Effect of age and dietary crude protein level on nitrogen excretion in dairy heifers

Abstract

The aim of this study was to examine the effect of age and dietary crude protein (CP) level on nitrogen (N) excretion in dairy heifers and to develop simple models to predict N excretion on farm based on nutrient intake parameters. Thirty-six Danish Holstein heifers were divided into 4 age groups; 8, 12, 16, and 20 months old (M8, M12, M16, and M20), respectively, with 9 animals per group. Heifers within age groups were blocked in groups of 3 according to age and pregnancy stage and were randomly assigned to dietary treatments in a 3 × 3 Latin square design. The dietary treatments consisted of 4 diets in total with CP concentrations of 9.8, 12.2, 14.6, and 17.0%, respectively, of which the 3 diets with the highest CP concentrations (i.e. 12.2, 14.6, and 17.0%) were provided to M8 and M12 heifers, and the 3 diets with the lowest CP concentrations (i.e. 9.8, 12.2, and 14.6%) were provided to M16 and M20 heifers. The experiment was divided into 3 periods, each period lasting for 2 weeks of which the first 11 days were used as adaptation and the last 3 days were used for sampling and recording. Feed and water intake were recorded daily, and feces (n = 6, pooled) and urine samples (n = 2, pooled) were collected and analyzed for chemical composition. Fecal output of N increased with increased dietary CP level, and this was probably due to the increase in dry matter intake (DMI) and N intake observed when increasing dietary CP level. Urinary N excretion increased with increased CP level in the diet, and was greatest in older heifers. Thus, for dairy heifers, urinary N output appeared to be strongly correlated with dietary N intake and seemed to increase with age. A dietary CP level of 12.2% seemed sufficient for heifers in all age groups to maintain adequate rumen microbial digestion, underpinning the potential of reducing urinary N by dietary manipulation. However, due to the short experimental periods, this study cannot state whether this CP level fulfills the N requirement for growth during the whole rearing period of dairy heifers. Furthermore, a simple prediction equation was established for N excretion in feces: N feces (g/d) = 0.054 × N intake + 6.52 × DMI and was considered a reliable proxy for N excretion in replacement heifers for application on farm.