Recovery of ovulation rate in ewes following their removal from an oestrogenic lucerne forage

Abstract

Grazing ewes on lucerne crops that contain elevated coumestrol concentrations before and during their mating period can decrease ovulation rate, which results in fewer multiple birth lambs. In this experiment, the ovulation rate of ewes grazing a moderately oestrogenic (29.1 ± 2.88 mg coumestrol/kg DM) lucerne crop for 6 weeks was compared with those transferred to grass pasture for 1 or 3 weeks, and a Control group on grass throughout. Ordinal regression predicted an exponential relationship with a sharp initial increase in ovulation rate from 0 to 14 days on grass that levelled off when ewes were on grass for durations greater than 3 weeks. Of importance, moderate coumestrol levels in lucerne remained throughout the experiment, and liveweight was unaffected by grazing treatment. This experiment quantified the decrease in risk of impaired lambing performance due to moderately oestrogenic lucerne consumption in relation to time on grass before ovulation. Specifically, removal of ewes from lucerne 2 weeks before ovulation mitigated the risk of decreased lambing performance.