Evaluation of the utilization of dietary nitrogen by dairy cows based on urea concentration in milk.

The objective of this study was to determine whether milk urea concentration is a valuable tool to monitor the utilization of dietary N by dairy cows. Data from 11 feed trials (n = 2828 observations of 356 cows) were used in this study. Dietary protein utilization was evaluated according to the Dutch DVE-OEB system. A close correlation (0.8) was found between rumen-degraded protein balance in the ration and urea concentration in milk. The effects of the balance of true protein digested in the small intestine and net energy on milk urea concentration were small but significant. Parity and stage of lactation did not significantly influence milk urea concentration. Because of the large variation among and within cows, the monitoring of protein utilization of an individual cow was inaccurate. However, milk urea concentration in bulk milk is a valuable tool to monitor the rumendegraded protein balance in the ration.