The relationship of milk urea nitrogen to urine nitrogen excretion in Holstein and Jersey cows'

The objectives of this study were to assess the relationship between urinar y nitrogen excretion (UN, g/d) and milk urea nitrogen concentration (MUN, m g/dl) and whether the types of carbohydrates fed interacts with the dietary CP and the breed (size) of cows to affect this relationship. Eight multipa rous cows (four Holstein and four Jersey) were fed four different diets in a 2 x 2 factorial arrangement of levels of crude protein (13 and 17%) and l evels of neutral detergent fiber (30 and 40%). The experimental design was a split plot Latin square with breeds forming the main plots and diets form ing the subplots. Experimental periods were 3 wk in length, with d 1 to 14 used for adjustment and d 15 to 19 used for a total collection of urine and feces. Crude protein concentrations had a significant effect on milk, milk fat and protein production, plasma urea N, MUN, and on N balance measureme nts (N intake, fecal and urinary N excretion, milk N production, N retentio n, apparent N digestibility, and N efficiency). Neutral detergent fiber lev els had no effect on any production parameters or N balance measurements. T he relationship between urinary N and MUN was linear over the range of MUN values observed and different for the two breeds. The breed effect on the U N-MUN relationship was no longer significant (P = 0.63) when body weight (B W) was included in the model. The optimal allometric coefficient for BW was 0.96 and was not different from 1.0. Therefore, the following equation is proposed to predict UN excretion based on MUN and BW: UN (g/d) = 0.0259 (+/ - 0.0006) BW (kg) x MUN (mg/dl).