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Synchronization of Ewes during Anestrus: Influence of Time of Year and Interval to Onset of Estrus on Conception Rate¹

US Department of Agriculture^2,3,, Clay Center, NE 68933

Abstract

The effects of progestogen-gonadotropin treatment on estrous response, ovulation rate and lambing response of crossbred ewes were examined at two times during the anestrous season (May-June and July-August). Each ewe group was treated with progestogen pessaries for 16 days and injected with 750 IU pregnant mare serum gonadotropin (PMSG) on days 17 and 32 to induce two consecutive estrous periods. Data accumulated from single -sire matings of 500 ewes indicated that estrous, ovulation and lambing responses did not differ (P<.10) between ewes treated in May-June (92%, 2.5, 66%) and ewes treated in July-August (93%, 2.5, 67%). We concluded that there were no significant differences between reproductive responses of ewes synchronized during mid-anestrus (May-June) and those of ewes treated during late anestrus (July-August), For the May-June and July-August groups combined, estrous, ovulatory and lambing response were higher (P<.05) during the first induced estrous period (76%, 3.0, 62%) than during the second (33%, 1.9, 43%). In both the first and second induced estrous periods, as interval from PMSG injection to onset of estrus increased, significant declines in ovulation rate, conception rate and ram marking activity were observed, particularly when onset of estrus occurred 72 hr or more after PMSG injection. These results indicate that the reduced lambing responses reported for ewes synchronized during anestrus may be due to increased asynchrony of reproductive events in the ewe, or decreased ram aggressiveness and fertility, or a combination thereof, as the interval from PMSG injection to the onset of estrus increases.

¹ The authors gratefully acknowledge the technical assistance of Mr. Lee Peshek, Mr. Basil Farber, Mr. Peter Garcia and Ms. Gail Gilstrap with the collection of animal data. Cooperation of the Nebraska Agr. Exp. Sta., Univ. of Nebraska, Lincoln, is acknowledged.

² Roman L. Hruska US Meat Animal Research Center, AR, SEA.

³ Mention of a trade name, proprietary product or specific equipment does not constitute a guarantee or warranty by the USDA and does not imply its approval to the exclusion of other products that may also be suitable.

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