Truncation selection requires that each bull used in a breeding program exceed specified minimums for each of the several traits considered by a breeder. To alter the importance of traits, the breeder must adjust the lowest acceptable level of each trait. The major weakness of this approach is that there is no consideration given to the bull that is outstanding for all but one trait. Real inefficiency results when the bull is barely below standards for that one trait. Survivors of truncation selection often are bulls without strengths or weaknesses.
Index selection is the second approach to choosing sires.
Under this approach, different weights are applied to each trait considered important to dairy producers. Variation in traits is equalized through the application of weights. The numbers representing the weighted traits are added to produce a single number. This number is used to rank the bulls, and the higher ranking bulls are selected as sires. This approach allows strength in an important trait to offset weakness in some other trait and is more efficient than truncation selection.