THE EFFECTS OF DOSE AND DURATION OF ADMINISTRATION OF PFSH DURING THEFIRST FOLLICULAR WAVE ON THE OVULATION RATE OF BEEF HEIFERS

Four experiments were carried out to examine the effects of administra tion of pFSH (Vetrepharm) from Day 3 of the estrous cycle in conjuncti on with PG on Day 5 on follicular populations and ovulation rate in he ifers. In Experiment 1, 47 heifers were allocated to 1 of 4 treatment groups (n=11 to 12 per group): a) control, b) 1.5 mg pFSH, c) 2.0 mg p FSH or d) 2.5 mg pFSH until estrus. Heifers assigned to the 3 treatmen ts had a higher ovulation rate than the controls (P < 0.05). In Experi ment 2, 45 heifers were allocated to 1 of 5 treatment groups (n = 8 to 10 per group): a) control, b) 1.0 mg pFSH until PG, c) 1.0 mg pFSH un til estrus, d) 1.5 mg pFSH until PG or e) 1.5 mg pFSH until estrus. Fr om Day 5, heifers assigned to pFSH treatments had more large follicles than the controls (P < 0.05). There was no effect of treatment on the incidence of twin ovulations. In Experiment 3, 43 heifers were assign ed to 1 of 3 groups (n=11 to 16 per group): a) control, b) 1.0 mg pFSH until estrus or c) 1.5 mg pFSH until estrus. At slaughter, 14 d after administration of PG, the incidence of twin ovulations was 0/11, 7/16 and 8/16 for Groups a, b and c, respectively (P = 0.011). In Experime nt 4, pFSH (1.5 mg) was administered to 3 groups during the developmen t of the first dominant follicle: a) growth phase (n = 19); b) static phase (n = 17); and c) decline phase (n = 17). All pFSH-treated heifer s had a higher ovulation rate than the controls (P < 0.05); heifers as signed to Group c had a higher ovulation rate than those in Groups a o r b (P < 0.05). More heifers assigned to Group c (7/17) superovulated than in the other 2 groups (P < 0.05). In conclusion, administration o f 1.0 or 1.5 mg pFSH twice daily beginning at Day 3 of the estrous cyc le in association with the induction of luteolysis increased the ovula tion rate significantly when pFSH treatment was continued to onset of estrus. The ovulation rate and the occurrence of multiple ovulations w ere significantly higher when pFSH was administered at the time that t he first dominant follicle was in decline. (C) 1998 by Elsevier Scienc e Inc.