Effect of Nitrogen Solubility on Nitrogen Lactating Dairy Cattle,

Three nitrogen-balance trials were used to measure nitrogen utilization in lactating dairy cows. In each trial, the basic diet was a complete feed containing 55% corn silage and 45% grain mixture (dry-matter basis). The crude protein contents of the complete diets in trials 1, 2, and 3 were 12, 13, and 15%, respectively. Four high-producing Holstein cows similar in age and producing ability and within 30 to 45 days postpartum were used in each 12-wk trial in a Latin-square experimental design. Urea was the treatment variable supplying approximately 0, 8, 16, or 24% of the total dietary nitrogen. The measured solubility of the nitrogen in the diets ranged from 31.5 to 48.7%. The data were fitted to a multiple linear-regression model to determine coefficients of nitrogen utilization. The utilization coefficients for soluble nitrogen and insoluble nitrogen were relatively constant across the three trials, but in each trial the utilization coefficient for insoluble nitrogen was greater than for soluble nitrogen.