Postpartum reproductive function of daity cows as influenced by energy and phosphorus status

To determine if energy or phosphorus status would affect postpartum reproductive performance, we assigned primiparous heifers, six per group, to either high (135% NRC) or low (85% NRC) energy, and high (138% NRC) or low (98% NRC) phosphorus in a 2 x 2 factorial design. Treatments were continued from parturition through 84 days postpartum, after which standard diets were fed. The entire experiment was repeated. Reproductive function was almost identical among energy and phosphorus groups. However, no cow with peak serum progesterone below 2.7 ng/ml before insemination conceived to that insemination. Interval from parturition to first observation of serum progesterone (larger than or equal to 3 ng/ml) was correlated with serum phosphorus (r = -.34), glucose (r = -.33) and and growth hormone (r = .51). In addition, body weight change, milk production and glycemia did not seem to affect reproduction markedly. These data suggest that neither energy nor phosphorus status, within the ranges studied, influences reproduction of dairy cows.