Milk urea is a product of nitrogen breakdown in the body and is highly correlated to dietary nitrogen and nitrogen balance of a cow. Dietary protein is the major determinant of MUN concentrations. If protein in the diet is deficient relative to the cow’s requirements, MUN concentrations will be low. Conversely, if protein in the diet is in excess of the cow’s requirements, MUN concentrations will be high. If this were the whole story, using MUN to indicate nitrogen efficiency would be simple. Unfortunately, several additional factors influence MUN concentrations. These include time of milk sampling, season of the year, body weight, days in milk, breed, level of production, and other nutritional factors.
Starch is commonly suggested as one of the nutritional factors controlling MUN, and, indeed, it will alter MUN, but only if it impacts milk protein production. If we take a hypothetical cow being fed 50 lb/day of dry matter (DM) with 17 percent crude protein (CP) and a moderate level of dietary starch, nitrogen intake will be 1.36 pounds/day. If she is producing 80 lb/day of milk at 3.0 percent protein, she is secreting 2.4 pounds/day of milk protein or 0.38 pounds/day of milk nitrogen. Much of the remaining 0.98 pounds/day of nitrogen that was consumed but not converted to milk protein will be converted to urea and eventually excreted. If we increase the dietary starch content by the addition of finely ground starch so that it is ruminally available, it very likely will stimulate microbial growth in the rumen, which will use more of the waste nitrogen generated in the rumen, and thus less ammonia will be absorbed and converted to urea by the cow. However, the resulting extra microbial protein flows to the small intestine where it is mostly digested and absorbed as amino acids. If those amino acids are not used to make more milk protein, the cow will simply degrade them and convert the nitrogen to urea. So the defining event is an increase in milk protein production. In the absence of a milk protein production response, nothing has been gained, and the blood and MUN contents will be the same for both levels of dietary starch.
The target level for MUN across herds is generally 12 mg/dl. If MUN is greater than that, the herd is likely fed protein in excess of needs. If MUN is below 12 mg/dl, the herd may be experiencing a protein deficiency. However, there are differences among herds and among cows within a herd after all of the above factors have been considered, suggesting that the genetic makeup of the herd may play a role in determining herd and cow MUN levels. This is confirmed by the observation that MUN is genetically heritable. Thus, two hypothetical herds with the same breed and same-sized cows fed the same diet and with the same milk production, at the same stage of lactation, could have different MUN concentrations. Therefore, reducing MUN levels below 12 mg/dl without losing milk production may not be possible for all herds. Conversely, some herds may be able to achieve levels below 12 mg/dl and are wasting nitrogen at 12 mg/dl. To achieve maximum nitrogen efficiency and minimize ration costs, herds should establish their own specific targets for MUN.