Responses of Milk Urea Nitrogen Content to Dietary Rume 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 rumen 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