Response of bt and near-isoline corn hybrids to plant density

Transgenic Bt (Bacillus thuringiensis) corn (Zea mays L.) hybrids with resistance to corn rootworm (CRW; Diabrotica spp.) or European corn borer [ECB; Ostrinia nubilalis (Hübner)] can have greater tolerance to water and nutrient stress, and thus may have higher optimum plant densities. Experiments were conducted following soybean [Glycine max (L.) Merr.] 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 affected 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 ha^-1 following soybean in Illinois, 87,100 to 93,400 plants ha^-1 following soybean in Iowa, and 87,400 to 95,700 plants ha^-1 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.