EVALUATION OF DIETARY NITROGEN-UTILIZATION IN DAIRY-COWS BASED ON UREA CONCENTRATIONS IN BLOOD, URINE AND MILK, AND ON URINARY CONCENTRATION OF PURINE DERIVATIVES

The effects of level and degradability of dietary protein on urea in b lood, urine and milk, and on the urinary purine derivatives and creati nine in dairy cows, were studied. Diurnal variation in urinary concent ration of urea, allantoin and creatinine was also studied. A total of 24 multiparous lactating dairy cows were selected from a production ex periment and divided into two 12-cow groups according to their lactati on stage (LG1, 14th-16th week of lactation; and LG2, 26th-30th week of lactation). Within groups, cows were fed four different rations (thre e cows per treatment). Four concentrate mixtures with two levels of pr otein [19 and 14% of crude protein on dry matter (DM) basis] and two d ifferent rumen degradabilities (72 and 52%) of the dominating protein feed rapeseed meal, were fed to the cows together with pasture silage and hay. In both groups of cows the urea concentration in plasma, urin e and milk increased (p < 0.05) with increasing protein intake. Based on the pooled data from both lactation groups, significant (p < 0.001) correlations were found between the intake of PBV (protein balance va lue in the rumen) and urea concentrations in plasma (R(2) = 0.73), uri ne (R(2) = 0.53) and milk (R(2) = 0.63). Also significant (p < 0.001) relationships were observed between plasma urea and urea in urine (R(2 ) = 0.78) and milk (R(2) = 0.73), and between milk urea and urinary ni trogen losses (R(2) = 0.64). In LG2 there was an increase in urinary a llantoin excretion associated with an increase in DM intake. A clear d iurnal pattern of concentrations in urine of urea, allantoin and creat inine was observed in LG1 and LG2. The use of allantoin to creatinine ratio in spot samples of urine appeared to be an useful index of the u rinary allantoin excretion in intact animals.