I. Differences between seasons and between livestock improvement associations

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Abstract
Over the 1968 to 1972 seasons the average conception rate (C. R.) for all six Livestock Improvement Associations varied between 63.5% and 66.4%. The percentage of short return intervals (1 to 17 days) increased from 6.2% to 7.0%, whereas the percentage of long return intervals (25 to 49 days) decreased from 7.8% to 6.4%. The increase in the percentage of short return intervals was primarily due to errors in oestrus diagnosis and animal identification, which produced a rise in the proportion of cows returning to service within 10 days, and only partly due to an increasing incidence of genuine short oestrous cycles of 8 to 10 days' duration. Although the overall percentage of normal return intervals (18 to 24 days) tended to vary, the percentage of 22-to24-day return intervals steadily declined. This probably reflects the effects of improved semen extenders, which increase in vivo sperm livability, thus preferentially raising conception rates (C. R.) among cows inseminated in early oestrus. Conception rate differences between the six Livestock Improvement Associations (60.6% to 67.2%) were greater than seasonal C. R. differences. Detailed analyses for the Wellington-Hawke's Bay and the Taranaki Associations showed that the 1972 C. R. difference of 5.69%, in favour of the latter Association, was the result of (i) fewer errors in oestrus detection and diagnosis, producing a lower percentage of short return intervals (5.78 v. 7. 33); (ii) fewer cows being inseminated in early oestrus, thus reducing the percentage of 22-to 24-day intervals (6.05 v. 7. 97); and (iii) a lower percentage of long return intervals, probably due to the less intensive use of A. B. for a shorter period (6.12 v. 7.81). The C. R. differences for first inseminations between the two Associations increased as the season progressed, as there was a significant linear increase in C. R. (P < 0.0001) recorded by the Taranaki Association from 2 October to 10 December, 1972. This increase was probably due to a progressive increase in the number of unrecorded returns to first insemination inflating the recorded C. R. s. The results demonstrate the value of return interval analyses in comparing results between seasons and between Livestock Improvement Associations.