Non-return rate (NRR) to artificial insemination (AI) gives a measure of bull fertility early in the mating season. Calving rate can be biased due to culling and is available too late in the season to be useful for detecting sub-fertile bulls. A current limitation, in the New Zealand dairy industry, to calculating the NRR for bulls is that it is largely based on raw averages of binary data (returned or not) and using only qualifying matings. For example, for an 18-24 day NRR, a mating would qualify if the insemination occurred at least 24 days prior to the end of the observation period and this would utilise only about 42% of the available mating information. Currently no account is taken of environmental factors that may affect NRR (e.g. region, breed of cow) and of the number of inseminations for each bull. The variable of interest is the time interval between the mating date for a cow and an end-point. For a cow returning to service during the AI mating period, the end-point is the date of return to service. For non-returns within the mating period, the end-point is the final AI mating date for the herd. The former type of mating record is uncensored while the latter is a censored record in the sense that a possible return event may not be observed.

DL, Johnson

Proceedings of the New Zealand Society of Animal Production, Volume 64, Hamilton, 138-139, 2004
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