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Some Factors Affecting Response of Anestrous Ewes to Hormone Treatment¹

U.S. Department of Agriculture, Dubois 83423 and Oregon State University, Corvallis 97331

Abstract

Several experimental, range and farm flocks were used to study the effects of various environmental factors on response of the anestrous ewe to hormone therapy. Lactating ewes started on estrogen, progesterone and repeat PMS treatment beginning 18 to 24 days post-partum were significantly more fertile than ewes started on treatment 2 to 8 days post-partum. Ewes which lambed in June following the treatment described above were more fertile than ewes which lambed in February. However, both those lambing in June and February were more fertile than those lambing in April. Anestrous ewes suckling single lambs proved generally to be more fertile than ewes suckling twin lambs. In another study dry ewes (failed to lamb) were more fertile than ewes which lambed but did not raise their lambs. Ewes which lambed but failed to raise their lambs were in turn more fertile than ewes raising singles which in turn more fertile than ewes raising twins. A treatment including an intramuscular injection of 2 mg estradiol-17ß at the time of progesterone implantation resulted in an apparent improvement in fertility in one flock and a significant improvement in fertility in another flock over other hormone treated ewes not receiving this estrogen pretreatment. Estradiol-17ß (.025 mg) reduced fertility at the first estrus when it was given at the same time as the first PMS. The overall treatment effect did not, however, differ from other treatments. Young early post-partum ewes (1, 2 or 3 years old) whether lactating or nonlactating were significantly more fertile following hormone therapy than mature ewes of the same status. Dorset x Targhee crossbred ewes appeared to be more fertile following hormone therapy than Rambouillet, Targhee or Columbia ewes of the same ages. The response of Suffolk ewes to hormone treatment was significantly lower than other breeds studied in the same flock. Hormone treated lactating anestrous ewes given a high level of nutrition appeared to be more fertile than those on a standard level of nutrition.

¹ The authors wish to acknowledge the technical assistance and cooperation of H. D. Frederiksen, H. D. Willes, H. Hollandsworth, D. Sperry, S. K. McIlvanie and Don Gnos.

The authors also wish to thank the Abbott Laboratories, North Chicago, Illinois for supplying the Progesterone-Silicone Elastomer Implants used in these studies.

² The U.S. Sheep Experiment Station, Sheep and Fur Animal Research Branch, Animal Science Research Division, A.R.S., Dubois, Idaho in cooperation with the University of Idaho, Moscow, Idaho.

³ Department of Animal Science, Corvallis, Oregon.