A STATISTICAL EVALUATION OF ANIMAL AND NUTRITIONAL FACTORS INFLUENCING CONCENTRATIONS OF MILK UREA NITROGEN

Data from 35 trials with 482 lactating cows fed 106 diets were used to study the effects of animal and dietary factors on the relationship b etween milk and blood urea N and the value of milk urea N in the asses sment of protein status. In two trials, urea N in whole blood and in b lood plasma were closely related (r(2) = 0.952); the slope was not sig nificantly different from 1.0, and the intercept was not significantly different from 0. Regression of milk urea N on blood urea N with a mi xed effects model using all 2231 observations yielded the equation: mi lk urea N (milligrams of N per deciliter) = 0.620 x blood urea N (mill igrams of N per deciliter) + 4.75 (r(2) = 0.842); this model accounted for a significant interaction of cow and blood urea N. Single factors that yielded significant regressions on milk urea N with the mixed ef fects models were dietary crude protein (CP) (percentage of dry matter ; r(2) = 0.839), dietary CP per megacalorie of net energy for lactatio n (NEL) (r(2) = 0.833), excess N intake (r(2) = 0.772), N efficiency ( r(2) = 0.626), and ruminal NH3 (r(2) = 0.574). When all factors were a nalyzed at once, 12 were significant in a mixed effects model. Blood u rea N, body weight, yield of fat-corrected milk, dietary CP content, e xcess N intake, dry matter intake, and days in milk were positively re lated to milk urea N, and parity, milk and fat yield, dietary CP per u nit of NEL content, and NEL intake were negatively related to milk ure a N. In one trial, the mean urea concentration was 35 times greater in urine than in milk; lower proportions of total urea excretion in milk were observed in the a.m. sampling (1.8%) than in the p.m. sampling ( 3.3%). Measuring urea N in a composite milk sample from the whole day substantially improved reliability of data. The number of cows fed a s pecific diet that must be sampled to determine mean milk urea N within 95% confidence intervals with half widths of 1.0 and 2.0 mg of N/dl w as estimated to be 16 and 4, respectively.