Effect of Short-Term Progestogen Treatment on Induction of Estrus and Lambing in Anestrous Ewes

Two-hundred-fifty-six crossbred and Targhee ewes which lambed in February and March were randomly assigned to a 38- or 73-day postpartum weaning group and within weaning group to a control, an 8- or a 16-day progestogen treatment. Treatment was an intravaginal progestogen pessary for 7 days after weaning or for 15 days beginning 8 days before weaning with a 10 mg injection of progesterone on the day of pessary removal, followed by injections of 750 IU PMS on Days 1 and 16 after pessary removal. Ewes treated with progestogen and PMS showed a marked increase in estrous response and lambing rate when compared with control anestrous ewes. During a 34-day breeding period, 96% of the progestogen- and PMS-treated ewes exhibited estrus but only 23% of the control ewes were marked. After the first PMS injection, the difference in percentage of estrous response between ewes treated with progestogens for 8 or 16 days was slight (80 vs 89%). Anestrous ewes treated with progestogen and PMS also showed a marked increase in lambing rate when compared with control ewes. Lambing rate from matings after the first PMS injection averaged 38%, whereas lambing rate from matings after the second PMS injection averaged 66%. However, a reduction in length of progestogen treatment from 16 to 8 days did not increase or decrease lambing rates. Average lambing rate for all progestogen- and PMS-treated ewes was 63%; that for control ewes was 5%. Early weaning and short progestogen treatments separately or in combination may be advantageous from a management standpoint.