@U_Stormcrow mentioned flax too...
From another thread (this is me typing):
Flax is good protein, high fiber, and massive fat. Its amino acid profile is one of the best in the seed family. Unfortunately, its fat content is TEN TIMES the recommended level for poultry. Its about as healthy in quantity as you taking a multivitamin in a stick of butter each day.
Now this is in reference to someone trying to build their own complete feed with store bought flax seed (and others). As a forage, the seeds are so dang tiny, and the chickens have to work hard at them (assuming you haven't simply planted a field of flax, but rather have it mixed amongst other greens), I'm actually positive on the stuff. Only in big quantity do I have concerns with its very high fat levels, just as I would with a field of sunflowers.
I have, in fact, planted flax (with lots of other plants) in my acres of weeds. And I'll be updating that today, soon as I detatch my cell phone from my computer.
Alfalfa, btw, is a tolerable substitute for soy in many ways - its one of the better legumes. Unfortunately, I've had no success with it on my property, and make due with various clovers.