Plain ole green cabbage, never tried the purple. I also tie the bunch of collards together with the bungee and let that hang, they pick at it all day too. Maybe you could try a head of lettuce?
No way could I ever feed my chickens, chicken, that is so gross I could gag thinking about it. I have given them leftover vegetable soup with ground hamburger in it and they went nuts over it. Mine love cabbage to, I hang it on a bungee and they work on it all day. They also like collard...