Variation in the ability of melatonin implants inserted at two different times after the winter solstice to restore reproductive activity in reduced seasonality ewes

The purpose of the present study was to assess the effects on reproductive activity of melatonin implants inserted immediately after the winter solstice compared with the classical implantation around the spring equinox. The experiment was conducted at latitude 41degreesN in the absence of rams. Twenty-one ovariectomised, oestradiol-treated Rasa Aragonesa ewes received (M1, n = 7, 15 January; M2, n = 7, 15 March) or not (C, n = 7) a single implant containing 18 mg melatonin. Blood samples were collected twice weekly from 15 January (M1) and from 15 March (M2, C), and assayed for luteinizing hormone. Six M1 ewes resumed their reproductive activity by 20 April, 95 days after implant insertion, although the reproductive season induced only lasted for 45 days. Melatonin implants inserted on 15 March did not induce a resumption of reproductive activity (0 ewes; P < 0.05). It is concluded that, in the absence of males, the response of Mediterranean ewes to melatonin implants is better when implantation takes place immediately after the winter solstice than around the spring equinox. Two subsequent field experiments with the same breed and year were carried out. Animals were implanted (M) or not (C) with melatonin on 29 December (M, n = 156; C, n = 159) and on 20 March (M, n = 150; C, n = 128). Results confirmed the efficacy of the treatment with melatonin immediately after the winter solstice in ewes with a short seasonal anoestrous period. Implants induced a mean productivity increase of 0.30 (December) and 0.36 (March) lambs per treated ewe compared with those in the control groups.