Effects of Grazing Crop Residues of bt-Corn Hybrids on Performance of Pregnant Beef Cows

To determine the effects of grazing crop residues from bt-corn hybrids on performance of pregnant beef cows, one non bt-corn hybrid (Pioneer 3489) and three bt-corn hybrids (Pioneer 34RO7 and Novartis NX6236 with the Yieldgard event and Novartis N64Z4 with the Knockout event) were planted in duplicate 7.1-acre fields in 1998 and 1999. Thirty Angus x Charolais x Simmental cows in midgestation were allotted amongst two drylots or the eight crop residue fields to strip-graze for 126 days. Cow body condition scores were visually estimated biweekly and alfalfa-grass hay was supplemented to maintain a mean body condition score of 5 on a 9-point scale. Corn stalks and ear shanks from Pioneer 3489 corn had a higher (P\u3c.05) infestation of corn borers than did any of the bt-corn hybrids. Mean yields of harvested grain, dropped ears or grain, or crop residue dry matter (DM) or organic matter (OM) over the two years were not significantly affected by corn hybrid, but values for bt-corn hybrids were not equivalent to the non bt-corn hybrid. At grazing initiation, crop residues from Novartis NX6236 and N64Z4 had higher (P\u3c.05) concentrations of in vitro digestible organic matter (IVOMD) and lower (P\u3c.05) concentrations of acid detergent fiber (ADF) and acid detergent lignin (ADL) than Pioneer 3489 or Pioneer 34RO7. Rates of change in the concentrations of IVOMD, neutral detergent fiber (NDF), ADF, ADL and crude protein (CP) over winter were not significantly affected by corn hybrid, but rates for bt-corn hybrids were not equivalent to non bt-corn hybrids. Forage selected during feeding by fistulated steers did not differ in IVOMD concentrations between winter feeding systems or NDF, ADF, CP or acid detergent insoluble nitrogen (ADIN) concentrations between corn hybrids. Intakes of forage digestible DM, NDF and ADF did not differ between winter feeding systems, but were not equivalent to the non bt-corn hybrid. Mean amounts of hay required to maintain body condition score of cows maintained in a drylot were greater than cows grazing crop residues (3199 vs 825 lb DM/cow), but did not differ between corn hybrids