Application of Milk Urea Nitrogen Values

In dairy cows, dietary crude protein (CP) that can is not captured as milk protein is largely excreted as N in manure. Of course, some protein-N is retained for growth in first and second lactation animals and in the calf and fetal tissues during pregnancy; however, these amounts are small relative to the proportion of absorbed N going into milk. Cows and other mammals convert N from absorbed amino acids and other nitrogenous compounds that are not utilized mainly into urea, the principal N excretory form. Most of this urea is produced in the liver and carried via the blood to the kidneys, ' where it is concentrated into urine. Blood urea N (BUN) distributes rapidly throughout 1 the cow's body water, including milk. That is why milk urea N (MUN) also serves as a good index of urinary N excretion in dairy cows. Anything impairing CP utilization, whether excessive ruminal protein degradation, reduced milk protein synthesis due to inadequate supply of a limiting AA or energy, or any other factor, will increase both urinary N and MUN. Concentrations of MUN are valuable for monitoring dietary CP intake and N-efficiency because: 1) excess N intake may impair reproductive performance (Carroll et al., 1988; Ferguson and Chalupa, 1989), possibly through elevated ammonia in the urogenital tract (Jordan et al., 1983); 2) consuming excess CP increases digestible energy requirements by 13.3 kcal/g of excess N (NRC, 1989): 3) protein supplements are generally the most costly feed ingredients; and 4) excess N excretion has a negative environmental impact. We will present some of the evidence of how impaired N efficiency and excessive urinary N excretion are reflected in MUN concentrations and secretions. Plus, we will make a few recommendations on how MUN can be used to improve profitability and reduce N losses to the environment.