<P> A bull calf with high genetic potential may be reared for breeding purposes . It may be kept by a dairy farm as a herd bull, to provide natural breeding for the herd of cows . A bull may service up to 50 or 60 cows during a breeding season . Any more and the sperm count will decline, leading to cows "returning to service" (to be bred again). A herd bull may only stay for one season since over two years old their temperament becomes too unpredictable . </P> <P> Bull calves intended for breeding commonly are bred on specialized dairy breeding farms, not production farms . These farms are the major source of stocks for artificial insemination . </P> <P> The dairy cow will produce large amounts of milk in its lifetime . Production levels peak at around 40 to 60 days after calving . Production declines steadily afterwards until milking is stopped at about 10 months . The cow is "dried off" for about sixty days before calving again . Within a 12 to 14 - month inter-calving cycle, the milking period is about 305 days or 10 months long . Among many variables, certain breeds produce more milk than others within a range of around 6,800 to 17,000 kg (15,000 to 37,500 lbs) of milk per year . The Holstein Friesian is the main breed of dairy cattle in Australia, and said to have the "world's highest" productivity, at 10000L of milk per year . The average for a single dairy cow in the US in 2007 was 9164.4 kg (20,204 lbs) per year, excluding milk consumed by her calves, whereas the same average value for a single cow in Israel was reported in the Philippine press to be 12,240 kg in 2009 . High production cows are more difficult to breed at a two - year interval . Many farms take the view that 24 or even 36 month cycles are more appropriate for this type of cow . </P> <P> Dairy cows may continue to be economically productive for many lactation cycles . In theory a longevity of 10 lactations is possible . The chances of problems arising which may lead to a cow being culled are high, however; the average herd life of US Holstein is today fewer than 3 lactations . This requires more herd replacements to be reared or purchased . Over 90% of all cows are slaughtered for 4 main reasons: </P>

Do cows need to be pregnant to milk