From BIF Guidelines Wiki

Selection is the process of determining which animals will produce the next generation of offspring. Selection can happen at various stages of an animal's life. Selection occurs for both males and females and includes decisions such as bull and semen purchases, within-herd heifer retention or replacement heifer purchases, embryo donor cow selection and cow culling decisions. Many factors can be involved in selection decisions such as conformation, productivity, color patterns, horned, polled or scurred, or other factors.

The progress made through selection is dependent on the genetic variation of the trait, accuracy of selection, selection intensity and generation interval. Traits with greater genetic variation relative to their average are able to realize greater rates of genetic change. The ability to identify superior animals or increased accuracy of selection accelerates the rate of genetic change. Selection intensity refers to the difference in the average genetic merit of the selected population compared to the entire population. Therefore, using a group of sires that average in the top 10% of the population for a trait would result in greater genetic change than using a group of bulls that averaged in the top 20% for that trait. Generation interval refers to the average age of the breeding animals; therefore, if a greater percentage of young animals are retained each year and older animals are culled then generation interval is decreased which will result in greater genetic change.