The reproductive performance of naturally mated and artificially inseminated ewes treated with Fecundin at various intervals before the start of breeding

Two experiments were conducted to determine whether an interval longer than 2 wk between the last injection of Fecundin and the start of breeding would improve the reproductive performance of mature Merino ewes. In both experiments the ovulation and twinning rates of the immunized ewes were significantly higher than in the untreated ewes irrespective of whether the interval was 2, 3 or 4 wk. In Experiment 1, ovulation rates (ORs) for untreated and 2 and 4 wk were 1.02, 1.24, and 1.17, respectively (P < 0.001). Percentage lambing ewes with twin lambs for untreated and 2 and 4 wk: 5.8, 21.2, 17.4, respectively (P < 0.05). In Experiment 2, ORs for untreated and 2, 3, and 4 wk were 1.45, 1.92, 1.90, and 1.98, respectively (P < 0.01). Percentage of lambing ewes with twin lambs for untreated and 2, 3, and 4 wk were 36.8, 93.3, 63.0 and 68.6, respectively (P < 0.001). There were significantly more barren ewes when the interval between the last injection and the start of breeding was only 2 wk compared with either the untreated ewes or those injected 4 wk before the start of breeding (Experiment 1, 11.0, 32.5, 11.5% and Experiment 2, 7.4, 20.2, 9.5% for the untreated, 2- or 4-wk groups, respectively). An interval of approximately 4 wk should avoid a depression in the reproductive performances of immunized ewes.