Present and future uses of selection index methodology in dairy cattle.

Selection indexes have been extensively applied in the estimation of breeding value of dairy cattle for single traits as well as for combinations of traits for selection purposes. Milestones in methodology, such as multiple-trait evaluation procedures by BLUP, (co)variance component estimation, nonlinear models, discounted gene flow, dynamic programming, and international sire evaluations, together with increased computing power and the development of integrated AI and recording schemes, have contributed to efficient implementation of selection indexes and are reviewed in this article. Results of an international survey on evaluation practices and breeding programs are presented, demonstrating wide adoption of index selection for total merit and the need for further applications. Results from a simulation study on the efficiency of index selection for total merit are also presented; when the breeding goal includes, in addition to production traits, functional nonproduction traits such as mastitis resistance and fertility, failure to consider these traits in the selection index decreases efficiency 15 to 25%. Future applications are also discussed in view of advances in the areas of genome mapping, marker detection, and international comparisons. Further research should focus on functional nonproduction traits.