36 MILK PRODUCTION COST ACCOUNTS 



The total, 19,716, divided by the number of exitants, which 

 is 2,590, gives the average age at exit as 7I years. The aver- 

 age age at entry we found above to be 23 years, so that 

 average life is 4H years. It is probable that if the present 

 standard is maintained the years of usefulness wiU increase as 

 the associations become older and as better care is given the 

 cows. This, however, furnishes a figure which should be very 

 near what may be expected, and if the cost of cows is computed 

 on the basis of depreciation which replaces them every 5 years 

 the herd will be maintained. There are many factors that 

 shorten the economic life of a cow. Death is a small factor, 

 and only accounts for 1.69 per cent in Pennsylvania dairies 

 and 1. 3 1 per cent in Michigan.' Udder troubles, tuberculosis, 

 failure to breed, and accidents are some of the other causes 

 for a shortened period of usefulness in milk production. Some 

 valuable cows which continue to breed are kept after they are 

 not profitable as milk producers, but this number is very 

 small. In the formula below account is not taken of the cows 



* U. S. Dept. of Agr., Professional Papers, Bull. No. 341, p. 65. 



