The present invention includes a method of formulating ruminant feed to selectively change a composition of milk produced by the ruminant. The method includes providing a fistula directly into an abomasum of the ruminant. A selected feed component at a selected level is introduced directly into the abomasum through the ingress. The selected feed component thereby bypasses other portions of the ruminant's stomach. The direct introduction of the selected feed component allows for changing a selected characteristic of the composition of milk. The composition of the milk corresponding to the selected feed component is then analyzed. The direct introduction of the selected feed component is repeated, if necessary, at other selected levels. The milk produced corresponding to the other selected levels is then analyzed. A selected quantity of the rumen inert feed component to be fed orally to the ruminant is determined by utilizing the analysis of the composition of the milk corresponding to the introduction of the selected feed component into the abomasum of the ruminant.