Effectiveness of short-term progestogen primings for the induction of fertile oestrus with eCG in ewes during late seasonal anoestrus

Abstract Two hundred and seventy-seven ewes were used in three experiments during late seasonal anoestrus to evaluate short-term progestogen primings for equine chorionic gonadotropin (eCG)-induced oestrus. In experiment 1, medroxy-progesterone acetate (MAP) sponges were inserted for 6, 9 or 13 days in nulliparous ewes. Percentage of ewes in oestrus (92.3, 85.7 and 96.4%, respectively) and conception rates (66.7, 75.0 and 63.0%, respectively) did not differ among groups (T > 0·05). In experiment 2, MAP sponges were inserted for 1, 2, 3, 6 or 12 days in multiparous ewes. Percentages of ewes in oestrus were higher with 3, 6 or 12 days than with 1 or 2 days of priming (72.2, 93.8 and 87.5% v . 25.0 and 33.3% respectively, P < 0.05) and maximum response was with 6-day priming. From both experiments we conclude that short-term progestogen (6 days) priming is as effective as long-term traditional primings for inducing oestrus. In experiment 3, level of progestogen priming was compared among treatments using a new progesterone releasing device (CIDR) for 6 days, a 6-day used CIDR for 6 days, or an 11-day used CIDR for 6 days. While percentages of oestrous response were similar among groups (95.9, 93.6 and 88.9%, respectively), pregnancy rates were higher with a new CIDR than with one that had been used for 11 days (57.1% v . 27.8%, P < 0.05), with the 6-day used CIDR being intermediate (51.1%). We conclude that a 6-day progestogen priming combined with eCG can be used successfully to induce fertile oestrus in seasonal anoestrous ewes if progestogen levels during priming are kept sufficiently high.