Cows are fed very, very little corn. Corn is not cheaper than grass! Grass is free and widely available in the great American plains. More than 90% of a cow's diet over its lifetime is grass and hay. Less than 10% is grain or non-human edible leftovers from corn stalks, such as corn silage:

https://www.progressiveforage.com/forage-types/silage/what-i... https://beef.unl.edu/cattleproduction/forageconsumed-day