plantings showed weed seedbank additions are influenced by species and planting date. Crop tolerances in growth and yield were variable in the mid-April and both May plantings, and the crop was least affected by weed interference in the mid-June and early-July planting dates. As the planting season progressed from late-May to early-July, sweet corn accounted for a great proportion of the total crop–weed biomass. Based on results from Illinois, a risk management perspective to weeds should recognize the significance of planting date on sweet corn competitive ability. This work suggests risk of yield loss from weed control failure is lower in late-season sweet corn plantings (June and July) than earlier plantings (April and May).

Nomenclature: Corn, Zea mays L.