Influence of exogenous melatonin and altered day length on reproductive performance of Polypay ewes

Two experiments were designed to evaluate postlight treatment use of melatonin as a method of overcoming photorefractory response by measuring the lambing percentage of early-postpartum Polypay ewes bred either late in the breeding season or during anestrus. In Trial 1, pregnant ewes (n = 140) were assigned to one of three groups: 1) ambient photoperiod (control), 2) extended light (20 h) from October 21 to December 27, and 3) extended light as in Group 2, followed by supplemental feeding of 10 mg melatonin/head (hd) daily until April 6. Breeding started on February 3 and ended April 9. A greater percentage of ewes given extended light plus melatonin (54%) and extended light alone (45%) lambed than controls (24%) (P = 0.06). In Trial 2, pregnant ewes (n = 158) were assigned to groups as in Trial 1, except extended light was given to Groups 2 and 3 from January 1 to March 11, and Group 3 ewes were supplemented with 10 mg melatonin daily from March 12 to June 10. Breeding started April 18 and ended June 10. Lambing percentage was increased (P < 0.01) by extended light plus melatonin (54%) compared to controls (6%) or ewes given only extended light (10%). The shift from artificially extended light to the shorter ambient light with or without melatonin enhanced the lambing percentage of early postpartum ewes on an accelerated lambing program during breeding late in the season. However, only the more pronounced shift from long days to short days, accomplished with extended light plus melatonin, was effective in inducing out-of-season breeding in Polypay ewes.