Grass would make up the bulk of their diet. Note that corn is a grass.

Dairy cows have a diet that is strictly designed by a nutritionist. When cows are fed corn, it isn't just the corn seed, it is the entire plant. Dairy cows get the more expensive premium feed, and soy is not a large part of that.

Their diets are probably evolving but we're the ones changing the environment. Instead of grazing pasture, we feed them with genetically modified (not a bad thing per se, but not naturally available on the environment) and so much washed out that we have to fortify with B12 otherwise they would get sick.