Implants of melatonin can improve reproductive performance in sheep but timing of joining is important. In this study, 531 Coopworth ewes were used in a 2x4 factorial design to investigate the effects on reproductive performance of Regulin (melatonin) implants and time of joining. Implanted ewes were treated about 1 month prior to joining on either 1 December, 15 December, 29 December or 12 January. Control ewes were untreated. Matings with Poll Dorset rams were recorded fortnightly until the rams were removed on 5 February. About 10% of the implanted and control ewes joined on 1 December mated by 29 December. By the end of joining, over 90% of these early joined ewes had mated. Melatonin treatment had no effect on mating pattern in these 2 joining groups. By contrast, melatonin treatment improved the total number of ewes mated when joined on 29 December (100% v 58% P<0.001). About 50% of the treated and control ewes joined on 12 January had mated by the end of joining, but not melatonin treatment affected return to service rates. Melatonin treatment reduced the pregnancy rate in ewes joining on 1 December and 15 December (69% v 83% P<0.01). In contrast, melatonin treatment increased the pregnancy rate for 29 December joinings (72% v 41% P<0.001) but not 12 January joinings (49% v 46%). The incidence of multiple pregnancies was increased by melatonin treatment (40% v 30% P<0.05). The melatonin treatment trend was for fewer lambs per ewe joined for early joining times to more lambs per ewe joined for later joining times. These results demonstrate that reproduction responses to melatonin implants are very sensitive to time of joining. With a late December to mid January joining time and an early February end of joining, between 20 and 40 more lambs per ewe joined was achieved in this study. The onset to the breeding season in these Coopworth ewes is about 2 months earlier than expected. The use of Poll Dorset rams may be an important factor in this early onset.
Proceedings of the New Zealand Society of Animal Production, Volume 49, , 15-20, 1989
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