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Effect of Season of Lambing on Postpartum Ovulation, Conception and Follicular Development of Four Fat-Tailed Iranian Breeds of Sheep¹

Pahlavi University², Shiraz, Iran

Abstract

A total of eighty-seven ewes from four breeds, Karakul, Mehraban, Naeini and Bakhtiari, were randomized within breed into two groups for mating in the spring and fall. Of these, 76 ewes lambed and were used in this experiment. Ewes lambing either in the fall or spring were laparotomized on day 23 postpartum and if they had not ovulated at this observation they were laparotomized a second time 21 days later. In both seasons of lambing all ewes were exposed to fertile rams 48 hr after lambing until 100 days from the date the last ewe lambed.

Thirty-eight ewes lambing October 16 to December 9, and 38 ewes lambing April 4 to April 29 were allotted to the fall-lambing and spring-lambing groups, respectively. There were no significant differences among breeds for date of lambing, percentage of ewes conceiving after lambing and interval between lambings. Duration of lambing, starting from the first lambing date, was longer (P<.05) in the fall-lambed ewes than in the spring-lambed ewes (32.26 vs 12.80 days). There was a nonsignificant increase in the percentage of ewes conceiving following lambing in the fall than in the spring (72 vs 50%). The interval between lambings was shorter (P<.05) in fall-lambed than spring-lambed ewes (187.98 vs 251.00 days).

Greater numbers (P<.05) of follicles were observed in spring-lambed ewes as compared to fall-lambed ewes.

No breed differences were found in the incidence of postpartum ovulation. The percentage of ewes which had ovulated by day 23 after lambing was higher (P<.05) for fall-lambed ewes (80 vs 0%). Since none of the spring-lambed ewes in any breed had ovulated by the time of the first laparotomy, a second laparotomy was performed 21 days later. Only one of the Karakul ewes had ovulated by the second laparotomy.

Uterine involution as judged by the length of the post-gravid uterine horn and the diameter of the uterine body was completed earlier (P<.05) in spring-lambed than fall-lambed ewes. No significant breed x season interaction was found for any of the characteristics studied except for the duration of lambing and number of follicles with a diameter of >10 millimeters.

¹ Supported by Pahlavi University Agricultural Research Center.

² Department of Animal Science.