SUPPLEMENTATION OF RUMEN-UNDEGRADABLE PROTEIN TO THE DIETS OF EARLY LACTATION HOLSTEIN COWS ON GRASS PASTURE

The objectives were to evaluate supplements differing in rumen-undegra dable protein (RUP) on the performance of early lactation cows on gras s pasture. Twenty-four Holstein cows averaging 68 d of lactation and 3 9.8 kg/d of milk were rotationally grazed on predominantly Dactylis gl omerata pasture for 8 wk. Cows were blocked according to parity, milk yield, and days of lactation and were randomly assigned to a grain mix ture with a low Or high RUP content. Pasture and the low and high RUP grain mixtures averaged 25.6, 14.7, and 13.7% crude protein and 4.0, 7 .0, and 8.4% RUP, respectively (dry matter basis). Grain was fed twice daily at 1 kg/4 kg of milk, pasture provided all forage in the diet, and grain consumption was similar (8.9 kg/d per cow) for cows on both treatments. Total dry matter intakes estimated using chromic oxide wer e 19.9 and 20.9 kg/d for cows fed low and high RUP grain mixtures. Mil k yields did not differ between treatments; means were 34.2 and 35.5 k g/d for cows fed low and high RUP grain mixtures. Multiparous cows ten ded to yield more milk (36.2 vs. 34.5 kg/d) and milk protein (1.06 vs. 0.98 kg/d) when fed the high RUP grain mixture. Concentrations of pla sma urea N and nonesterified fatty acids were unaffected by treatment and averaged 18.7 mg/dl and 307 mu eq/L, respectively, Results indicat ed that a supplemental grain mixture with a high RUP content did not a lter milk yield of high yielding cows when pasture was the sole forage ; however, milk protein yield tended to be greater for multiparous cow s fed the high RUP grain mixture.