Fine-tuning Test-day MUN Records for DHI-related Variables

The liver of the cow is a cross-road of nitrogen metabolism in dairy cows. Urea is synthesized in the liver as a results of unused degradable protein in the rumen, the absorption of excess amino acids from rumen undegradable protein, and from the catabolism of amino acids that are not utilized for productive purposes. Dietary crude protein (CP, % of DM) is the one single dietary factor most closely associated with milk urea nitrogen (MUN). From a nutrition perspective, MUN of approximately 12 mg/dl associated with a diet of approximately 16.5% CP, indicates an optimal situation that does not penalize milk production, but avoids unnecessary losses of urinary nitrogen. Current data suggest also that MUN increases by approximately 2.0 mg/dl per percentage unit of increase in dietary CP in the range of 15.0% to 18.5%. Target values should be used cautiously to assess test-day MUN on farm because of large variations due to breed, parity, sample type (a.m. vs. p.m.), season, level of production, and in Holstein cows the interactions between milking frequency and season and between milking frequency and level of milk production. Target test-day MUN may not apply to records collected the first 35 days of lactation. During this period, MUN may be more reflective of the energy and protein balance of the cow than the adequacy of dietary inputs. The use of adjustment coefficients to standardize test-day MUN to a common base may facilitate interpretation and improve its dependability as a tool to fine-tune rations on commercial herds. For herds that manage DHI test-day MUN in combination with dietary data from frequent ration analysis, it might be possible to recommend target MUN in the range of 10-12 mg/dl. However in herds without such combined information, we would recommend not to “raise a flag” until MUN is above 14 mg/dl.