Manipulation of reproduction in sheep

Active immunization of cyclic ewes against androstenedione (using melatonin, Fecundin) has been shown to increase reproductive output by about 30 extra lambs per 100 ewes put to the ram in UK sheep flocks. However, this technique has not been widely adopted within the industry. One possible reason for this is that it results in an unacceptable incidence of triplet births, even though the litter size distribution is similar to that obtained for untreated flocks with the same overall mean lambing percentage. The use of progestagen sponges plus pregnant mares' serum gonadotrophin (PMSG) for the induction of out-of-season breeding in ewes is associated with a number of recognized shortcomings. A possible reason for the variability in conception rates is the high incidence of complete embryo loss or fertilization failure in ewes that are induced to superovulate after treatment with the doses of PMSG required to promote a high oestrous response. Recent studies have indicated that slow release implants of melatonin (Regulin) can advance the onset of the breeding season of commercial sheep flocks, but only by about 4 weeks. This means that the optimum treatment window varies according to the natural breeding season characteristics for the breed. Nevertheless, this treatment appears to overcome a number of the shortcomings of progestagen sponges plus PMSG. It also produces a modest increase in litter size per ewe pregnant (about 15 extra lambs per 100 ewes) without the wide variation in birth types and birth weights associated with PMSG. The development of a laparoscopic procedure that improves conception rates after intrauterine insemination of frozen semen allows comparisons of genetic merit across flocks. This should promote far greater rates of genetic improvement for traits of high economic importance than those achieved by within-flock selection and in the last two years has allowed the development of sire-referencing schemes within the major terminal sire breeds. Laparoscopic procedures for the collection and transfer of embryos in sheep now have the potential to overcome many of the limitations of more invasive surgical approaches, although the unpredictability of superovulatory responses remains a particular obstacle to full exploitation of multiple ovulation embryo transfer (MOET) for genetic improvement through the female line.