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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±6hr 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 relationship of the onset of standing oestrus, preovulatory LH surge and time of ovulation in beef heifers. The purpose of Exp. 2 was to examine the normality of ovum and blastocyst development within the first 15 days after mating.