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Fertility Rate and Embyronic Death Loss in Ewes Early in the Breeding Season¹

Kentucky Agricultural Experiment Station

Abstract

In this study one hundred and eighty crossbred yearling ewes were paired as they came in heat and each was inseminated with 0.2 ml. of a split portion of undiluted semen. One ewe from each pair was slaughtered three days after breeding to determine the ovulation rate, the fertilization rate, condition of the ova and the presence of any structural abnormality which would prevent fertilization. The remaining ewes made up the control group and were checked for return to heat and allowed to go to lambing time.

All ewes (90) in the slaughter group had ovulated; the ovulation rate was 1.47 per ewe and 96.2 percent of the ova were recovered. Cleavage was evident in 55.1 percent of the ova. Including both the cleaved and uncleaved ova, 20.5 percent of all ova were classified abnormal by the third day after breeding. Of the cleaved ova, 14.3 percent were abnormal and 28.1 percent of those not showing cleavage were abnormal.

No structural abnormalities of the reproductive tract which would be a barrier to fertilization were found. Insemination with semen of high quality as judged by motility significantly increased the fertilization and lambing percentages but did not affect the embryonic death rate.

Using the ovulation and fertilization rates from the slaughter group as estimates in the control group, lambs born represent 41.7 percent of the ova recovered, and the embryonic death rate was found to be 32.7 percent. The reproductive performance of the ewes as a group may be classified as follows: (1) ova not fertilized, 38.9 percent; (2) death of embryo, 20.0 percent; and (3) lambed, 41.1 percent.

Failure of the ova to become fertilized was the most important factor in accounting for the low lambing rate.

¹ The investigation reported in this paper is in connection with a project of the Kentucky Agricultural Experiment Station and is published by permission of the Director.