Seasonal variation in oestrous and ovarian activity of five breeds of ewes in Otago

Abstract
Oestrous and ovarian activity of 1½ and 4½-year-old Romney, Coopworth, and Perendale ewes were recorded from January 1974 to April 1975. The mean date that oestrus was first detected was earlier (P < 0.01) in Coopworth (19 March) than in Romney or Perendale ewes (25 and 26 March respectively). Duration of the breeding season for Romney, Coopworth, and Perendale 1½-year-old ewes was 117, 126, and 136 days respectively, and for 4½-year-old ewes was 128, 147, and 128 days respectively (breed × age interaction P < 0.01). Length of the oestrous cycle was similar between breeds and age of ewes, and did not change during the breeding season. Approximately 10% of ovulations during the breeding season in these three breeds of ewe were not associated with detected oestrus. Rams did not detect oestrus in conjunction with 28% and 19% of recorded ovulations in 4½-year-old Merino and Dorset Horn ewes respectively, included in the experiment from March 1974. Also, in 1975 more than half of the Merino and Dorset Horn ewes had two silent ovulations before the first detection of oestrus, whereas 87–95% of the ewes in the other three breeds had only one silent ovulation. Seasonal and breed effects on ram and ewe behaviour could account for these differences. Although natural twice-a-year lambing would not be possible in Otago, lambing three times in 2 years would be feasible.