What I have found to be the Best forage crop for chickens and rabbits , not certain for ducks...is ALFALFA. I start my chicks on alfalfa greens when first start pecking around. This conditions them to have a taste for it as adults. I give my flock alfalfa bales, leave the strings on and they devour the
hay. As the bale gets smaller, I cut the strings and snug them up again on the bale and re-tie. Rabbits love alfalfa too. You can
feed "store bought' alfalfa pellets to rabbits, why not a bale? I have weeder geese to clear
irrigation ditches...I give them grass hay in the winter to supplement with the wheat that I place in their water pan.
Of interest? I have found that the "plants" that you give baby chicks and geese to eat, they will develop a taste for as adults. I have a friend who did not give her chicks alfalfa to eat as babies, she tried to give them a bale of alfalfa as adults, they had no idea they could eat it.