Days to calving in artificially inseminated cattle

Heritability and cow permanent environmental effect (c²) were estimated for various definitions of fertility for artificially inseminated (AI) cows. The most useful cow fertility trait was found to be the number of days from the date the first cow in the contemporary group was AI'd until each cow calved, with a penalty value for non-calvers. This trait mimics the equivalent trait for natural mating, the first AI date in a contemporary group being treated as the 'bull-in' date. Means and variances of the AI trait depend on how contemporary groups are defined, so various methods of defining contemporary groups are compared and discussed. Estimated heritability of AI days to calving (AIDC) was 5% with c² of 7-8%, broadly similar to current results for the equivalent BREEDPLAN trait for naturally mated cows. AIDC is therefore a potentially useful cow fertility trait; breeders interested in this trait should be encouraged to formally record contemporary groups of cows at mating.