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Dairy cattle (also called dairy cows or milk cows) are cattle cows bred for the ability to produce large quantities of milk, from which dairy products are made. Dairy cows generally are of the species ''Bos taurus''.〔http://www4.uwsp.edu/biology/facilities/vertebrates/Mammals%20of%20Wisconsin/Bos%20taurus/Bos%20taurus%20page.htm〕 Historically, there was little distinction between dairy cattle and beef cattle, with the same stock often being used for both meat and milk production. Today, the bovine industry is more specialized and most dairy cattle have been bred to produce large volumes of milk. The United States dairy herd produced 83.9 billion kg (185 billion lbs) of milk in 2007, up from 52.6 billion kg (116 billion lbs) in 1950,〔.〕 yet there only about 9 million cows on U.S. dairy farms—about 13 million fewer than there were in 1950.〔 == Management == Dairy cows may be found either in herds on dairy farms where dairy farmers own, manage, care for, and collect milk from them, or on commercial farms. Herd sizes vary around the world depending on landholding culture and social structure. Dairy cow herds in the United States range in size from small farms of a dozen animals to large herds of more than 15,000.〔James M. McDonald ''et al''., "(Changes in the Size and Location of U.S. Dairy Farms )," ''Profits, Costs and the Changing Structure of Dairy Farming, ERR-47'' Sept. 2007.〕 The United Kingdom dairy herd overall has nearly 1.5 million cows, with about 100 head reported on an average farm.〔"(Milk and Milk Products, UK Dairy Industry )," Department for Environment Food and Rural Affairs 3 Sept 2010.〕 In New Zealand, the average herd has more than 375 cows, while in Australia, there are approximately 220 cows in the average herd.〔"(New Zealand Dairy Statistics 2009-2010 )," Dairy NZ 2010.〕〔"(Industry Statistics )," Dairy Australia. Retrieved 5 Jan. 2011.〕 To maintain lactation, a dairy cow must be bred and produce calves. Depending on market conditions, the cow may be bred with a "dairy bull" or a "beef bull." Female calves (heifers) with dairy breeding may be kept as replacement cows for the dairy herd. If a replacement cow turns out to be a substandard producer of milk, she then goes to market and can be slaughtered for beef. Male calves can either be used later as a breeding bull or sold and used for veal or beef. Dairy farmers usually begin breeding or artificially inseminating heifers around 13 months of age.〔Department of Animal Science," (Cattle Teaching and Research Center )," Michigan State University 4 Nov. 2010.〕 A cow's gestation period is approximately nine months.〔Dewey, T. and J. Ng., "''(Bos taurus ),''" Animal Diversity Web, 2001. Retrieved January 13, 2011.〕 Newborn calves are removed from their mothers quickly, usually within three days, as the mother/calf bond intensifies over time and delayed separation can cause extreme stress on both cow and calf.〔Dale More ''et al''., "(Calf Housing and Environments Series )," ''Veterinary Medicine Extension'' Dec. 2010.〕 Domestic cows can live to 20 years, however those raised for dairy rarely live that long, as the average cow is removed from the dairy herd around age four and marketed for beef.〔 In 2014, approximately 9.5% of the cattle slaughtered in the U.S. were culled dairy cows: cows that can no longer be seen as an economic asset to the dairy farm. These animals may be sold due to reproductive problems or common diseases of milk cows such as mastitis and lameness.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dairy cattle」の詳細全文を読む スポンサード リンク
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