Barley crops are a good counter-example. There are three main purposes: malting, milling & animal feed. A good chunk of the feed barley on the market is barley grown for malting or milling but wasn't good enough for those purposes and is fed to chickens. The alternative to feeding it to chickens is to throw it away. That's hardly environmentally friendly.

Some barley is purposefully grown for feed rather than aiming for the higher price that malting or milling will get. Yields on such variants can be as much as 3 times that of a nice malting or milling variant.

Given that a chicken can convert 1.5 grams of barley protein into 1 gram of meat protein, you can see how it be more environmentally efficient to "eat" barley by running it through a chicken first.