As a rule, the animals from which meat is derived do not eat vegetables.
(...) livestock consume 6 billion tonnes of feed (dry matter) annually – including one third of global cereal production – of which 86% is made of materials that are currently not eaten by humans [1]
In particular, much of meat consumed by us is that of ruminants, which depend on eating foods which must be fermented internally to be assimilated, such as grass.
The global livestock sector ingested an estimated 6.0 billion tonnes of feed (DM) in 2010. The three major feed materials were grass and leaves (46% or 2.7 million tonnes, Fig. 2 and Table SI 2 in Supplementary Information), followed by crop residues such as straws, stover or sugar-cane tops (19% or 1.1 billion tonnes DM). At global level, human-edible feed materials represented about 14% of the global livestock feed ration. [1]
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.
