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Relationship Between Ovulation Rate and Reproductive Performance in Sheep

U. S. Department of Agriculture and Utah State University⁴

Abstract

The relationship of ovulation rate at one estrus to ovulation rate at a second estrus approximately 35 days later was studied in 342 mature ewes by means of laparotomy. Ewes which had one (105 head), two (207 head), three (29 head) and four (2 head) corpora lutea (CL) in early October, 1964 and 1965 averaged 1.65, 1.96, 2.21 and 2.50 CL, respectively, about 35 days later (P<.01). These same ewes were bred before the second observation. The average number of embryos per ewe and per pregnant ewe at approximately 23 to 31 days after breeding in November were 1.13, 1.35, 1.24, 2.00; and 1.45, 1.70, 1.80 and 2.00 for the groups which had one, two, three and four CL per ewe in October, respectively (P<.05). The percent of pregnant ewes, number of embryos per ewe and per pregnant ewe at about 23 to 31 days postbreeding were 69, 80, 88; .68, 1.40, 1.68; 1.00, 1.74 and 1.90, respectively, for the groups which had 1, 2 and 3 ovulations for the estrous cycle at which they were bred (November). One hundred thirty-eight of the ewes studied had an opportunity to lamb during each of the 4 consecutive years for a total of 552 ewe years. These ewes were used to study the relationship of ovulation rate to previous lifetime lamb production. The number of ewes in this group having an average of 1 or less, 1.5, 2.0 and 2.5 or more CL for the two observation periods were 13, 36, 72 and 17, respectively. The numbers of lambs born per ewe for these four groups were 1.23, 1.33, 1.47 and 1.56 (P<.01).

¹ U. S. Sheep Experiment Station, Dubois, Idaho.

² Utah State University, Logan, Utah.

³ MAP used in this study courtesy of The Upjohn Company, Kalamazoo, Michigan.

⁴ From the U. S. Sheep Experiment Station, Sheep and Fur Animal Research Branch, Animal Husbandry Research Division, ARS, 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. 596.