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Variations in the Length of Estrus Cycles and the Breeding Season in Ewes

Department of Animal Science, University of Illinois, Urbana

Abstract

A flock of 100 Western ewes was checked for heat daily for three consecutive years. The data showed no tendency of individual ewes to be consistently early or late with regard to the onset of the breeding season. Therefore no progress can be made in sheep of this genetic background in attempting to select for prolonged or continuous breeding seasons.

Of 2,321 cycles, 32.5% were longer than 19 days or shorter than 14 days. The great majority of these "abnormal" cycles was observed very early or very late in the breeding season, when the frequency of abnormal cycles was significantly higher than those of the cycles falling within 14–19 day range.

Ewes which do not come into heat until late August or September tend to have first cycles within the normal range of 14 to 19 days. The earlier a ewe terminates the breeding season the less likely she is to have terminal cycles of abnormal length.

¹ Data from Ph.D. thesis of senior author.