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

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(2 alpha) 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 x 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(2 alpha) synchronizat ion systems. All heifers were fed MGA (0.5 mg/head/d) for 14 d, and PGF(2 a lpha) was administered either 17 or 19 d after the completion of MGA feedin g. Heifers were inseminated artificially for 30 d followed by 30 d of natur al 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(2 alpha) on Day 17 after MGA had first-service conception rates of 75.9% compared with 81.4% for heifers receiving PGF(2), on Day 19. In re sponse to the PGF(2 alpha) injection, 99% of the Day 19 heifers that were d etected in estrus were inseminated artificially by 72 h after the PGF(2 alp ha) injection, whereas 74% of the heifers in the Day 17 treatment were inse minated by that time. Average interval to artificial insemination (AI) afte r PGF(2 alpha) 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 concepti on rates or overall pregnancy rates occurred; however, heifers receiving PG F(2 alpha) on Day 19 after MGA had shorter intervals to estrus, and a great er proportion was inseminated within 72 h after PGF(2 alpha), thus possibly facilitating successful timed insemination of the remaining heifers not ye t inseminated by that time. (C) 2000 by Elsevier Science Inc.