Ewes from five genotypes (Australian Merino, New Zealand Merino, Romney, Australian Merino x Romney, New Zealand Merino x Romney) were examined over 2 years from April 1986 to March 1988. Maiden 2-tooth ewes were used in the first year and mixed aged ewes in year 2. Ewes were treated with CIDRs (14 d) and joined with vasectomised rams every 6 weeks throughout the year. The incidence of oestrus and ovulation and ovulation rate were recorded. In year 1, there were no discernible differences between the five genotypes in the pattern of ovulation response which ranged from about 100% in April to 70% in August, dropping to 3% in September and then to 0% until a recover to 100% in the 4 Merino lines and 70% in the Romney by March. In year 2, the incidence of ovulation for all 5 genotypes was 100% for April, May and July. In August, 70% of the Merino lines and 55% of the Romney and Romney cross lines ovulated. Only the Australian Merino ovulated in November (8%) while both Merino lines ovulated in December (Australian 4%, New Zealand 14%). However in contrast to Year 1, in January the pure Merino lines showed 100% incidence of ovulation, the cross breed lines 60% and Romney 10%. By March, the cross breed incidence had reached 100% and the Romney 80%. These data indicate a marked seasonal pattern of ovulation in all 5 genotypes with some indication of an earlier onset to the breeding season in Merino ewes.

TG, Harvey, and RL Baker

Proceedings of the New Zealand Society of Animal Production, Volume 49, , 215-220, 1989
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