With modern factory farming that is true, but older subsistence style farming is different. Cows eat grass and weeds, I however can't eat those. Chickens can be fed scraps and inedible seeds and bugs, I can eat some of that but not all of it and chickens will actively look for their own food if possible requiring little work on my part. Pigs will eat literal shit and rotten food like it was a jelly doughnut.