PHYSIOLOGICAL LIMITS TO FURTHER IMPROVEMENT IN THE EFFICIENCY OF ESTROUS SYNCHRONIZATION IN GOATS

The variability between animals in the timing of oestrus after adminis tration of a synchronization treatment seems to explain the low rate o f fertility in goats inseminated at a predetermined time after progest erone withdrawal. Two experiments were performed during the breeding s eason to test whether the variation was due to the exogenous hormone r egime or to the endogenous physiology of the animals. Twenty-one goats were given a synchronization treatment consisting of a vaginal sponge impregnated with 45 mg of fluorogestone acetate (FGA) for 11 days ass ociated with intramuscular injection of 400 LU. of equine chorionic go nadotrophin (eCG) and 50 mu g of cloprostenol 48 h before sponge remov al. Progesterone concentrations were measured during the subsequent cy cle and the patterns were modelled to allow precise determination of t he onset of luteolysis. Oestrus and the luteinizing hormone (LH) surge began 33.0+/-6.8 h and 76.0+/-33.0 h after sponge withdrawal, v. 43.4 +/-5.7 h and 90.0+/-36.0 h after natural luteolysis. For both observat ions, the between-goat variability was larger during the natural than during the synchronized oestrus (P < 0.05). The duration of the oestro us cycle was independent of the number of corpora lutea (CL), whereas the duration of luteal phase was shorter in goats with 2-3 CL (16.4+/- 0.9 day than in those with 1 CL: 17.7+/-1.3 day; P < 0.05). In the sec ond experiment, 20 goats were ovariectomized and given a vaginal spong e as described above. Sixteen h after sponge removal, they were inject ed with 50 mu g of oestradiol benzoate (ODB). This treatment was repea ted with the second sponge being inserted 1-2 days after observation o f oestrus. Oestrus and LH surge were observed: 32.8+/-6.8 h v. 27.8+/- 7.8 h after the first ODB injection, and 36.6+/-7.3 v. 34.3+/-4.8 h af ter the second ODB injection. No relationship was observed between dat a of the two experiments. In both cases, the variability in the occurr ence of oestrus and LH surge was of the same order as observed in the first experiment. This study shows that the timing of oestrus and LH s urge is less variable after progestagen treatment than during a natura l oestrous cycle. Moreover, a significant proportion of variability is inherent in the delays following the oestradiol peak, suggesting that further improvement in the synchronizing capacity of treatment based on progestagen administration is unlikely.