Different modes of using the information on the α_s1 casein genotype of goats, in order to estimate breeding values and to select sires, are compared through Montecarlo simulations. The criterion used for the comparisons was the breeding value reached after 20 generations of selection. Two traits, differently affected by the α_s1 casein genotype, were considered. Results showed that genotyping all animals could be meaningful only in the case of low polygenic heritability and very strong effect of the α_s1 casein genotype on the trait.