X. Seasonal trends in fertility and in the distribution of return intervals: Differences between deep-frozen and liquid semen

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Abstract
Insemination records from 557 herds were classified into: cows detected in oestrus before or during afternoon milking but inseminated next morning (p.m. cows); and cows detected in oestrus before or during morning milking but inseminated the same morning (a.m. cows). There were more a.m. cows than p.m. cows (53.7% v. 46.3%), frequency of a.m. cows being greatest during the peak of the seasonal breeding programme. Daily conception rate (C.R. = 49-day non-return rate) increased linearly from 1 October to 14 December, mainly because of reduction in frequency of short (1–17 days) and long (25–49 days) return intervals. On any particular date frequency of short and long return intervals was not related to semen fertility. The consistently lower C.R. of a.m. cows was due mainly to insemination of more cows in early oestrus, to re-insemination of more cows on consecutive days, and to a higher frequency of errors in oestrus detection. The apparently higher C.R.s or inseminations with semen from recognised beef sires arose partly because of the semen's greater use after the main breeding period when the incidence of short return intervals was reduced. Incidence of short return intervals was less with deep-frozen than with liquid semen (4.9% v. 7.2%), possibly because the higher cost of frozen semen encouraged farmers to identify oestrus cows more carefully. When results were adjusted by eliminating short return intervals, the major difference between deep-frozen and liquid semen was in C.R. of a.m. cows (63.6% and 66.5% respectively).

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