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Effects of recent sexual experience and melatonin treatment of rams on changes in plasma testosterone concentration and sexual behaviour and ability to induce ovulation in seasonally anoestrous ewes

Agricultural and Animal Production

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Reference

Rosa, H. J. D., Juniper, D. and Bryant, M. J. (2000). Effects of recent sexual experience and melatonin treatment of rams on changes in plasma testosterone concentration and sexual behaviour and ability to induce ovulation in seasonally anoestrous ewes. Journal of Reproduction and Fertility, 120, 169-176.

Summary

ABSTRACT The aim of this study was to establish if advancing the seasonal changes associated with the rams by treatment with exogenous melatonin and allowing the rams prior sexual experience would increase the proportion of anoestrous ewes ovulating in early July. Two-hundred-and-twenty-five North Country Mule ewes were blocked by live weight and body condition score and randomly allocated from blocks to the following treatments: (1) isolated from rams (n=25); (2) introduced to rams; (3) introduced to rams which had mated with ewes during the previous two days; (4) introduced to rams implanted with melatonin; (5) introduced to rams implanted with melatonin which had mated with ewes during the previous 2 days. Treatments 2-5 were replicated (2x25 ewes) and two rams were introduced to each replicate. Introductions began on 4 July and were completed by 11 July. The rams were withdrawn from the ewes after 8 days. Melatonin was administered as a subcutaneous implant (Regulin®) on 22 May and again on 20 June. Blood samples were taken from all rams to establish melatonin and testosterone (19 samples over 6 hours) concentrations in plasma. The behaviour of the sheep was continuously videotaped during the first 3 hours of ram introduction. Occurrence of ovulation was detected by a rise in plasma progesterone concentration from <0.5 ng ml-1 to >0.5 ng ml-1. Mean (se) plasma melatonin concentrations were 18.3(2.40) and 649.7(281.40) pg ml-1 for non-implanted and implanted rams, respectively (P<0.001). Melatonin implants also increased plasma testosterone concentration from 4.3(1.88) to 10.1(1.10) ng ml-1 (P<0.01), libido of rams and the proportion of ewes ovulated in response to rams presence (43 and 56% vs 24%). It was concluded that implanting rams with melatonin prior to introduction to seasonally anovular ewes increases the proportion of ewes that ovulate in response to ram introduction but previous sexual experience of rams appears to have little or no effect.

Sunday, 31 December, 2000

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