Abstract: Rumen performance and ruminant growth in growing ruminants is studied through rumen morphological studies and growing ruminant developmental studies. Based on the studies, a feed composition containing an effective amount of milk replacer, supplemental feed or both is administered to the ruminant over a growing period to optimize rumen growth. A method of formulating a feed formulation for a ruminant involves studying at least rumen morphology of one ruminant ingesting a diet with an amount of milk replacer, supplemental feed or both, and formulating the feed ration for another ruminant based on the study. A feed composition formulated based on rumen morphological studies includes an effective amount of nutrients to optimize rumen growth in the growing ruminants characterized by one or more of optimized rumen size, rumen papillae density, rumen papillae surface area, rumen papillae histology in the ruminant by twenty-four weeks of age.