北海道ホルスタイン検定牛群の泌乳曲線形状の実態とその泌乳・繁殖特性、及び除籍理由

The current shapes of lactation curves and their relationships with other milk production and reproductive traits as well as reasons for culling were investigated in order to determine the characteristics and causes of annual changes in shapes of lactation curves as well as the characteristics due to differences in shapes of lactation curves, and to discuss the concepts of LP (lactation persistency) from cow management practices with respect to the breeding of LP. Data for lactation curves, milk production, reproduction in 1st～3rd lactation Holstein cows and culling information obtained from the Hokkaido Dairy Milk Recording & Testing Association were used for analysis. The data were analyzed for annual changes in 305-d milk yields and shapes of lactation curves for cows born in the period from 1975 to 2009, for shapes of lactation curves and milk production and reproductive traits for each of 3 levels of 305-d yield and LP and 9 levels of their combinations, for sizes of herds on farms and areas of Hokkaido in cows born in 2005～2009, and for culling reasons for levels of herd size of farms and areas of Hokkaido in 2009～2011. The results obtained were as follows. 1. Annual changes in lactation curves of cows born from 1975 to 2009 showed that the LP value (=240 days in milk (DIM) -60 DIM+100) decreased from 1975 with the rate of decrease becoming slower with delay in the peak period (DIM of peak yield) from 1985 and that LP increased as the rises in 305-d milk yield and peak yield became mild from 1995, so that lactation curve tended to be flatter. The extent of flatness of lactation curve was in the order of 1st parity > 2nd parity > 3rd parity. 2. Sperman correlations between 305-d yield and lactation curve traits were in the order of 0.86～0.88 with peak yield > 0.29～0.35 with initial growing yield (=35 DIM-10 DIM) > 0.23～0.31 with peak period > -0.28～0.05 with LP value. 3. Cows with flatter lactation curves, due to lower initial growing yield and peak yield, higher LP value and longer days open, showed greater total milk yields due to longer lactation period and shorter dry period. 4. Cows in large-scale dairy farms tended to be younger at 1st calving and have greater 305-d and total milk yields with shorter days open, lactation and dry periods. Primiparous cows showed the flattest lactation curves and multiparous cows showed lactation curves with higher peaks and less persistency in cows on large-scale dairy farms with over 200 cows. 5. Cows on farms in upland dairy areas such as Ishikari, Tokachi and Abashiri tended to have higher 305-d and total milk yields than those of cows on farms in grassland dairy areas such as Nemuro and Kushiro. Cows in higher areas of farms with free stall barns had shorter days open, higher 305-d and total milk yields and longer peak period (P<0.05), and primiparous cows had the flattest lactation curves with highest LP values (P<0.01) 6. Cows in large-scale dairy farms had lower average and culling calving numbers, higher rates of planned culling, higher incidences of hoof and digestive disorders and lower incidences of mammary disease. From the results obtained, both the flatter lactation curve with lower peak yield and higher LP from the 1990s and the phenotype flatness of lactation curve can be explained by the following genetic and environmental factors : (1) possibility that cows with genetic flatter lactation curves that are suitable for labor-saving herd management were gradually selected, (2) increase in the use of concentrates due to widespread use of total mixed ration and labor-saving herd management, (3) increase in the incidence of reproductive disorders due to longer calving interval such as increases of high-producing cows, (4) record hot summers and longer heat stress periods or increase in the incidence of reproductive disorders caused by heat stress, (5) effects of shortened dry period, and (6) effects of frequent milkings. Cows with flatter lactation curves are thought to be particularly observed in one ration group throughout the lactation period in free-stall housing. The cows thereby tend to have a relatively risk of high BCS (body condition score) in the late stage of lactation because of high energy ration and long calving interval. Therefore, mating with bulls with high LP is recommended. The phenotype of flatter lactation curve might be caused by factors such as delayed conception, heat stress and use of a large amount of concentrates at late lactation. In that case, the BCS are not necessarily levelled. BCS together with total milk yield and LP should be considered as selection traits in terms of feeding and management, corresponding to the breeding of LP with indirect selection of levelled BCS. Analysis of data for cows in herd of different sizes and in different areas suggested that shapes of lactation curves reflect differences in feeding methods. Integration of survey on feeding method and milk records of Dairy Herd Improvement should contribute to the establishent of efficient breeding and management methods.