Milk urea nitrogen (MUN) is related to dietary protein intake, as well as protein-energy ratio in the dairy cow's diet. The normal/target values for MUN are within the range from 10 to 15 mg/dl. High values typically indicate inefficient protein utilization, either because of feeding of excess protein or insufficient amount of energy in relation to protein in the diet. Low values, on the other hand, may indicate insufficient protein feeding.
The Department of Veterinary Preventive Medicine at the OSU College of Veterinary Medicine and Ohio DHI collaborated in a study, evaluating MUN concentrations in 24 randomly selected Ohio dairy herds over a one-year period. Half of the herds (n = 12) were defined as low producers (Rolling Herd Average (RHA) milk production < 16,000 lb) and half (n = 12) as high producers (RHA milk production > 23,000 lb). The MUN concentration was measured in the DHI laboratory from individual cow's monthly test day milk samples. The study herds comprised of 1681 cows altogether.
Based on these Ohio data, MUN concentrations, on average, were higher in the high than in the low-producing herds. The herd level MUN concentrations varied between 5.0 and 15.1 mg/dl in the low producing herds (overall average 11.3 mg/dl) and between 10.1 mg/dl and 19.2 mg/dl in the high producing herds (overall average 13.9 mg/dl). The MUN concentrations varied with month of lactation, as well as with the season of the year. Great variability in MUN was observed from test day to test day in both production groups, and therefore, it is advisable to monitor MUN monthly to establish a baseline MUN concentration for a herd. If the average for the herd (or for a particular group of cows) is outside the target range, it is a good idea to try to determine the cause. The data from this study indicated that herds with RHA milk production over 24,000 lb can have overall MUN as low as 10 to 11 mg/dl. This suggests that using MUN measurements as a practical monitoring tool might provide an opportunity to improve protein feeding efficiency, reduce feed costs, and improve profitability of the herd.
The association between MUN and fertility of dairy cows was also evaluated from these data. Increasing MUN concentrations were negatively correlated with fertility of cows and were associated with a lower likelihood of detectable pregnancy at herd checks. Cows with MUN concentrations below 10.0 mg/dl in early lactation were 2.4 times more likely and cows with MUN levels between 10.0 and 12.7 mg/dl were 1.4 times more likely to be confirmed pregnant than cows with MUN values above 15.4 mg/dl (the values are adjusted for cow's milk yield). Negative effects on reproduction have previously been reported when MUN concentration is above 19 or 20 mg/dl. The results from this Ohio study would suggest that the levels of MUN that are adversely associated with fertility are likely lower than reported earlier. However, when evaluating reproductive problems in a herd, it is always important to consider MUN in the context of the entire herd management and not to target at low MUN just to improve reproduction without considering the associations between MUN, nutrition, milk production, and reproduction in a herd.