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OESTRUS, LH, OVULATION AND FERTILITY IN BEEF HEIFERS

University of Nebraska, and U.S. Department of Agriculture, Clay Center, Nebraska 68933, U.S.A.

(Received 18th November 1974)

The pregnancy rate resulting from matings at a single oestrus in beef cattle averages 70% for natural mating (Turman, Laster, Renbarger & Stephens, 1971) and 60% for artificial insemination (Laster, Glimp & Gregory, 1972). The sources and causes of low pregnancy rates have not been defined, but might include variation in time of ovulation relative to deposition of spermatozoa in the female reproductive tract, fertilization failure, ovum transport, abnormal ova and blastocyst development, and implantation failure. The interval from onset of oestrus to ovulation in dairy cattle has been reported to average 29±6 hr with a range from 16 to 42 hr (Swanson & Hafs, 1971). Such a large interval could account for a high percentage of the losses.

This study included two experiments. The purpose of Exp. 1 was to determine the

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