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Effect of progestagen treatments on the incidence of oestrus and pregnancy rates in ewes

1991; Elsevier BV; Volume: 24; Issue: 1-2 Linguagem: Inglês

10.1016/0378-4320(91)90086-f

1873-2232

T.F. Crosby, M.P. Boland, Ian Gordon,

Animal Behavior and Welfare Studies

In experiment 1, a 4 × 3 × 2 factorial design was used to examine the effect of intravaginal progestagen (60 mg medroxyprogesterone acetate; 30 mg fluorogestone acetate; 500 mg progesterone or a controlled internal drug release device with 12% w/w (0.4 g) progesterone), pregnant mare serum gonadotrophin (PMSG) dose level (500, 750 or (1000) IU) and mating management systems (hand or paddock) on the induction of oestrus and pregnancy in 247 anoestrous ewes and 100 ewes during the breeding season. The intravaginal treatments employed in the anoestrous group did not affect oestrous response (96%), proportion of ewes lambing (72%) or litter size (1.74). When applied in cyclic ewes, a higher proportion (P < 0.025) of ewes lambed to the controlled oestrus following synthetic progestagens (84% v 62%). Increasing PMSG from 500 to 1000 IU depressed litter size in cyclic animals. There were indications in the anoestrous ewes that the minimum effective dose of PMSG may be higher when combined with progesterone treatments, rather than with synthetic progestagens. Mating method did not affect the percentage of ewes mated or lambing, although there were indications that the paddock mating system resulted in a higher litter size. In experiment 2, 854 lowland ewes, on 10 commercial farms, were synchronised using either synthetic (FGA; n = 426) or natural (CIDR; n = 428) progestagens in late July. A lower proportion (P < 0.05) of ewes treated with CIDR lambed to the induced oestrus (67% v 77%). There were no treatment differences in litter size. Results of these experiments suggest that synthetic progestagens are likely to be more successful than progesterone, particularly in the breeding season, in obtaining high pregnancy rates.