<P> Dairy cows may be found either in herds or 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 . The United States has 9 million cows in 75,000 dairy herds, with an average herd size of 120 cows . The number of small herds is falling rapidly with the 3,100 herds with over 500 cows producing 51% of U.S. milk in 2007 . The United Kingdom dairy herd overall has nearly 1.5 million cows, with about 100 head reported on an average farm . In New Zealand, the average herd has more than 375 cows, while in Australia, there are approximately 220 cows in the average herd . </P> <P> Pasteurization is the process of heating milk to a high enough temperature for a short period of time to kill the microbes in the milk and increase keep time and decrease spoilage time by killing the microbes, decrease the transmission of infection and eliminates enzymes that reduce the quality and shelf life . Pasteurization is either completed at 63 ° C for 30 minutes or a flash pasteurization is completed for 15 seconds at 72 ° C . </P> <P> 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 . A cow's gestation period is approximately nine months . 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 . </P> <P> 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 . </P>

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