Effect of dietary N-carbamoylglutamate on milk production and nitrogen utilization in high-yielding dairy cows.

The objective of the current study was to investigate the effect of N-carbamoylglutamate (NCG) supplementation on milk production and nitrogen (N) utilization in Chinese Holstein dairy cows. Sixty multiparous cows (78±17.3 d in milk, 635±61.00kg of body weight, and 41.9±7.9kg/d milk yield; mean ± SD) were blocked by parity, days in milk, and milk yield and randomly allocated to 1 of 4 groups, each of which was fed a dietary treatment containing 0 (control), 10, 20, or 30g of NCG/d. Milk yield was recorded weekly. Dry matter intake, milk composition, plasma variables, and urea N contents in plasma, urine, and milk were determined every other week. Blood samples were collected from the coccygeal vein. Rumen microbial protein synthesis was estimated based on the purine derivatives in the urine. Dry matter intake was found to be similar between the treatments. Addition of 20g of NCG/d tended to increase milk yield (40.2 vs. 38.1kg/d) and increased the content (2.83 vs. 2.74%) and yield (1.12 vs. 1.02kg/d) of milk protein compared with the control. The yield and content of milk fat were similar between the treatments, whereas the contents of lactose and total solids increased linearly with an increase in NCG. Dietary supplementation of NCG linearly increased the plasma nitric oxide level and decreased the plasma ammonia N level. Compared with the control, the plasma Arg concentration in cows fed 10, 20, and 30g of NCG/d was increased by 1.1, 10.4, and 16.0%, respectively. The urea N concentrations in the milk, plasma, and urine decreased with the addition of NCG, although the lowest urea N concentrations were observed with the addition of 20g of NCG/d. The conversion of dietary crude protein to milk protein exhibited quadratic trends of improvement by NCG supplementation, with a peak at 20g of NCG/d. The rumen microbial protein synthesis was not altered by NCG supplementation, but the metabolizable protein tended to show a quadratic increase in cows fed 20g of NCG/d. In conclusion, supplementation of 20g of NVG/d may alter the plasma metabolites, optimize the AA profile, increase the metabolizable protein utilization, and thereby improve the lactation performance and N utilization of high-yielding dairy cows.