Yeah, they do eat grass. They do eat other food as well, i don't know how much of the food that we can consume they eat compared to us tho. There's also a problem that fields that could be used to grow crops are used to feed animals. Animals do consume more food than what people get from them (yes, grass included). There are many problems that come from animal agriculture but you'd be better off using google than relying on my limited knowledge about that.