Milk Urea Nitrogen as an Indicator of Nitrogen Metabolism Efficiency in Dairy Cows: A Review

The milk urea nitrogen concentration can be used as a tool management of the nutritional strategies in dairy farm and of improving proteins utilization efficiency by dairy cows. The level of dietary crude protein is the most nutritional factor that influence the milk urea nitrogen concentrations in lactating cows. Recent researches suggest that the milk urea nitrogen content depends mainly on the nitrogen/energy ratio in the diet, but other many factors may affect ureogenesis. Level and quality of protein contents, milk yield, season effects or lactation stage, parity and lactation number, weight and breed, feeding frequency and water intake are factors associated with the variation of milk urea concentration. Several studies have suggested that measuring milk urea nitrogen may serve as indicator to monitor nitrogen efficiency in dairy cows and to improve milk nitrogen production. However, the targeted milk urea nitrogen values for optimizing the nitrogen utilization efficiency are different from those required for milk protein production. Thus, an increase in milk protein production can be expected at milk urea nitrogen levels >11 mg/dL, while protein utilization efficiency is below this level (<11 mg/dL). Normal MUN values range from 10 to 14 mg/dL, but for many countries, the recommended milk urea nitrogen values for cow’s milk are ideally ranged from 10 to 16 mg/dL of milk. Finally, to ensure a balance between milk protein production and reducing urea nitrogen excretion in urine and milk, recent studies suggest to include 16.5% of crude protein supply in dairy cows diets.