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

The aim of this study was to determine whether advancing Me seasonal change s associated with rams by treatment with exogenous melatonin and allowing t he rams previous sexual experience would increase the proportion of anoestr ous ewes ovulating in early July. North Country Mule ewes (n = 225) were gr ouped by live body weight and body condition score and allocated randomly t o the following treatments: (i) isolated from rams (control; n = 25); (ii) introduced to rams (treatment 2); (iii) introduced to rams that had mated w ith ewes during the previous 2 days (treatment 3); (iv) introduced to rams implanted with melatonin (treatment 4); and (v) introduced to rams that wer e implanted with melatonin and had mated with ewes during the previous 2 da ys (treatment 5). Treatments 2-5 were replicated (2 x 25 ewes) and two rams were introduced to each replicate group. Introductions began on 4 July and were completed by 11 July. The rams were withdrawn from the ewes after 8 d ays. Melatonin was administered as a subcutaneous implant (Regulin(R)) on 2 2 May and again on 20 June. Blood samples were taken from all rams to deter mine plasma melatonin and testosterone concentrations (19 samples in 6 h). The behaviour of the sheep was videotaped continuously during the first 3 h after the ram was introduced. Ovulation was detected by an increase in pla sma progesterone concentrations from < 0.5 ng ml(-1) to > 0.5 ng ml(-1). Me an +/- SE plasma melatonin concentrations were 649.7 +/- 281.4 and 18.3 +/- 2.4pg ml(-1) in rams with and without melatonin implants, respectively (P < 0.001). Melatonin implants also increased plasma testosterone concentrati ons from 4.30 +/- 1.88 to 10.10 +/- 1.10 ng ml(-1) (P < 0.01), the libido o f the rams and the proportion of ewes that ovulated in response to the rams (43 and 56% (treatments 4 and 5) versus 24% (treatments 2 and 3)). In conc lusion, implanting rams with melatonin before introducing them to seasonall y anoestrous ewes increases the proportion of ewes that ovulate in response to introduction of a ram, but previous sexual experience of rams appears t o have little or no effect.