title: Effect of prebreeding body weight or progestin exposure before breeding on beef heifer performance through the second breeding season.
abstract: Two experiments evaluated prebreeding target BW or progestin exposure for heifers developed lighter than traditional recommendations. Experiment 1 evaluated the effects of the system on heifer performance through subsequent calving and rebreeding over 3 yr. Heifers (229 kg) were assigned randomly to be developed to 55% of mature BW (299 kg) before a 45-d breeding season (intensive, INT; n = 119) or 50% of mature BW (272 kg) before a 60-d breeding season (relaxed, RLX; n = 142). Prebreeding and pregnancy diagnosis BW were greater (P <or= 0.006) for INT than RLX heifers. Overall pregnancy rate did not differ (88.4%; P = 0.51), but RLX heifers had later calving dates (7 d; P < 0.001) and lighter calf weaning weights (194 +/- 4 vs. 199 +/- 4 kg; P < 0.07) compared with INT heifers. Calf birth weight, calving difficulty, second-calf conception rates, and 2-yr-old retention rate did not differ (P > 0.15) between systems. Cost per pregnant 2-yr-old cow was less for the RLX than the INT heifer development system. Of heifers that failed to become pregnant, a greater proportion (P = 0.07) of heifers in the RLX than in the INT system were prepubertal when the breeding season began. Therefore, a second 2-yr experiment evaluated melengestrol acetate (MGA, 0.5 mg/d) as a means of hastening puberty in heifers developed to 50% of mature BW. Heifers were assigned randomly to the control (n = 103) or MGA (n = 81) treatment for 14 d and were placed with bulls 13 d later for 45 d. Prebreeding and pregnancy diagnosis BW were similar (280 and 380 kg, respectively; P > 0.10) for heifers in the control and MGA treatments. The proportion of heifers pubertal before breeding (74%), pregnancy rate (90%), calving date, calf weaning weight, and second breeding season pregnancy rate (92%) were similar (P > 0.10) between treatments. Developing heifers to 50 or 55% of mature BW resulted in similar overall pregnancy rates, and supplementing the diets of heifers developed to 50% of mature BW with MGA before breeding did not improve reproductive performance.
authors.affiliation: University of Nebraska West Central Research and Extension Center, North Platte, Nebraska 69101, USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Martin
authors.forename: K W
authors.initials: KW
authors.lastname: Creighton
authors.forename: J A
authors.initials: JA
authors.lastname: Musgrave
authors.forename: T J
authors.initials: TJ
authors.lastname: Klopfenstein
authors.forename: R T
authors.initials: RT
authors.lastname: Clark
authors.forename: D C
authors.initials: DC
authors.lastname: Adams
authors.forename: R N
authors.initials: RN
authors.lastname: Funston
mesh_terms: D000822:Animal Husbandry / Q000379:methods; D000818:Animals; D000831:Animals, Newborn / Q000254:growth & development / Q000502:physiology; D001724:Birth Weight / Q000502:physiology; D001835:Body Weight / Q000502:physiology*; D001947:Breeding / Q000379:methods; D002417:Cattle / Q000254:growth & development / Q000502:physiology*; D005260:Female; D005938:Glucocorticoids / Q000494:pharmacology; D008552:Melengestrol Acetate / Q000494:pharmacology*; D011247:Pregnancy; D018873:Pregnancy Rate; D045167:Progesterone Congeners / Q000494:pharmacology*; D011897:Random Allocation; D012098:Reproduction / Q000187:drug effects / Q000502:physiology; D012621:Seasons
chemical_list: D005938:Glucocorticoids; D045167:Progesterone Congeners; D008552:Melengestrol Acetate