Optimum use of subcutaneous melatonin implants to enhance the reproductive performance of seasonal and non-seasonal sheep joined in spring and early summer

Abstract

This paper reviews data from five research trials and 108 clinical trials conducted in three countries to validate the optimum use of melatonin to advance seasonal breeding patterns of a variety of breeds of sheep. In order to define the optimum time for treatment in breeding flocks, ewes of three different breeds were treated with controlled-release 18-mg melatonin implants (Regulin®), with treatments commencing at various times ranging from 9 to 3 weeks prior to joining with fertile rams. Mating and conception patterns were recorded by regular observation for raddle marks and fecundity was assessed by mid-pregnancy ultrasonography. These research trials were conducted at a latitude of 38°S in Merino ewes of slight seasonality, in Merino-based cross-breeds of intermediate seasonality and in highly seasonal British breed Romney Marsh ewes. A single implant treatment, commencing at 30–40 days prior to spring or early summer joining, resulted in coincidence in timing of the peak of the inductive phase of the ovarian response to melatonin with the peak of oestrus and mating which arose from the pheromonal influence of the rams. Thus, this treatment strategy maximises the potential advantages to be obtained from both the melatonin treatment and the rams. Subsequent clinical trial programmes in Australia, the UK and New Zealand confirmed this treatment strategy and tested the available periods of the year during which effective responses could be obtained under farm conditions. It was shown that, whereas Merino and Merino cross-breeds were responsive to melatonin treatment over a wide part of spring and early summer, the British breed ewes responded only during a relatively short period of the season around the time of the summer solstice. Treatments which commenced too early in British breeds were ineffective or gave negative results, possibly because of a failure of such breeds to be able to fully interpret the pharmacological short-day signal before they have experienced an adequate period of long-day priming. For all breeds, treatments given late in summer at a time of decreasing natural daylight were also ineffective since, by this time, ewes were already experiencing a naturally occurring inductive photoperiodic signal. Overall, the studies reviewed in this paper showed that melatonin pretreatment of spring and early summer joined ewe flocks resulted in both a modest decrease in the number of barren ewes and an increase in the number of multiple births. The effects were seen in flocks of high or low basal performance and in both maiden and mature ewes, but in no case were the responses associated with the induction of supraphysiological litter size. Mean increases in litter size, for different breeds of ewe, ranged from 14.3 to 28.6 additional lambs born per 100 ewes treated, with variation between trials largely accounted for by expected sampling errors associated with the precise measurement of the discontinuous variable of litter size in relatively small sample sizes of 50–200 animals per group. Treatment of mothers with melatonin prior to joining had no effect on the sex ratio, birth weight or appearance of offspring. Melatonin pretreatment of breeding ewes was also associated with an earlier mean conception (lambing) date and a compaction of the conception (lambing) patterns but the magnitude of these measured effects was dependent upon the extent to which untreated control flocks were responsive to the pheromonal influence of rams. Overall the data extends, to a practical field level, the proper use of melatonin to at least partially overcome the seasonal constraint to breeding in sheep, and provides a sound basis for future research and clinical experimentation in this area.