Reproductive performance of Japanese Black cattle: Association with herd size, season, and parity in commercial cow-calf operations.

The Japanese Black is the most common breed of beef cattle in Japan. However, only limited data are available on the associations of season, parity, and herd size with reproductive performance in Japanese Black cattle. Therefore, the objective of the present study was to determine the associations of these factors with reproductive performance parameters, such as the calving to first service interval (CFSI) and first service conception rate in Japanese Black cattle. Data were collected from 34,763 calvings in 13,186 animals from 826 commercial cow-calf operations in the Miyazaki prefecture, which is located on the south eastern coast of Kyushu, Japan. This region has a temperate climate with warm humid summers and cold winters. All cattle were reared intensively, and the animals were housed in free stalls throughout their lives. The mean number of cows per farm was 18 (range, 1-454). All animals were bred by artificial insemination. Herds were classified into three groups based on size: small (≤10 cows), intermediate (11-50 cows), and large (≥51 cows). The mean (±SD) parity, CFSI, and the first service conception rate were 4.9 ± 2.9, 80.0 ± 46.2 days, and 53.5 ± 49.9%, respectively. Cows that calved in the spring (March to May) and winter (December to February) had the longest CFSI (P < 0.05). The CFSI in first-parity cows was shorter than in cows at parity 7 or higher (P < 0.05). Cows in large herds had an approximately 10 days shorter mean CFSI than those in small herds (P < 0.05). Cows inseminated in the winter or spring had an approximately 5% points lesser first-service conception rate (FSCR) than those inseminated during the summer (June to August) or autumn (September to November; P < 0.05). As parity increased from 1 to 9, FSCR decreased from 60.0% to 43.1% (P < 0.05). Cows in small herds had a lesser FSCR than those in intermediate and large herds (P < 0.05). In summary, decreased reproductive performance in intensively reared Japanese Black cattle was associated with calving and artificial insemination during the winter and spring, greater parity, and small herd size.