Growth Performance and Microbial Activity Positively Responded to Interactive Effect of Nitrogen and Phosphorus in Young Holstein Dairy Calves.

We hypothesized that nitrogen (N) and phosphorus (P), as two pivotal nutrients that contributed to the growth of growing animals, may have interactive effect on growth performance and microbial development in young calves. For this purpose, feeding two starter protein contents (20% [20CP] vs. 24% [24CP], DM basis) and two phosphorus supplementation levels (0.35% [0.35P] and 0.70% [0.7P], DM basis) was evaluated on growth performance, health indicators, digestibility of nutrients and microbial protein synthesis (MPS), the latter estimated from urinary purine derivatives (PDs). Forty-eight female Holstein calves (3 days of age) were assigned randomly to the following treatments: 20CP-0.35P, 20CP-0.7P, 24CP-0.35P and 24CP-0.7P (n = 12, each). Milk feeding schedule was identical among treatments until weaning (d 59), but study lasted until d 73. Feeding 24CP compared to 20CP diet tended to improve starter intake, increased average daily gain (ADG) during pre-weaning period (p < 0.05), and final body weight (p = 0.01). Higher P supplementation has marginal effect on starter intake but improved ADG (pre-weaning; p = 0.02) and microbial activity by greater MPS (p = 0.02) compared to 0.35P. The greatest starter intake, the most favourable general appearance score, the highest withers height, the highest hip height, the greatest organic matter and neutral detergent digestibility, and the greatest MPS were found when calves fed diets contained 24CP along with 0.7P in starter. Based on the current study results, N and P have separate effects on the growth performance of young calves; however, their positive interaction can be attributable to growth performance and microbial development, especially during the pre-weaning period.