What’s more sustainable than an animal that helps to grow its own feed?
That’s exactly what a dairy cow does. The farmer feeds the cow. The cow makes manure (poops). The manure is then fertilizer for the fields in which the farmer grows feed like corn and hay for the cow.
The dairy cow eats that hay and corn along with several by-products of human consumables, like wheat middlings from making flour or brewer’s grains from making beer. Southern cows might eat orange pulp or cotton hulls. From that, she makes delicious, nutritious milk full of nine essential nutrients!