Effect of age and dietary crude protein level on nitrogen excretion in Holstein bull calves

Abstract

The aim of the present study was to investigate the effect of age and dietary crude protein (CP) level on nitrogen (N) excretion in Holstein bull calves. Thirty-six Danish Holstein bull calves were divided into 4 age groups; 12, 22, 32, and 42 weeks old (W12, W22, W32, and W42), respectively, with 9 animals per group. Within age group, calves were blocked in groups of 3 according to age and weight, and randomly assigned to the dietary treatments in a 3 × 3 Latin square design. The dietary treatments consisted of 4 diets in total, with CP concentrations of 10.9, 14.1, 16.9, and 20.2%, of which the 3 diets with the highest CP concentrations (i.e. 14.1, 16.9, and 20.2%) were provided to W12 and W22 calves, whereas the 3 diets with the lowest CP concentrations (i.e. 10.9, 14.1, and 16.9%) were provided to W32 and W42 calves. The experiment was divided into 3 periods, all lasting for 2 weeks in which the first 11 days were used as adaptation, while the last 3 days were used for sampling and recording. Feed and water intake were recorded daily, and feces and urine samples were collected and analyzed for chemical composition. Intake of DM increased for all age groups with greater dietary CP level (P < 0.01), and was greatest in the oldest bull calves (P < 0.01). Urinary N excretion was positively correlated with N intake (P < 0.01) and age of the calves (P < 0.01). Fecal output of N increased with increasing dietary CP level (P = 0.05), but might mainly be explained by the greater feed intake observed with increasing CP level in the diet. When increasing the dietary CP level, the proportion of N intake excreted in urine increased (P < 0.01), whereas the proportion of N intake excreted in feces decreased (P < 0.01). In addition, the proportion of N intake excreted in urine was greatest in the oldest bull calves (P < 0.01). Furthermore, simple prediction equations for N excretion in urine and feces, respectively, were established and considered reliable proxies for N excretion in Holstein bull calves for application on farm.