Responses of Milk Urea Nitrogen Content to Dietary Rumen Degradable Protein Level in Lactating Holstein Dairy Cows

Nine multiparous lactating cows averaging 171 days in milk were divided according to days in milk and milk production into three 3×3 Latin squares with three 3-week periods to investigate the effect of rumen degradable protein on milk urea nitrogen. Diets were formulated to provide 3 concentrations of dietary ru-men degradable protein (9.8, 10.8, and 11.8% of dry matter), while rumen undegradable protein (4.6% of DM) remained constant. Each period was 3 weeks in length, with 2 weeks used for adjustment and one week used for sampling. Rumen degradable protein levels had a low effect on milk yield (P > 0.05), but a significant effect on protein percentage and milk urea nitrogen content. There was linear increase in the milk urea nitrogen content of cows fed diets one to three (P < 0.01). Milk urea nitrogen is a simple and non-invasive measurement that can be used to monitor nitrogen efficiency in dairy cows. These results indicated that milk urea nitrogen might be used as a parameter to monitor the change in dietary protein levels.