Aspects of reproduction in New Zealand dairy herds. 2. Calving interval, breeding period and non-pregnancy rates.

Summary Records for over 28 000 cows in 316 herds serviced by the Bay of Plenty Livestock Improvement Association showed the average calving interval (CI) was 364 days (S.D. = ±31 days). Between-herd variation accounted for less than 1% of the total variation. The CI for cows conceiving to first insemination was 358 days compared to 367 days for conceptions to second inseminations, but the CI for this latter group of cows was significantly influenced by the interval between inseminations. When the between-service interval was 17 days or less the ensuing CI was 352 days, compared to 374 days for a between-service interval of 18 to 24 days. The results indicate that while the high incidence of short-return intervals to first insemination in New Zealand dairy cattle will reduce non-return rates, the factors producing these short-return intervals are not increasing CI's. The estimated intervals from calving to first service varied from 57 to 76 days. Although the A.B. service was used for an average of only 3...