Abstract
Fifty-eight anestrous Suffolk ewes, raised in Hokkaido, were randomly divided into 2 groups, with or without melatonin feeding as follows: Group 1 ewes (n=33) were fed melatonin daily for 45-90 days from the early or mid-anestrous season (late March to June) and Group 2 ewes (n=25) were not. All the ewes were introduced to fertile rams for 6-12 weeks starting between April and July. The incidence of estrus in Group 1 ewes during the joining of the rams was higher than that in Group 2 ewes (100% vs. 32%, p<0.01). The first estrus in Group 1 was observed 7-10 weeks after the beginning of melatonin feeding. The intervals from the ram introduction to the onset of estrus in 8 ewes that showed estrous response in Group 2 were not distinguishable from those in the ewes fed melatonin for over 5 weeks before the ram introduction in Group 1. The conception rate in Group 1 was higher than that in Group 2 (93.9% vs. 8%, p<0.01). The prolificacy in the Group 1 and 2 ewes was 148% and 8%, respectively. These results indicates that Suffolk ewes were responsive to the 'male effect' 5 weeks onward after the start of the melatonin feeding during early and mid- anestrous season.
