Breeding decisions made today will have no impact in the bulk tank for at least three years. If you are not satisfied with a cow's performance, you cannot change her genetic ability as you could change her ration. Your only resource is to sell her and replace her with an animal of improved genetic ability. Consequently, herd breeding objectives must be carefully planned, clearly stated, and rigorously pursued for several years. The dairy herd represents a pool of genes, some desirable, some unprofitable.
Management has some control over what kind of genes flow into and out of a herd.
Gene flow into a herd comes from purchase of replacement animals and from semen purchases. Gene flow out of a herd results from cow culling. Trading cows is expensive. Furthermore, cows with desirable or undesirable combinations of genes cannot be identified with much accuracy. Thus, buying and selling cows has limited impact on changes in the genetic ability of a herd.