Reproductive Response in Seasonally Anestrous Suffolk Ewes After a Treatment Combining Melatonin Feeding and the 'Male Effect'.

Abstract

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.

Induction of fertile estrus without the use of steroid hormones in seasonally anestrous Suffolk ewes

“…There are two studies in which results comparable to those in the present study were achieved in Suffolk ewes induced to cycle by combining the male effect with a non-steroid hormone. In those studies, melatonin feeding was combined with the male effect [25,26]. Both the melatonin treatments used in those studies and the GnRH/PGF2α treatments used in this work can be considered as clean alternatives [10,25], as all these hormones degrade quickly within the animal organism, leaving no bioactive residues.…”

“…In those studies, melatonin feeding was combined with the male effect [25,26]. Both the melatonin treatments used in those studies and the GnRH/PGF2α treatments used in this work can be considered as clean alternatives [10,25], as all these hormones degrade quickly within the animal organism, leaving no bioactive residues. However, the studies of Kusakari and Ohara [25,26] involved daily oral administration of melatonin for up to 90 days, as well as an interval of 37 to 47 days from the onset of melatonin feeding to initial exposure to the males.…”

“…Both the melatonin treatments used in those studies and the GnRH/PGF2α treatments used in this work can be considered as clean alternatives [10,25], as all these hormones degrade quickly within the animal organism, leaving no bioactive residues. However, the studies of Kusakari and Ohara [25,26] involved daily oral administration of melatonin for up to 90 days, as well as an interval of 37 to 47 days from the onset of melatonin feeding to initial exposure to the males. Thus, the first estrus in those studies occurred around day 60 after initiation of melatonin feeding, and pregnancy occurred around 76 days after the onset of the treatments [25].…”

“…However, the studies of Kusakari and Ohara [25,26] involved daily oral administration of melatonin for up to 90 days, as well as an interval of 37 to 47 days from the onset of melatonin feeding to initial exposure to the males. Thus, the first estrus in those studies occurred around day 60 after initiation of melatonin feeding, and pregnancy occurred around 76 days after the onset of the treatments [25]. In contrast, in the present study it was only necessary to administer between zero (ME group) and three (GPG group) injections to the animals, and both estrus and conception occurred on average 20 days after the onset of the experiment.…”

“…However, the efficiency of estrus detection was significantly improved by the incorporation of GnRH into the induction protocol, since estrus was detected only in 60% of the ovulations that were preceded by a normal-length luteal phase in the ME group, while in the groups treated with GnRH the efficiency of estrus detection ranged from 85% to 100%. It has been reported that an important cause of failure to respond to the male effect in Suffolk ewes is the presence of silent ovulations [18,25], and that their incidence may be reduced with additional measures, such as the administration of melatonin [25] or the use of very active males of low-seasonality breeds [18]. In the present study the administration of GnRH also reduced the incidence of non-detected estrus during male-induced ovarian activity.…”