Dr. Paul Crosson:

Except that live weight is an important part of the index in terms of the cost of the cow. If we look across the index, it can be divided the cow traits and the calf traits. The cow traits account for approximately 60% of the total value within the index and calf traits account for approximately 40%, within the replacement index. That is why there is the terminal index; to balance out the terminal traits you want in the progeny.