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Effects of Season and Breed of Sire on Incidence of Estrus and Ovulation Rate in Sheep^1,2,3,

Oregon State University, Corvallis 97331

Abstract

The effects of season and breed of sire on estrous activity and ovulation rate were estimated in 50 mature ewes, 10 each sired by North Country Cheviot, Dorset, Finnsheep, Romney and Suffolk rams and born from Suffolk dams. Estrus was checked with vasectomized rams from July 1979 through December 1980. From May through December of 1980, ovulation rate was determined by the counting of corpora lutea (CL) during laparoscopic examinations of ovaries. A breed x month interaction (P<.01) was found for incidence of estrus. Finnsheep-sired ewes were slower to enter anestrus and had the shortest anestrus period. Cheviot-sired ewes had the longest anestrus period. Ovulation rate, measured as the number of CL/ewe examined per month from August through December of 1980, was highest (P<.05) in October and November. Finnsheep-sired ewes tended to have higher ovulation rates (2.04) per ewe ovulating than did Romney- or Cheviot-sired ewes (1.48). Suffolk- and Dorset-sired ewes were intermediate. Ovulation rate, measured as the number of CL/ewe available per month from May through December of 1980, was highest (P<.05) in September, October and November. Suffolk-, Finnsheep- and Dorset-sired ewes had higher (P<.10) ovulation rates (.95) per ewe available than did Romney- and Cheviotsired ewes (.67). Monthly repeatability of ovulation rate from August through December of 1980 was .41 ± .25.

¹ Technical Paper No. 5870, Oregon Agr. Exp. Sta. Contribution to North Central Regional Project NC-111, "Increased Efficiency of Lamb Production."

² The authors wish to express their appreciation to the Oregon Sheep Commission for financial support of this research.

³ Dept. of Anim. Sci.

⁴ Present address: Dept. of Anim. Sci., Univ. of Nebraska, Lincoln 68583.

⁵ Present address: Dept. of Anim. Sci., Univ. of Illinois, Urbana 61801.