Some less desirable genes enter the herd through complementary sire selection, but these can be removed by culling females if necessary. Individual sire selection for many traits seldom allows truly superior genes for single traits to enter the herd. Opportunities for substantial progress in one trait are sacrificed to prevent minor deficiencies. Disadvantages of Complementary Sire Selection Two criticisms are frequently leveled at complementary sire selection. The first is that such selection decreases uniformity.
Variation is the key to genetic progress.
If all cows were identical, no selection could take place. First, differences between cows are only partly due to genetics. Sire selection only partly controls half the genetic variation in a herd. The only truly effective means of increasing uniformity in a herd is through female selection. The second frequently mentioned criticism of complementary sire selection is that it decreases the opportunity for corrective matings.