The effect of roughage to concentrate ratio in the diet on nitrogen and purine metabolism in dairy cows

Four lactating dairy cows were used in two experiments to study the effects of the roughage to concentrate ratio in the diet on nitrogen balance, plasma urea, urinary urea, milk urea and urinary purine derivatives. The use of the allantoin to creatinine ratio in spot samples of urine as an index of the urinary allantoin excretion was also evaluated. Four isoenergetic and isonitrogenous diets were formulated according to a 2 × 2 factorial arrangement. Factor I was concentrate content. The roughage to concentrate ratios were 65:35 and 35:65 for the high roughage and high concentrate diets, respectively. Factor II was fat content, which was 2.8% and 5.8% for the low and high fat diets, respectively. In Experiment 1 cows were fed diets with low fat content, and in Experiment 2 cows were fed diets with high fat content. In both experiments, diets were fed according to a change-over design. Nitrogen balance was not affected by the treatments. In cows fed high concentrate diets the amount and the proportion of nitrogen excreted in milk, as well as milk production was higher than in cows fed the high roughage diets. In both experiments, as an overall effect, the urea levels in plasma, urine and morning milk were higher, although the total urinary excretion of urea was lower, for the high concentrate diets. Urinary allantoin excretion was higher, although not significantly in Experiment 1, for the high concentrate diets. The allantoin to creatinine ratio in spot samples of urine showed the same pattern as the total allantoin excretion. Urinary creatinine excretion appeared to be affected by the diet.