Field experiments were conducted from 1987 to 1990 to investigate the combination of cultural and chemical methods for control of Amaranthus spp. in corn. The tolerance of corn hybrids to herbicides or herbicide combinations was dependent on application rate. The early season threshold level for a mixed population of Amaranthus spp. occurred 3.5 wk after emergence. Density of Amaranthus spp. from 155 to 495 plants/m2 caused corn yield reduction of 50%. A quadratic model was derived relating percentage of yield reduction vs. duration of weed competition. The results emphasize the importance of using herbicides, cultural practices, and competition thresholds for an integrated weed control approach of Amaranthus spp. in corn.
