Fertility Traits in Japanese Black Cows in Farms Classified by Calving Intervals Using Epidemiological Methods.

The calving interval (CI) is an important determinant of reproductive performance in Japanese Black cattle. The present study aimed to compare individual fertility traits among farms classified by CI. In total, 49,780 artificial insemination (AI) records for 7327 cows from 2011 to 2014 were obtained from 622 farms located in Miyazaki, Japan. The farms were classified into three groups (referred to as farm CI groups) using the lower and upper 25 percentiles of farm CI as thresholds. The mean CI of individual cows was 403.5 ± 70.9 days. The interquartile ranges of CI for individual cows were 49 days for short CI farms and 114 days for long CI farms. The interval between calving and the first AI was associated with farm CI group and parity (p < 0.05) but not with calving season and gestation length. The conception rate (CR) at the first AI in cows on long CI farms decreased as the interval from calving to the first AI decreased (p < 0.05). Additionally, no differences in CR among farm CI groups were observed for AI number ≥ 4. In long CI farms, cows with two AI treatments had longer interservice intervals than those for AI values of 3 and ≥ 4 (p < 0.05). In summary, fertility characteristics for long CI farms differed from those of farms with short or middle CI, and standard operating procedures need to be improved to decrease the CI.