MILK UREA NITROGEN AS A TOOL TO MONITOR THE PROTEIN NUTRITION OF DAIRY-COWS

The relationship between protein nutrition and milk urea N was investi gated in three experiments with a total of 125 cows. After 4 wk of pre treatment, cows received 1 of 13 diets with different ratios of protei n to energy for 16 wk. Milk was sampled individually for urea analyses during pretreatment and during wk 1, 5, 10, and 15 of treatment. Resu lts were compared with N losses estimated from rumen fermentation and with N losses of metabolic origin. The mean milk urea N concentration was 12.6 mg/100 ml of milk (range, 9.0 to 18.3 mg). For bulk samples e specially, the rumen efflux of crude protein intake was the main deter minant of the variation in milk urea N (r(2) = 0.81; residual SD = 1.1 ). However, N losses from the rumen explained only about 50% of the va riation in the milk urea N content of samples from individual cows. Th e N losses of metabolic origin, which, in these experiments, were resp onsible for 47 to 100% of urinary N losses, were not related to milk u rea N. Results showed that regular measurement of milk urea N in bulk samples can be used to monitor N losses from rumen fermentation. Howev er, the value does not give an indication of the efficiency with which the absorbed protein is utilized.