IV. Differences in the distribution of return intervals between herds

Abstract

Records from some dairy herds, whose owners used the A. B. service provided by the Auckland Livestock Improvement Association in 1973, were used to show how factors influencing a herd's conception rate alter the distribution of return intervals to first insemination. An above average conception rate may be recorded, in spite of an abnormal distribution of return intervals, if the herd owner fails to record many of the returns to service after using A. B. for only 4 weeks or less.

The incorrect submission of a cow for insemination will result either through the misinterpretation of behavioural symptoms associated with oestrus (faulty diagnosis), or because an oestrous cow is incorrectly identified and another non-oestrous animal is presented (animal identification). The distinction between these two types of errors is important in considering recommendations to improve conception rates.

The conception rate distribution for 4342 herds with 10 or more first inseminations showed that 14.9% of the herds had results of 55% or less compared to the Association herd average of 66.7%. Binomial variation accounted for 34.2% of the total between-herd variation. The widespread adoption of improved methods of oestrus diagnosis, such as vasectomised bulls fitted with the chin-ball mating harness or pressure-activated devices, should produce the greatest increases in the Association's average conception rate.