Effects of slow-release urea and rumen-protected methionine and histidine on performance of dairy cows.

This experiment was conducted with the objective to investigate the effects of slow-release urea and rumen-protected (RP) Met and His supplementation of a metabolizable protein (MP)-deficient diet (according to NRC, 2001) on lactation performance of dairy cows. Sixty lactating Holstein cows were used in a 10-wk randomized complete block-design trial. Cows were fed a covariate diet for 2 wk and then assigned to one of the following treatments for an 8-wk experimental period: (1) MP-adequate diet [AMP; 107% of MP requirements, based on the National Research Council (NRC, 2001)]; (2) MP-deficient diet (DMP; 95% of MP requirements); (3) DMP supplemented with slow-release urea (DMPU); (4) DMPU supplemented with RPMet (DMPUM); and (5) DMPUM supplemented with RPHis (DMPUMH). Total-tract apparent digestibility of dry matter, organic matter, neutral detergent fiber, and crude protein, and urinary N and urea-N excretions were decreased by DMP, compared with AMP. Addition of slow-release urea to the DMP diet increased urinary urea-N excretion. Dry matter intake (DMI) and milk yield (on average 44.0±0.9kg/d) were not affected by treatments, except DMPUMH increased DMI and numerically increased milk yield, compared with DMPUM. Milk true protein concentration and yield were increased and milk fat concentration tended to be decreased by DMPUMH, compared with DMPUM. Cows gained less body weight on the DMP diet, compared with AMP. Plasma concentrations of His and Lys were not affected by treatments, whereas supplementation of RPMet increased plasma Met concentration. Plasma concentration of 3-methylhistidine was or tended to be higher for DMP compared with AMP and DMPU, respectively. Addition of RPHis to the DMPUM diet tended to increase plasma glucose and creatinine. In conclusion, feeding a 5% MP-deficient diet (according to NRC, 2001) did not decrease DMI and yields of milk and milk components, despite a reduction in nutrient digestibility. Supplementation of RPHis increased DMI and milk protein concentration and yield. These results are in line with our previous data and suggest that His may have a positive effect on voluntary feed intake and milk production and composition in high-yielding dairy cows fed MP-deficient diets.