Experiments in Denmark and America and studies in this country have shown that the law of diminishing returns applies to the complex relationship between the food intake of the cow and her milk yield, that is, as the food intake of the cow increases, milk yield increases but at a constantly declining rate, so that a higher level of feeding produces a greater additional quantity of milk when the plane of nutrition of the cow is already low. The optimal level of feeding dairy cows for profit cannot therefore be fixed for it must vary with both the cost of the cow’s food and the selling price of milk.