I have rabbits and I grow one 1020 tray of barley/oat (8/1) fodder each day. I share some of that with hens.
My birds are caged off the ground. We've got coyotes, weasels and raptors all over the place. I feed my hens all of the yard weeds or garden greens they will each eat. Then I give...
Some will some won't. Chickens prefer to dig in most of what they eat. They would have both feet in their feeders if they could get them in it. I have one hen that gobbles down fodder & weeds like mad and the other eats a little and makes a mess with the rest. I used a door mounted fodder...
Remember, watercress is a major "filtering" plant. It typically grows wild and often out of control, pulling not only nutrients out of the water but physically filtering with its root systems but also causing the plant to absorb all kinds of chemicals (toxins?) that might flow through that body...