Transgenic Bt corn hybrids with resistance to corn rootworm or European corn borer can have greater tolerance to water and nutrient stress, and thus may have higher optimum plant densities. Experiments were conducted following soybean over nine site-years in Illinois to determine whether the response to plant density for corn grain yield and net return to seed cost differ among near-isoline hybrids with no insect resistance, Bt resistance to CRW, or Bt resistance to CRW plus ECB. Similar experiments were conducted over three site-years in Iowa following both soybean and corn for near-isoline hybrids with Bt resistance to ECB or ECB plus CRW. Larval CRW injury was low in Iowa and stalk lodging was minimal in all experiments. Across site-years in Illinois and in both crop sequences in Iowa, grain yield and net return to seed cost were not af ected by hybrid. Net return to seed cost within $2.50 ha-1 of the maximum occurred with densities of 76,300 to >98,600 plants following soybean in Illinois, 87,100 to 93,400 plants following soybean in Iowa, and 87,400 to 95,700 plants following corn in Iowa. Yields within these optimum plant densities were 15.9, 16.1, and 15.4 Mg ha-1, respectively. When CRW and ECB are managed or are at low levels, optimum plant density is similar between hybrids with or without resistance to these pests.
