Experimental cattle aided agricultural scientists throughout the nineteenth and twentieth centuries in their efforts to produce beef and milk more efficiently for the growing human populations of the United States. Feed experiments were especially important for understanding how and what cattle needed to eat to better produce this food. However, as experts dedicated their time toward creating the most “economical” rations, their organism of focus shifted. This essay describes how scientific efforts to understand feed conversion in livestock became increasingly focused (...) on the role of ruminant microorganisms over the course of the twentieth century. Highlighting media coverage of fistulated cows and the design of artificial rumens, I argue that the scientific shift from macro- to microorganism was contemporaneously embraced and, in turn, funded by agricultural chemical companies to better market animal feed products by the postwar period. (shrink)