A comparison of two methods of oestrous synchronisation of hair sheep in the tropics.

Two trials were conducted to evaluate the efficacy of oestrous synchronisation procedures in St. Croix White, Barbados Blackbelly hair and Florida Native wool ewes. In Trial 1 (conducted in June), 27 ewes were treated with controlled internal drug release (CIDR) devices for 12 days (CIDR1) and 29 untreated ewes served as controls (CONT). The CIDR devices were removed on the same day that intact rams equipped with marking harnesses were placed with the ewes. Time to oestrus after ram introduction was shorter (P < 0.0001) in CIDR1 than CONT ewes. Within 3 days of ram introduction 100% of CIDR1 ewes but only 37.9% of CONT ewes had been in oestrus (P < 0.0001). Conception rate at first oestrus after ram introduction was 74.1% overall, with no effect (P > 0.10) of treatment, but days to conception were shorter (P < 0.001) in CIDR1 than CONT ewes. Ovulation rate at first oestrus after ram introduction was not different (P > 0.10) between CIDR1 and CONT ewes. The CIDR1 ewes lambed earlier (P < 0.004) in the lambing season than CONT ewes, but there was no difference in the number of lambs born per ewe (P > 0.10). In Trial 2 (conducted in October), 14 St. Croix White ewes were treated with CIDRs as in Trial 1 (CIDR2) and 14 St. Croix White ewes were given two i.m. injections (15 mg) of prostaglandin F2 alpha (PGF) 10 days apart. Intact rams were introduced on the day of CIDR removal or the second PGF injection. The CIDR2 ewes exhibited oestrus earlier (P < 0.01) than PGF treated ewes. The conception rate to breeding at the synchronised oestrus was similar (P > 0.10) between CIDR2 and PGF treated ewes. Progesterone concentration on Day 10 after the synchronised oestrus was not different (P > 0.10) between CIDR2 and PGF treated ewes. These results indicate that oestrous synchronisation procedures can be used in sheep in the tropics without adversely affecting fertility. Due to a lack of seasonal anoestrous these procedures have the potential to be used during all times of the year.