Feeding value of corn silage estimated with sheep and dairy cows is not altered by genetic incorporation of Bt176 resistance to Ostrinia nubilalis

A genetically modified Bt176 corn hybrid (Rh208Bt)-providing control of European corn borer damage-and the conventional isogenic hybrid (Rh208)-harvested as whole plant silage-were evaluated in three separate feeding trials to verify that the in vivo feeding value was substantially equivalent among modified and conventional hybrids. In the first trial, after a week of preexperiment, two sets of six Texel sheep, housed in digestibility crates, were fed silage sources of Rh208 and Rh208Bt; hybrids, and silage of three additional control varieties of low, intermediate, and high feeding value (Rh289, Adonis, and Adonis bm3) for 1 wk. Feed offered to sheep was adjusted to maintenance requirements based on metabolic body weight. Agronomic and biochemical traits were similar among the Rh208 and Rh208Bt; hybrids. Organic matter digestibility (67.1 and 67.6%), crude fiber digestibility (52.9 and 54.2%), and neutral detergent fiber digestibility (50.2 and 49.0%) were not significantly different among Rh208 and Rh208Bt hybrids. In the second trial, two sets of 24 Holstein cows were fed silage from Rh208 and Rh208Bt corn hybrids for 13 wk, 9 wk after calving, and including 2 wk of preexperiment. Fat-corrected milk yield (31.3 and 31.4 kg/d), protein content (31.7 and 31.6 g/kg) and fat content (36.7 and 37.0 g/kg) in milk of dairy cows were unaffected by hybrid source. Body weight gains of cattle were not different. However, intake was significantly higher in cows fed Rh208Bt silage. In the third trial, five midlactation multiparous Holstein cows were successively fed the silage from Rh208 and Rh208Bt corn hybrids 2 or 3 wk. Data were considered only for the last week of each period. There were no significant effects on protein fractions, fatty acid composition, or coagulation properties of milk between Rh208 and Rh208Bt fed cattle. Cattle and sheep can perform equally well with a conventional or a genetically modified Bt176 corn silage.