Prolonging the MGA-prostaglandin F 2α interval from 17 to 19 days in an estrus synchronization system for heifers

Abstract

Our objective was to determine whether extending the interval from 17 to 19 d between removal of melengestrol acetate (MGA) feed and administration of PGF 2α would alter conception rates, pregnancy rates and the degree of synchrony in replacement beef heifers. A commercial heifer operation in north-central Kansas purchased 591 Angus × Hereford heifers from 12 sources. Prior to the spring breeding season, 14% of the heifers were culled. The remaining heifers were assigned randomly to 2 MGA-PGF 2α synchronization systems. All heifers were fed MGA (0.5 mg/head/d) for 14 d, and PGF 2α was administered either 17 or 19 d after the completion of MGA feeding. Heifers were inseminated artificially for 30 d followed by 30 d of natural mating. Based on each source, first-service conception rates ranged from 66 to 90%, whereas overall pregnancy rates ranged from 91 to 100%. Heifers given PGF 2α on Day 17 after MGA had first-service conception rates of 75.9% compared with 81.4% for heifers receiving PGF 2α on Day 19. In response to the PGF 2α injection, 99% of the Day 19 heifers that were detected in estrus were inseminated artificially by 72 h after the PGF 2α injection, whereas 74% of the heifers in the Day 17 treatment were inseminated by that time. Average interval to artificial insemination (AI) after PGF 2α was greater (P < 0.01) for the Day 17 heifers (73.1 ± 1.1 h) than for the Day 19 heifers (56.2 ± 1.1 h). No differences in conception rates or overall pregnancy rates occurred; however, heifers receiving PGF 2α on Day 19 after MGA had shorter intervals to estrus, and a greater proportion was inseminated within 72 h after PGF 2α, thus possibly facilitating successful timed insemination of the remaining heifers not yet inseminated by that time.