An Analysis of Age Patterns of Dams and their Relationships with Age of Home-bred and Foreigen-bred Sires in Hokkaido Dairy Herds

The age patterns of dams of dairy cows in Hokkaido herds as well as age patterns in mating to home-bred and foreign-bred bull were investigated. The 658, 810 cows used were born in Hokkaido from 1972 to 1983 and their dams were also produced in Hokkaido from 1960 to 1981. The cows were divided into three groups according to the districts where their sires were produced (Hokkaido, the USA and Canada). The dams mated to Hokkaido sires produced the most cows at 3 years of age and tended to decrease in production after that, while the number of cows from dams mated to American sires showed a distribution with a wide peak followed by a relatively mild decrease. Dams mated to Canadian sires showed a pattern peaking at 4 years of age intermediate among the other two groups. The generation intervals from dams to cows were estimated from these results at 4. 6, 6. 1 and 5. 5 years respectively for Hokkaido, American and Canadian sires' groups. The general tendency was that the proportion of cows produced by dams aged 2 and 3 years decreased, but, on the other hand, that of dams aged 5 to 7 years increased year by year. This yearly trend was consistent among the 3 groups. The yearly average ages of dams mated to Hokkaido sires were consistently younger than those of dams mated to American and Canadian sires, though the differences were only 4 to 6 months.