Abstract
The synchronization of estrus with synthetic progestins or progesterone (P 4) results in the development of a large, persistent ovarian follicle. The objectives of the present study were to determine if development of a persistent ovarian follicle during synchronization of estrus suppresses recruitment of additional follicles during FSH treatment. On Day 5 of the estrous cycle (estrus = Day 0), beef cows were treated with 0.5 or 2.0 P 4 releasing intravaginal devices (PRIDs) for 8 d (Experiment 1, n = 20), 5 or 2 d (Experiment 2, n = 44) before initiation of FSH treatment. Prostaglandin F 2α (25 mg) was administered on Days 5 and 6. Superovulation was induced with 24 mg of recombinant bovine FSH (rbFSH, Experiment 1) or 28 mg of FSH-P (Experiment 2) over a 3- or 4-d period, respectively. The PRIDs were removed concurrently with the 5th injection of rbFSH or FSH-P. There was a treatment-by-day interaction (P < 0.001) for the concentration of 17β-estradiol in cows treated for 8, 5 or 2 d before FSH treatment. In Experiment 1, FSH treatment initiated 8 d after insertion of a 0.5 PRID did not affect the number of CL (6.9 ± 1.4 vs 6.7 ± 1.6), ova/embryos (3.7 ±1.3 vs 3.0 ± 1.3) and transferable embryos (2.4 ± 0.9 vs 3.0 ± 0.9) compared with that of the 2.0 PRIDs. In Experiment 2, FSH treatment initiated 5 d after insertion of a 0.5 PRID decreased the number of CL (4.0 ± 0.5 vs 8.3 ± 0.8; P < 0.001), ova/embryos (3.0 ± 0.6 vs 5.9 ± 1.2; P < 0.03) and transferable embryos (2.3 ± 0.6 vs 5.1 ± 1.0; P < 0.03) compared with that of a 2.0 PRID, respectively. Initiation of FSH treatment 2 d after insertion of a 0.5 PRID compared with a 2.0 PRID had no affect on the number of CL (8.0 ± 2.1 vs 8.7 ± 1.2), total ova (4.8 ± 1.4 vs 6.9 ± 1.4) and transferable embryos (2.9 ± 1.2 vs 6.1 ± 1.7). In conclusion, treatment with low doses of P 4 (0.5 PRID) for 5 d but not for 2 or 8 d before initiation of FSH treatment results in the development of a dominant ovarian follicle, which reduces recruitment of ovarian follicles, and the number of CL, total ova and transferable embryos.
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