Genotype × Environmental Interactions on Reproductive Traits of Bovine Females. II. Postpartum Reproduction as Influenced by Genotype, Dietary Regimen, Level of Milk Production and Parity3

Abstract

The interaction between dietary regimen and breed or breed of sire on postpartum reproduction was examined in two experiments. Exp. 1 was a 2 × 2 factorial involving two breeds (Hereford and Holstein) and two dietary regimens, high (H) and low (L). In Exp. 2, females out of Holstein dams and sired by bulls of four breeds, Angus (AF), Hereford (HF), Simmental (SF) and Chianina (CF) were used in a 4 × 2 factorial with two dietary regimens. Postpartum traits were observed for three consecutive postpartum periods (PP1, PP2, PP3), beginning after first calving. In Exp. 1, Holsteins tended to have longer intervals to first estrus (IE) and had longer intervals to conception (IC, P<.025) than Herefords in PP1 but not in PP2 or PP3. Cows fed H diets had significantly shorter IE and IC in PP1 and PP2, but not in PP3. Interval to conception was not recorded in PP3. There were breed × diet interactions for IE in PP1 (P<.06) and PP2 (P<.10) as differences in IE between L and H diets were more pronounced for the Holsteins than for the Herefords. Though not significant, Herefords fed L diets tended to have shorter intervals between estrus and conception than other groups in PP1 and PP2. In Exp. 2, breed of sire did not significantly affect any postpartum reproductive traits. Cows fed the H diet had shorter (P<.10) intervals to first estrus in PP1 and shorter (P<.05) intervals to first ovulation (10) in PP2 than L-fed cows. Interval to first ovulation was also shorter for H-fed cows in PP1, but the differences were not significant. The differences in IE between L and H diets tended to be greater for AF and HF cows than for SF and CF animals for PP1, but the interaction was not significant. The HF cows fed L diets had longer IE in PP2 than HF cows fed H diets, but the other breed - of - sire groups were not influenced by dietary regimen, resulting in a breed of sire × diet interaction (P<.10). Multiple regression analysis indicated that cows fed L diets that produced more fat-corrected milk and consumed less total digestible nutrients (TDN) from 0 to 56 d postpartum had longer IE during PP1. Fat-corrected milk production, TDN intake, weight at calving and change in body weight from 0 to 56 d postpartum were not significantly related to IE for cows receiving H diets during PP1. Data from PP2 and PP3 were pooled across diet groups for multiple regression analysis. None of the factors included in the model were significantly related to IE. We concluded that the influence of diet on postpartum reproductive function interacted with breed and parity and that the influence of level of milk production on IE varied with dietary level and parity. Because diet influenced reproduction more in Holsteins than Herefords, the use of dairy breeds in beef production may necessitate diets higher in energy to achieve maximum reproductive performance.