To determine the e .ects 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 ith the Knockout event) ere planted in duplicate 2.8-ha .elds in 1998 and 1999.Thirty Angus x Charolais x Simmental cows in midgestation ere allotted amongst t o drylots or the eight crop residue .elds to strip-graze for 126 d.Co body condition scores ere visually estimated biweekly and alfalfa-grass hay as supplemented to maintain a mean body condition score of 5 on a 9-point scale.Crop residue yields and compositions ere determined monthly in one 4-m 2 location in each grazed and ungrazed paddock. On t o consecutive days after 2 wkof grazing,forage selected during 2 hr of feeding by one .stulated steer per .eld or drylot as collected by ruminal evacuation.Simultaneously,DMI as calculated from the digestibility of forage determined with indigestible ADF and the fecal output estimated from the passage kinetics of Cr-mordanted .ber (2% Cr)in t o cows per .eld or drylot.Corn hybrid did not a .ect mean yields of harvested grain,dropped ears or grain,or crop residue DM or OM over the t o years.At grazing initiation,crop residues from Novartis NX6236 and N64Z4 had higher (P <.05)concentrations of IVOMD and lo er (P <.05)concentrations of ADF and ADL than Pioneer 3489 or Pioneer 34RO7.Rates of change in the concentrations of IVOMD,NDF, ADF,ADL and CP over winter did not di .er bet een corn hybrids.Forage selected by .stulated steers did not di .er in IVOMD concentrations bet een winter feeding systems nor NDF,ADF,ADL or ADIN concentrations bet een corn hybrids.Intakes of forage digestible OM,NDF and ADF did not di .er bet een winter feeding systems.Mean amounts of hay required to maintain body condition score of co s maintained in a drylot ere greater than cows grazing crop residues (1454 vs 375 kg DM/co ),but did not di .er bet een corn hybrids.