A model to investigate the selection practiced among dairy cows was presented. The level of random culling and the efficiency of the rational selections were estimated employing this model. The proportions culled for causes unrelated to phenotypic merit for the first to the fourth lactations for selection practiced intra-year-season-herd subclass were 0.12, 0.12, 0.18, and 0.22 when yield was expressed as a deviation from herd-mates. The estimates for the efficiency of rational selection as measured by the correlation between yield and an overall measure of merit were 0.64, 0.45, 0.46, and 0.44 for the first four lactations. Graphs based on parameter estimates are presented indicating the selective value of milk yield ignoring other factors.