The effect of mid-cycle insemination on subsequent conception rates of cattle

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Abstract
Extract Sir,—The proportion of return intervals after first insemination of less than 18 days' duration is between 16 and 20% in New Zealand dairy cattle (Macmillan, 1970 Macmillan, K. L. 1970. Return intervals to first insemination and conception rates to second insemination in New Zealand cattle. N.Z. Jl agric. Res., 43 in press [Google Scholar] ) whereas the comparable figure in American herds is 3.5% (Moeller and VanDemark, 1951 Moeller, A. N. and Van Demark, N. L. 1951. The relationship of the interval between inseminations to bovine fertility. J. Anim. Sci., 10: 988–992. [Web of Science ®] [Google Scholar] ; Flerchinger and Erb, 1954 Flerchinger, F. H. and Erb, R. E. 1954. Influence of treatment and fertility level of semen on distribution and non-return decline of repeat service intervals. J. Dairy Sci., 37: 949–949. [Crossref], [Web of Science ®] [Google Scholar] ). Since the incidence of these short cycles tends to increase with increasing: herd size, these short return intervals after first insemination may be the result of errors made by the farmer identifying or diagnosing oestrous cows. Such errors will arise because (1) the farmer correctly submits a cow in the first instance but then unnecessarily resubmits the same cow because of incorrect identification or diagnosis; (2) the initial incorrect submission is followed by correct diagnosis and submission; or (3) errors in detection and diagnosis are made on consecutive occasions.