Ruminally degradable protein requirement of gestating beef cows grazing native winter sandhills range

Two grazing trials were conducted to determine the ruminally degradable protein requirement of gestating beef cows in the last half of pregnancy grazing winter native Sandhills range. In Trial 1, 80 crossbred cows (522 +/- 9 kg) were assigned randomly to one of the following ruminally degradable protein (RDP) treatments: 50%, 75%, 100%, or 125% of the estimated supplemental requirement. In Trial 2, 80 crossbred cows (529 +/- 8 kg) were assigned to 29%, 65%, 100%, or 139% of the estimated supplemental RDP requirement. In Trial 1, daily gain and condition score (CS) were not different (P > .15) among treatments. In Trial 2, gain responded quadratically, being higher (P = .14) for the 65% level than for the 29%, 100%, and 139% levels (.18, .05, .06, and .01 kg/d, respectively). Condition score was maintained at 65% and lost at 29%, 100%, and 139% (cubic effect, P = .06; 0, -.2, -.4, -.3, respectively). In both experiments, forage intake did not differ among treatments although OM digestibility increased linearly (P = .08) in Trial 1 with increasing level of RDP but not in Trial 2. Diet samples were collected in both experiments to estimate total nutrient intake. Forage RDP intake was 322 g/d in Trial 1 and 279 g/d in Trial 2. We conclude that gestating beef cows grazing native winter Sandhills range need between 62 and 140 g/d of supplemental RDP to meet their daily requirement of 340 to 430 g/d or 7.1% of the digestible OM.