Inseminations were made with semen from first and second ejaculates used separately and mixed on a within-sire basis. The average conception rate (CR = percent 49-day non-return rate) for inseminations from first ejaculates did not differ significantly from the CR for second ejaculates (69.7 v. 68.8). The CR for the mixed samples was similar to the average CR predicted from the known CR's for the constituent ejaculates (69.7 v. 69.4). The mixing of semen resulted in a significant reduction in the residual variation for CR from that observed with CR's for first or second ejaculates. This reduction in variation was primarily the consequence of averaging CR differences between individual ejaculates by mixing before final processing. Consequently, experiments in which semen extenders are to be compared should be based on a split-ejaculate technique.
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