Twenty-four lactating dairy cows were assigned to a completely randomized block design to compare the effects of isoenergetic diets but different crude protein (CP) levels: P100- 100%, P133- 133% or P166- 166% of the NRC (1989) requirements on the concentrations of urea nitrogen in plasma (PUN) and milk (MUN). Increasing levels of CP were obtained by adding urea to the diet. Feeds, milk, and blood were all collected weekly from 30 days before to 60 days after parturition. Milk yield and body weigh were also registered weekly in this trial. Milk yield averaged 12.4, 13.3, and 13.4 kg/d, respectively, on P100, P133, and P166 treatments and increased linearly with the incremental levels of urea in the diet. Concentrations of PUN and MUN also increased linearly when rumen-degradable protein (RDP) was increased in the diet resulting, respectively, in the following equations: Y= 16.83 + 0.202 X and Y= 13.73 + 0.219 X. A highly significant correlation coefficient (r= 0.603) between PUN and MUN was observed in this study. It can be concluded that PUN and MUN are useful indicators of dietary RDP levels and that higher contents of dietary nitrogen improved milk yield.
dietary protein; milk yield; milk urea nitrogen; plasma urea nitrogen
