Influence of varied progestogen treatments on ovarian follicle status and subsequent ovarian superstimulatory responses in cows

The influence of ovarian follicle status and follicle dominance on the response to superstimulatory treatment with FSH was examined in cows. In Experiment 1, oestrus was synchronised using Crestar® and on Days 4–6 of the ensuing oestrous cycle cows were assigned to: Group NO (n = 9), control, endogenous CL and no treatment; Group N1 (n = 15), injected with a luteolytic dose of cloprostenol (500 μg) and implanted with one implant (3 mg) of the synthetic progestogen, norgestomet; Group N8 (n = 18), injected with 500 μg cloprostenol and implanted with eight (24 mg) implants of norgestomet. On Days 9–11, seven implants were removed from six cows in Group N8 and these cows, plus eight Group N1 and all Group N0 cows, were superstimulated with porcine FSH (Folltropin-V®) over 4 days (360 mg total dose). The remaining implants were removed from cows in Groups N1 and N8 on Days 11–13, and all cows received 500 μg cloprostenol. Numbers and sizes of ovarian follicles, and CL, were recorded by trans-rectal ultrasonography; the largest follicle > 10 mm in diameter was considered morphologically dominant (DF). On Days 9–11, the proportions of cows with a DF were: Group N0, 39; Group N1, 1415; Group N8, 018. Total follicles on the 4th day of FSH treatment were greater (P < 0.05) for cows in Group N1 (21.6 ± 4.2) compared with Group N0 (10.9 ± 2.4), with cows in Group N8 (13.2 ± 0.9) not different from the other two groups. Subsequent numbers of CL were lower (P < 0.05) for cows in Group N1 (5.0 ± 1.3) compared with Group N0 (9.4 ± 2.0), with cows in Group N8 (8.5 ± 1.0) not different from the other two groups. In Experiment 2, oestrus was synchronised in cows and on Days 4–6, cows were assigned to: Group C0 (n = 7), control, endogenous CL and no treatment; Group C3 (n = 6), received three CIDR-B® intra-vaginal devices that delivered progesterone. On Days 9–11, two CIDR-B® were removed from cows in Group C3 and all cows treated with FSH as in Experiment 1. The remaining CIDR-B® was removed from cows in Group C3 on Days 11–13 and all cows injected with 500 μg cloprostenol. Proportions of cows with a DF on Days 9–11 and diameter of largest follicle were: Group C0, 6/7 and 12.6 ± 0.9 mm; Group C3, 2/6 and 9.6 ± 0.8 mm. Numbers of CL on Day 8 after oestrus were: Group C0, 20.0 ± 7.1; Group C3, 14.8 ± 4.8 (P > 0.05). Exposure to low dose norgestomet allowed development of a persistent dominant follicle, resulting in a reduced response to superstimulation with FSH. High dose progestogen restricted follicle growth without apparent effects on responses to superstimulation.