What proportion of the bulls actually in breeding use in the United States are old enough to be proven sires? What proportion of the purebred cows from which the next generation of bulls must come are old enough that their real breeding worth may be judged in considerable part by the qualities of the offspring they have already produced? How many years of breeding use may one expect to get from bulls of various ages? Out of every hundred heifers which come into milk for the first time, how many may be expected still to be breeding at 5 years of age? at 7? at 10? The answers to these questions are but vaguely known at present.

This bulletin reports a study of the age distribution of several breeds and groups of cattle together with discussion of what the observed data mean about the present replacement rate among both cows and bulls and about the use which is being made of the progeny to estimate the real breeding worth of sires and dams. Suggestions are also included as to practical means for making increased use of proven sires.