VIII. Effects of non-oestrous inseminations and return patterns after second inseminations

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Abstract
The conception rate (C.R. = 49-day % non-return rate) for 863 605 first inseminations was 62.0% compared with 61.5% for 207 884 second inseminations. Most(135 759) second inseminations were made after a period between insemination (p.b.i.) of 18–24 days, a further 52 075 being made after a p.b.i. of 17 days or less. C.R. was 64.0% for the former proup of second inseminations and 55.5% for the latter. This lower C.R. was associated with a higher frequency of short return intervals (1–17 days) after these second inseminations and probably arose through more errors in oestrus diagnosis. Such errors will result in non-oestrous inseminations. Results obtained in one field trial with 1712 cows showed that if the non-oestrous intra-uterine insemination preceded the oestrous insemination, the C.R. for the latter insemination was not affected. In contrast, a second field trial with 65 cows indicated that if the non-oestrous second insemination was made within 3 weeks of the oestrous first insemination, the pregnancy rate to the first insemination was significantly reduced from 53.1% to 24.2%.