Association of interservice interval with conception rate in Japanese Black cattle.

This study aimed to clarify the association of interservice interval (ISI) with conception rate (CR) and to evaluate factors affecting ISI in Japanese Black cattle. Data used in this study covered 32,639 artificial insemination (AI) records on 972 farms. The mean ± SEM of ISI and CR was 57.0 ± 0.3 days and 44.6 ± 0.3%, respectively. The relative frequency of ISI differed significantly between parity groups (p < .05). For parity 0, the proportion of ISI at an 18- to 24-day interval was 43.6%, whereas the proportions in the other parity groups ranged from 27.2% to 29.1%. The CR was associated significantly with ISI, parity and AI number (p < .05), but not with season. Cows reinseminated at an 18- to 24-day interval had the highest CR (46.8%; p < .05). Cows reinseminated at a 39- to 45-day interval had a similar CR to those reinseminated at 25- 38-day and 46- to 59-day intervals. Cows reinseminated at 11-17, 60-66 and 67 days or longer intervals had the lowest CR (p < .05). Thus, cows returning to oestrus in a normal cycle tended to have a higher probability of conceiving compared with those that returned to oestrus after a normal cycle for any degree of parity and at any number of AI.