Invited review: selection on net merit to improve lifetime profit.

Genetic selection has made dairy cows more profit-able producers of milk. Genetic evaluations began with 2 traits measured on a few cows but now include many traits measured on millions of cows. Selection indexes from USDA included yield traits beginning in 1971, productive life and somatic cell score beginning in 1994, conformation traits in 2000, and cow fertility and calving ease in 2003. This latest revision of net merit should result in 2% more progress, worth 5 million dollars/yr nationally, with improved cow health and fitness, but slightly less progress for yield. Fertility and longevity evaluations have similar reliability because cows can have several fertility records, each with lower heritability, compared with one longevity record with higher heritability. Lifetime profit can be estimated more accurately if less heritable traits are evaluated and included instead of ignored. Milk volume has a positive value for fluid use, but a negative value for cheese production. Thus, multiple selection indexes are needed for different markets and production systems. Breeding programs should estimate future rather than current costs and prices. Many other nations have derived selection indexes similar to US net merit.