The effect of progestagen treatment on sheep reproductive performance at different phases of the oestrous cycle

Abstract

Two experiments were carried out in order to determine the effect of progestagen treatment to synchronise oestrus on sheep reproductive performance at different phases of the cycle, comparing artificial insemination and natural mating. In experiment 1, and after analysis of plasma progesterone concentrations 7 days before and at pessary insertions, animals were divided into four groups: Group F (n = 25), follicular phase at pessary insertion; Group L1 (n = 22), the onset of the luteal phase; Group L2 (n = 18), mid-late luteal phase and Group A (n = 11), anovulatory. The ewes were inseminated 54 h after pessary withdrawal. Mean percentage of fertility was low (30%), and when data from Groups L1 and L2 were pooled, and compared with F+A groups, percentage of fertility was significantly higher (P < 0.05) in the latter groups (20% vs. 39%, respectively). Similarly, the number of lambs bom per inseminated ewe was twice as high (P < 0.05) in these animals as compared with L1+L2 (F+A: 0.50±0.13; L1+L2:0.25±0.09 lambs bom per inseminated ewe). In experiment 2, oestrous cycles of 40 ewes were synchronised with two i.m. injections of 7.5 mg Luprostiol. Seventy-two hours after the second injection (day 0), progestagen pessaries were inserted in 20 ewes (Group 0), when the ewes were in the follicular phase of the cycle. The remaining 20 ewes received the pessaries on day 11, during the luteal phase of the cycle (Group 11). Animals were hand-mated at least twice by fertile rams 48 h later; the ewes were thereafter exposed to rams in the pen overnight. The percentage of fertility was 90% in both groups. No significant differences between groups were observed for litter size and fecundity. In conclusion, an onset of the synchronisation treatment with progestagen in a program of artificial insemination close to the follicular phase of the oestrous cycle can improve both fertility and the number of lambs born per inseminated ewe. If natural service is used, perhaps the stay of the rams with the ewes for at least 24 h might compensate for the asynchrony between ovulation and mating, leading to similar percentages of fertility, whatever the phase of the cycle when the synchronisation treatment began. Further studies involving a higher number of animals are necessary to confirm these results.