Abstract
Holstein cow records of milk production, fertility and management traits between 1979 and 2004 from Livestock Improvement Association of Japan were used. The herds were divided into three classes (high, middle and low levels) by the ranking of milk production in Hokkaido and the other area. High, middle and low classes consist of top, middle and bottom 20% of herds, respectively. Furthermore, the herds in each milk production class were divided into three classes (high, middle and low levels) by the ranking of calving number. Data on 18 groups containing 3,646,839 records of 2,191,204 cows (24,742 herds) were analyzed. The herd size annually increased. The increment of herd size in the class of high milk yield and low calving times was remarkably large in recent years. The amounts of milk productions were annually increased and those were higher in Hokkaido than in the other area (P < 0.01). The calving number reduced gradually from 1984 to 1988 and kept constant after 1997. That was larger in Hokkaido than in the other area (P < 0.01). In Hokkaido, the calving number in high class of milk yield were lower than in the other classes of milk yield in each level of calving number. The age at first calving became young with time and that was older in Hokkaido than in the other area (P < 0.01). In Hokkaido, the age at first calving was brought forward as milk yield was higher, or calving number was larger. However, these tendency was not shown in the other area. The calving interval increased after 1986 and that in Hokkaido was shorter than in the other area (P < 0.01). In the area other than Hokkaido, the calving interval decreased with milk yield, or calving number. These tendency was not shown in Hokkaido. The milking length increased with calving number. And that length was shorter in Hokkaido than other area (P < 0.01), and that increase after 1993. The body weight increased with time and that was heavier in Hokkaido than in the other prefecture (P < 0.01). The body weight increased as milk yield was higher. The amount of concentrate feed increased with time and that was smaller in Hokkaido than in the other area (P < 0.01). The amount of concentrate feed was larger in the higher class of milk yield, or the lower class of calving number. The increment of milk yield may cause the increase in calving interval. However, many other factors would affect to the calving number. The annual increment of calving interval seems to be caused by the increment of the milk production rather than the levels of milk production. In the high class of calving number, it seems that the cows would be not only good performance of reproduction but also small trouble in the milking period from the reduction of the risk of metabolic disease by reduction of the amount of the concentrate feed. These results suggested that the calving number and the fertility could be controlled by the development of the management technique.
