The artificial insemination of the Merino sheep following the synchronization of oestrus and ovulation by progesterone injected alone and with Pregnant Mare Serum Gonadotrophin (PMS)

Two groups (2 and 3), each of 112 stud Merino ewes, which were experiencing regular oestrous cycles, received daily injections of 10 mg progesterone for 16 days, in an attempt to synchronize oestrus and ovulation for artificial insemination. One group (3) received an additional injection of 500 i.u. pregnant mare serum gonadotrophin (PMS) the day after the final progesterone injection. Injections were staggered so as to bring between 15 and 20 ewes of each group into oestrus on each of six days, on which days approximately equal numbers of untreated oestrous ewes were inseminated (group 1). In group 2 (progesterone alone), 104 ewes were teased and inseminated within 4 days of the final injection, including 74 (71 per cent.) on the third day. Corresponding numbers of ewes for group 3 (progesterone-PMS) were 107, 96 (90 per cent.). Injection of PMS significantly advanced and improved the precision of time of onset of oestrus, and presumably of ovulation. Of 315 ewes inseminated in all groups, 265 received 0.10 ml undiluted semen; 33, 0.05 ml; and 17, 0.20 ml. There were no apparent differences in lambing rate. Ewes inseminated, ewes lambing, and lambs born were: group 1, 104, 58 (55 per cent.), 74 (71 per cent.); group 2, 104, 53 (51 per cent.), 64 (62 per cent.); group 3, 107, 61 (57 per cent.), 85 (79 per cent.); overall, 315, 172 (55 per cent.), 223 (71 per cent.). There were no significant differences between groups in the proportion of ewes which lambed, or in the twinning rate. Ewes in oestrus and inseminated 72 hr after the final injection of progesterone had a significantly higher lambing rate than did ewes inseminated a day later. It is concluded that progesterone, particularly if used in conjunction with PMS, is of value in synchronizing oestrus in the breeding season for precisely planned artificial insemination.