Energy supplementation has been shown to improve reproductive performance. However, results
appear to be dependent on the timing (pre or postpartum) of supplementation. Clanton and Zimmerman
(1970) reported that the level of energy fed during the winter had a large effect on the interval between calving and first estrus, and that heifers fed high-energy rations returned to estrus 90 d sooner than heifers fed lowenergy rations. Wiltbank et al. (1962) suggested that feeding supplements pre or post-partum affected different reproductive variables. The level of energy intake in mature cows pre-calving influenced the length of interval from calving to first estrus and the level of energy intake post-calving influenced the conception rate. Wiltbank et al. (1964) suggested that the postpartum level of energy intake is probably more critical than prepartum level when considering total reproductive performance. However, Marston et al. (1995) found that cows fed energy during gestation had greater pregnancy rates than cows fed protein, but the pregnancy rate was not influenced by postpartum supplementation.