Seasonal changes in ovulation rate in coopworth ewes maintained at different liveweights

Abstract

Seasonal changes in ovulation rate were studied in ewes run outdoors and maintained at different liveweights during the breeding season. Coopworth ewes were run with vasectomized rams and fed different amounts of ryegrass/white clover pastures from December (anoestrus, latitude 45°51′S). In February, before the beginning of the breeding season, the high and low nutrition groups weighed approximately 60 and 50 kg, respectively. The ewe liveweights were maintained during the breeding season and the mean differences between the nutrition treatments were 10.0 and 9.9 kg for the 2 years of the experiment. Mating marks were recorded each week and the ovaries observed by laparoscopy every two weeks for 11 months. Ewes began cycling 16 days earlier in 1986 compared with 1985 (median dates of first oestrus were 22 March and 6 March for 1985 and 1986, respectively). Despite this large difference in the onset of the breeding season, the effects of nutrition were consistent in both years. Differences in the median dates of first oestrus between the nutrition treatments were 2 and 1 days in 1985 and 1986, respectively. Differences in the median dates of last oestrus between nutrition treatments were 3 and 9 days for 1985 and 1986. There was a marked seasonal pattern in ovulation rate with the peak in ovulation rate recorded in April and May (before the shortest day) for both treatments. The seasonal pattern in ovulation rate was similar for the two treatments and the mean difference between the treatments over the duration of the breeding season was 0.15. In the high plane treatment, the peak ovulation rates of 2.09 and 1.98 recorded in the two years of the study were immediately followed by a decrease at the next cycle. The results from individual ewes suggest that ovulation rates fluctuated in successive cycles after the peak of the breeding season.