II. Differences between sires in their return interval analyses

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Abstract
Records were analysed for 41 sires used by the Auckland Livestock Improvement Association in the 1970 season. Each sire recorded at least 597 first inseminations, made on the day after semen collection and processing, with semen diluted in Caprogen and minimum dose rates of 2.5 million total sperm per insemination. The average conception rate (C. R. = 49-day non-return rate to first insemination) for Hereford sires (67.9) was significantly higher than for Jersey (61.8) or Friesian sires (59.3). There was a significantly higher incidence of short return intervals (1 to 17 days) among Friesian sires (7.6 v. 6.2%). C. R. and % normal returns (18 to 24 days) were negatively correlated (r = —0.98). The percentage of short return intervals was also negatively correlated with C. R. and was — 0.53, whereas between C. R. and long return intervals (25 to 49 days) it was only — 0.26. Correcting C. R. s for the percentage of short and/or long return intervals for individual sires did not alter their relative C. R. ranking. The results support the hypothesis that C. R. differences between sires are primarily the result of variation in fertilisation rates.