The average age at which the dairy cow In the United States either dies, Is butchered, becomes a non·breeder, or for some other reason ceases to be a producer, Is six years. To many this figure may appear low, for cows are sometimes productive at ten or twelve years and even at fifteen or sixteen years and older. These cases of exceptlonal longevity are notable and they appear to be numerous, but the number of cows which die previous to th~lr sixth year Is far greater. Studies of many herds, of both grades and purebreds, under all conditions, thruout the country, confirm the statement that the productive life of the average cow terminates when she 111 six years old. Dairy heifers become producers when two years old and thus their average production period continues for four years. There are In this country approximately 24,000,000 dairy cows; one-fourth, or 6,000,000, drop out of production each year and must he replaced. It Is safe to assume that 18,000,000 calves are born annually; probably one·half of them are bulls and from the remaining 9,000,000 heifer calves must come the two·year old heifers to replace the 6,000,000 discarded cows.
Weaver, Earl; Shaw, J. M.; and Ely, Fordyce, "Raising Dairy Calves" (1924). Circular. Paper 91.