Ovulation rates of synchronised Coopworth ewes over the peak of the breeding season

Abstract

Changes in ovulation rates over the peak of the 1988 breeding season were studied at Invermay Agricultural Centre (45°61′S). Two hundred Coopworth ewes (≈ 60 kg) were synchronised by insertion of controlled internal progesterone release devices in mid March and ovulation rates recorded by laparoscopy at four successive cycles following device removal. The ewes were run outdoors on ryegrass/white clover pastures with harnessed vasectomised rams for the first three cycles and harnessed entire rams for the final cycle. Ewe liveweights were recorded after an overnight fast at the beginning of synchronisation treatment and after each laparoscopic observation. Ewes weighed 62.0 kg (SE = 0.48) at the start of the experiment and liveweights were maintained between 61.4 and 63.7 kg over the experimental period. There was a marked change (P<0.05) in ovulation rate over the experimental period, but ewes maintained their peak ovulation rate over two cycles in April/May. The proportion of ewes with an ovulation rate of 2 was similar at all observations, but there were twice as many ovulations of 1 and only half as many ovulations of 3 at the first and last observations compared with the two peak observations (P <0.05). The results indicate that fluctuating ovulations did not occur at the peak of the breeding season in synchronised ewes in contrast to previous observations in unsynchronised ewes.

Keywords:

• ewes
• Coopworth
• synchronised
• breeding season
• ovulation rate
• liveweight
• sheep