title: Prolonging the MGA-prostaglandin F2 alpha 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 PGF2 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-PGF2 alpha synchronization systems. All heifers were fed MGA (0.5 mg/head/d) for 14 d, and PGF2 alpha 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 PGF2 alpha on Day 17 after MGA had first-service conception rates of 75.9% compared with 81.4% for heifers receiving PGF2 alpha on Day 19. In response to the PGF2 alpha injection, 99% of the Day 19 heifers that were detected in estrus were inseminated artificially by 72 h after the PGF2 alpha injection, whereas 74% of the heifers in the Day 17 treatment were inseminated by that time. Average interval to artificial insemination (AI) after PGF2 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 conception rates or overall pregnancy rates occurred; however, heifers receiving PGF2 alpha on Day 19 after MGA had shorter intervals to estrus, and a greater proportion was inseminated within 72 h after PGF2 alpha, thus possibly facilitating successful timed insemination of the remaining heifers not yet inseminated by that time.
authors.affiliation: North Central Experiment Station, University of Minnesota, Grand Rapids 55744, USA. clamb@tc.umn.edu
authors.forename: G C
authors.initials: GC
authors.lastname: Lamb
authors.forename: D W
authors.initials: DW
authors.lastname: Nix
authors.forename: J S
authors.initials: JS
authors.lastname: Stevenson
authors.forename: L R
authors.initials: LR
authors.lastname: Corah
mesh_terms: D000818:Animals; D002417:Cattle / Q000502:physiology*; D015237:Dinoprost / Q000008:administration & dosage*; D004971:Estrus / Q000187:drug effects; D004973:Estrus Synchronization; D005260:Female; D007315:Insemination, Artificial / Q000662:veterinary; D003341:Luteolysis / Q000187:drug effects; D008552:Melengestrol Acetate / Q000008:administration & dosage*; D011247:Pregnancy; D013997:Time Factors
chemical_list: D008552:Melengestrol Acetate; D015237:Dinoprost