Influence of concentration, duration and route of administration of melatonin on reproductive performance of spring-mated polypay and polypay-cross ewes

The effects of exogenous melatonin on reproductive performance of 737 Polypay and Polypay-cross ewes were evaluated during a late March and April breeding period. Different concentrations (2 or 10 mg) routes of administration (fed vs implanted) and durations of administration (20 or 40 d before breeding) were studied. Estrus was synchronized in all ewes using 60 mg medroxyprogesterone acetate (MAP) in a vaginal pessary. Number of mature ewes lambing/ewe present at lambing was increased (P less than .05) nearly 20% by implanting for 40 d with melatonin (75%) or by feeding either 2 (72%) or 10 mg (73%) melatonin for 40 d before spring breeding compared with untreated control ewes (54%) or ewes implanted for only 20 d before breeding (58%). Date of lambing, number of lambs born/ewes lambing and number of lambs born alive/ewe lambing were not altered significantly by treatments. Number of yearling ewes (n = 166 for ewes 1.5 yr old at lambing time) lambing/ewe present at lambing was lower (P less than .01; 26%) than that of mature ewes (n = 381, greater than or equal to 2.0 yr; 68%). We concluded that feeding 2 or 10 mg melatonin or implanting melatonin for 40 d enhanced reproductive performance and effectively overcame the restrictions of seasonality of breeding in mature ewes. In yearling ewes, 10 mg melatonin increased the number of ewes lambing.