Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/1997
Publication Date: N/A
Citation: Interpretive Summary: Urea is the primary form of waste nitrogen excreted by mammals including dairy cows. High levels of blood urea are an index of inefficient protein utilization. Urea equilibrates rapidly throughout body fluids, including milk. In the normal course of keeping production records, farmers routinely take milk samples from their cows for analysis of protein and fat. It would be much easier to analyze these milk samples for urea content than to take blood samples from the same cows. Concentrations of blood and milk urea were determined during the course of 35 trials with 482 lactating cows fed 106 different diets. Milk urea was found to be very closely related to blood urea, indicating that milk urea mirrors blood urea levels. Protein level was the single dietary factor found to be most closely related to milk urea; dietary protein content can be estimated from milk urea level. Interestingly, ammonia level, which often is used by researchers as an index of protein wastage in the rumen, was more poorly related to milk urea than were several other factors. To get an accurate measure of milk urea it was found that each of a cow's daily milkings had to be sampled. Because of cow-to-cow variation, at least 4 cows fed a specific diet must be sampled to get a reliable estimate of the milk urea level on that diet. Results of this study indicate that milk urea will serve as an easily sampled index of blood urea and analyzing milk for urea level will help farmers to see if too much, or too little, protein is being fed to their cows.
Technical Abstract: 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 between milk and blood urea N and the value of milk urea N for assessing protein status. In two trials, urea N in whole blood and blood plasma were closely related (R**2 = 0.952), with slope not significantly different from 1.0 and dintercept not significantly different from 0. Regression of milk urea N o blood urea N with a mixed effects model using all 2231 observations yielded the equation: milk urea N (mg N/dL) = 0.620 x blood urea N (mg N/dL) + 4.75 (P < 0.0001; R**2 = 0.842); this model accounted for significant interaction of cow-by-blood-urea-N. Single factors yielding significant regressions on milk urea N with mixed effects models were: dietary CP (% of DM; R**2 = 0.839), dietary CP/Mcal NEL (R**2 = 0.833), excess N intake (R**2 = 0.772), N efficiency (R2 = 0.626), and ruminal ammonia (R**2 = 0.574). When all factors were analyzed at once, 12 were significant (P < 0.10) in a mixed effects model: blood urea N, BW, FCM yield, dietary CP content, excess N intake, DMI, and DIM were positively related to milk urea N; parity, milk and fat yield, dietary CP/NEL content, and NEL intake were negatively related to milk urea N. In one trial, average urea concentration was 35 times greater in urine than in milk; lower proportions of total urea excretion were excreted in AM (1.8%) than in PM (3.3%) milk. Measuring urea N in milk representing the whole day substantially improved reliability of data. Numbers of cows that must be sampled on a specific diet to determine mean milk urea N, within 95% confidence intervals having half-widths of 1.0 or 2.0 mg N/dL, were estimated to be 16 and 4.