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Effects of Slow-release Urea, Rumen-protected Methionine, and Histidine on Performance of Dairy Cows Fed Metabolizable Protein-deficient Diets
The main objective of this experiment was to investigate the effects of slow-release urea and rumen-protected (RP) Met and His supplementation of a metabolizable protein (MP)-deficient diet on lactation performance of dairy cows. Sixty Holstein cows (DIM, 87 ± 40 and BW, 640 ± 70 kg) were used in a 10-wk randomized complete block design trial. After a 2-wk covariate period, cows were blocked by parity, DIM, and milk yield, and randomly assigned to 1 of 5 dietary treatments: MP-adequate diet [AMP; 107% of MP requirements (NRC, 2001)]; MP-deficient diet [DMP; 95% of MP requirements]; DMP supplemented with slow-release urea as Optigen (Alltech Inc.; DMPO); DMPO supplemented with RPMet as Mepron (Evonik Industries AG; DMPOM); and DMPOM supplemented with RPHis (Balchem Corp.; DMPOMH). The basal diet consisted of (DM basis): 43% corn silage, 8% grass hay, 4% cottonseed hulls, and 45% concentrate and contained 16.7, 15.8, and 14.8% CP for AMP, DMPO, and DMP, respectively. Total-tract apparent digestibility of nutrients, and urinary N and urea excretions were decreased (P < 0.01) by DMP compared with AMP. Relative to AMP, milk N secretion as a proportion of N intake tended to be higher (P = 0.07) for DMP. DMI was not affected by MP level but tended to be higher (P = 0.09) for the DMPOMH (28.4 kg/d) compared with DMPOM (27.0 kg/d). Yields of milk and milk fat were not affected by treatment, averaging 44.0 kg/d and 1.56 kg/d, respectively; milk fat content tended to be lower (P = 0.06) for DMPOMH (3.36%) than DMPOM (3.78%). Milk true protein content was increased (3.26 vs. 3.16%, P = 0.04) and milk protein yield was numerically increased (1.49 vs. 1.39 kg/d, P = 0.14) by DMPOMH, compared with DMPOM. Cows fed DMP gained 14 g/d BW whereas cows on all other treatments gained on average 267 g/d (P ≤ 0.10). Supplementation of the DMPO diet with RPAA increased (P = 0.03) plasma glucose and numerically increased (P = 0.12) plasma insulin. In conclusion, feeding a 5% MP-deficient diet did not decrease DMI and yields of milk and milk components, despite the reduction in nutrient digestibility. Supplementation of the DMPOM diet with RPHis tended to increase DMI and increased milk protein content. These results confirm previous data and suggest that His may have a positive effect on voluntary feed intake in high-yielding dairy cows.
Keywords:
metabolizable protein, slow-release urea, rumen-protected methionine, rumen-protected histidine