Records for 6012 inseminations of cows were analysed to determine the effect of the stage of oestrus at the time of insemination on conception rates for three groups of sires. All inseminations were completed between 8.30 hr and 11.30 hr using 2·5 million total spermatozoa in 0·5 ml of ‘Caprogen’ extender containing catalase. The designation of the stage of oestrus was based on the milking or milkings relative to the time of insemination at which each cow was observed in oestrus. Data from cows returning to service less than 18 days after insemination were excluded.
The average conception rates (18 to 49 day% non-return rate) for the three groups of sires were 72·9% (7 sires of above average fertility), 70·5% (5 sires of average fertility), and 68·3% (6 sires of below average fertility). With inseminations in early oestrus, the comparable results were 74·3, 62·7 and 58·4% respectively. In contrast, the results for the three groups of sires with post-oestrous inseminations were 73·3, 71·13 and 73·8% respectively.
The average interval from insemination to ovulation and fertilization is related to the stage of oestrus at insemination. This interval was the major factor contributing to the fertility differences between the groups of sires included in this experiment. These differences were also reflected in the percentage and distribution of normal (18 to 24 day) return intervals.