Livestock eat quite a bit of corn / maize, which we think of as both a vegetable (fresh) and a grain (dried). I took the original comment to be tongue in cheek

Livestock eat quite a bit of corn / maize, which we think of as both a vegetable (fresh) and a grain (dried). I took the original comment to be tongue in cheek though, and to just mean that eating plants is more efficient than eating plant eaters.