Tolkamp et al. (1996) reported that dairy cows were able to choose between complete foods with different MP:ME ratios, and it was concluded that the selection the cows made was not random. Diet selection may remove nutritional constraints on milk production if the cow can choose the appropriate protein level to attain the milk yield determined by her genotype. At a specific DUP content the diet of an individual cow will be correctly balanced. When the cow is offered two foods, one in excess of predicted requirements for DUP, the other deficient in DUP for the predicted level of production, then the cow has the opportunity to select a diet which exactly meets her requirements for DUP. Most diet selection experiments have been completed under conditions of little or no competition; the aim of the experiment was to study the effects of offering lactating cows a choice between two protein concentrates under semi-commercial conditions.