Have to say I hate the idea of systemic insecticides for chickens...that will wind up in my eggs most likely. And, it can't be good for the chickens. I am very protective of their immune systems...don't want them sick.
Good question tho, is there a spray that will keep chickens off, preferably a natural one? I rather doubt it...
I just blocked them out of my garden. In the winter they are allowed in. It is all a matter of proper husbandry IMHO.
My thinking is that they wouldn't even eat it. Of course you wouldn't want the chickens dining on it but you'd think it would be too terrible tasting for them to choose it over grass.