RUMINALLY DEGRADABLE PROTEIN REQUIREMENT OF GESTATING BEEF-COWS GRAZING NATIVE WINTER SANDHILLS RANGE

Two grazing trials were conducted to determine the ruminally degradabl e protein requirement of gestating beef cows in the last half of pregn ancy grazing winter native Sandhills range. In Trial 1, 80 crossbred c ows (522 +/- 9 kg) were assigned randomly to one of the following rumi nally 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 estimat ed 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 collect ed in both experiments to estimate total nutrient intake. Forage RDP i ntake 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.