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Ovulation Rate and Subsequent Lamb Production in the Nulliparous and Primiparous Ewe

U. S. Department of Agriculture, Dubois, Idaho and Utah State University, Logan^,2

Abstract

In October 145, 18-mo.-old and 188, 30-mo.-old Targhee range ewes were synchronized for estrus. Three or 4 days after estrus the ewes were laparotomized and the number of corpora lutea (CL) observed using a speculum technique. The ewes were mated to fertile rams at the second and third post-treatment estrus. Follow-up rams were with the ewes for an additional 21 days. Those ewes not returning to estrus after ram introduction were laparotomized about 28 days following breeding, CL were counted and pregnancy diagnosed by palpation of the uterus. Ewes which returned to estrus were laparotomized about 28 days after the return estrus and the same observations made.

The intra-ewe correlation (pooled over both ages) between the number of CL observed following first and second, and first and third estrus were 0.25 (P<.01) and 0.29 (P<.01), respectively. Ewes of both age groups having two or more CL at the first estrus had significantly more multiple births as a result of breeding at the second estrus. A higher proportion of multiple ovulating ewes also lambed. Ovulation rate at the ramexposed estrus was highly associated (P<.01) with multiple births in both age groups.

There was a consistent relationship in both age groups between the average number of CL at two estrous cycles and percent of ewes having multiple births 17 mo. later (P<.05 in 30-mo.-old group only). The intra-ewe correlations between ovulation rate and number of multiple births 17 mo. later were 0.21 (P<.05) and 0.27 (P<.01) in the 18-mo.-old and 30-mo.-old groups, respectively. Ninety-five percent of ewes bred to 12 fertile rams in 1966 lambed. Also, 96% of ewes diagnosed pregnant by uterine palpation at about 28 days postbreeding lambed to the observed mating. Ovulation occurred in the right ovary at a slightly higher frequency than in the left ovary (52.9% vs. 47.1%). Observations indicate that the ovary on which a CL is located at one estrous cycle has no influence on which ovary the CL will occur at the following cycle.

¹ The authors gratefully acknowledge the cooperation of the Upjohn Co., Kalamazoo, Michigan in providing the MAP used in this study.

² From the U. S. Sheep Experiment Station, Sheep and Fur Animal Research Branch. Animal Husbandry Research Division. A.R.S. Dubois, Idaho, in cooperation with the University of Idaho and from Utah State University. Published with the approval of the Director of the Utah Agricultural Experiment Station. Logan, as Research. Paper No. 762.