Abstract
The effects of herd, season, parity and age on reproductive efficiency and fluctuation of reproductive efficiency were evaluated. A total of 783 records of Hokkaido Dairy Herd Testing Association registered Holstein cows of 38 Holstein herds of the Obihiro Animal Relief Association was used. Only cows in which the first artificial insemination after parturition was done in 1979 were included. Reproductive efficiency was expressed by the conception rate at the first A. I. (conception rate), the number of A. I. per conception (no. of A. I.), the interval between the first A. I. and conception (service period), days open and calving interval. 1) Parity had a statistically significant effect on conception rate, no. of A. I. and service period. 2) Days open and calving interval were significantly influenced by herd, parity and age (month). 3) Linear partial regression coefficent of days open and calving interval on age were positive, but these two traits decreased with parity. These indicated that least squares constants of parity for these two traits were biased by the effects of selection or culling. 4) Days open and calving interval may have been shortened by the reduction of the period from parturition to first A. I. in view of herd practice. 5) Season had no effect on all traits.
