Response of Breeds to Exogenous Hormones in Ewes

Three experiments were conducted, using 1,488 ewes during seasonal anestrous and during the normal estrous period, to evaluate the influence of breed on response to exogenous hormones. Treatment was intravaginal, progestogen pessaries Days 1 to 16, 10 mg crystalline progesterone on Day 16, followed by 750 IU PMS on Day 17. In the anestrous ewes a second PMS injection was given on Day 32 in one experiment and in one experiment the progestogen followed by a single injection of PMS was repeated 10 days after the first PMS injection. Breeds included were RambouiUet, Targhee, Hampshire, Dorset, Suffolk, Corriedale, Coarse Wool and Finnsheep-cross (1/2). Parameters studied were ovulation rate, mating data and lambing performance. Breed influenced ( P < .05) response to exogenous hormones during the normal breeding season and during seasonal anestrous. There was also a significant breed by level of PMS interaction on ovulation rate and there was a good indication that a breed by treatment interaction exists for percentage ewes lambing and percentage lambs born per ewe lambing. Treatment of anestrous ewes with progestogens for 15 days followed by 750 IU PMS 1 and 16 days later in one experiment resulted in 63% of the ewes lambing with 1.01 Iambs born per ewe exposed to rams. A similar treatment the following year resulted in 26% of the ewes lambing with 0.41 Iambs born per ewe exposed to rams.