Success in any system of breeding requires that each generation approach more nearly the object in view than has all preceding generations. Such progress is possible only when proper judgment is exercised and the best methods employed. In cattle breeding, for economic reasons, the bull is usually expected to contribute more to this general improvement than is the cow. To make this possible, the bull must therefore have been subjected to more rigid selection than the cow in order to eliminate more of his undesirable characters. If he contains few undesirable characters, from the breeding standpoint, he must of necessity transmit more of the good to his offspring.
Selecting dairy bulls by performance is fast coming to be recognized as the only reliable method. By performance in this sense is meant the ability of the bull to endow his daughters with powers of high milk production. While authorities are now fairly well agreed that selection by performance is more certain to accomplish the desired results, no definite standard of measuring performance has as yet been generally adopted.
Carroll, W. E., "Bulletin No. 153 - Selecting Dairy Bulls by Performance" (1917). UAES Bulletins. Paper 119.