That depends entirely on where you are.

In India, for instance, dairy cattle are fed almost exclusively on crop residues and by-products. Crop residues being what's left over in the field after you harvest and by-products being what would otherwise be waste left over after you process for human use.

Elsewhere, in addition to crop residues/by-products, you also have natural grasslands that aren't planted or irrigated, legume feed grown between major crop seasons when you can't grow anything else that also replenishes the soil and feed grown on otherwise marginal land or barely managed land.

Certainly some crops grown for cows would be edible by humans or the land repurposed for growing crops edible for people, but there's often a cost involved like heavier fertilizer requirements, pesticide use, water requirements, added infrastructure and/or labor.