Effect of pre-breeding body weight or progestin exposure before breeding on beef heifer performance through the second breeding season

¹ University of Nebraska West Central Research and Extension Center
² Department of Animal Science, University of Nebraska-Lincoln
³ Department of Agricultural Economics, University of Nebraska-Lincoln

^* To whom correspondence should be addressed. E-mail: rfunston2{at}unl.edu.

	Abstract

Two experiments evaluated pre-breeding target weight or progestin exposure for heifers developed lighter than traditional recommendations. Experiment 1 evaluated effects of system on heifer performance through subsequent calving and re-breeding over 3 yr. Heifers (229 kg) were assigned randomly to be developed to 55% mature BW (299kg) before a 45-d breeding season (Intensive, INT; n = 119) or 50% mature BW (272 kg) before a 60-d breeding season (Relaxed, RLX; n = 142). Pre-breeding and pregnancy diagnosis BW were greater (P 0.006) for INT than RLX. 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 BW (194 ± 4 vs. 199 ± 4 kg; P < 0.07) compared to INT. Calf birth BW, 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 INT heifer development system. Of heifers that failed to become pregnant, a greater proportion of (P = 0.07) RLX than INT heifers were pre-pubertal 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% mature BW. Heifers were assigned randomly to receive control (n = 103) or MGA (n = 81) for 14 d and placed with bulls 13 d later for 45 d. Pre-breeding and pregnancy diagnosis BW were similar (280 kg and 380 kg, respectively; P > 0.10) for control and MGA. Proportion of heifers pubertal before breeding (74%), pregnancy rate (90%), calving date, calf weaning BW, and second breeding season pregnancy rate (92%) were similar (P > 0.10) between treatments. Developing heifers to 50 or 55% mature BW resulted in similar overall pregnancy rates and supplementing heifers developed to 50% mature BW with MGA before breeding did not improve reproductive performance.

Key Words: beef heifers, heifer development, progestin, target weight

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