Very true. We've switched our pigs onto Allen and Page's Sow Cakes, which do not contain soya, for this very reason. There is still some wheat, barley and oats in it, along with beans, grass, linseed, molasses, but no soya - and all GM-free. And all our herbivores now get Dengie All Stock Grass Pellets - containing just grass (and not all rye grass, either.). Also GM-free.
And the chickens get the A&P Layers Pellets - also soya-free and GM-free.
I often ask local vegans to tell me how to put together a locally-sustainable vegan diet. I haven't had any offers to date.