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 of water. Watercress from different locations can have different contents. I suspect some chickens will like it and others not depending on what the plant has pulled into it.
I plant it in an artificial water feature to detox the water. It has helped reduce the excessive nutrient level that was causing all kinds of moss issues. I have since planted water lilies, water hawthorn and duckweed. My stream and pond is now in balance. I see all kinds of water creatures in it as well. It is as healthy as a natural stream. Since the cress grows so fast, I pull out huge plants all of the time and start new.
I am just getting to the point where I would trust human and animal consumption of the cress, so that I would check out here to see who had experience with feeding it to their chickens. What results fed back here are not surprising. I could imagine it going both ways. I did not know, however, if there was a basic ingredient in the plant that could be harmful. That does not appear to be the case if some do eat it. I don't think it really matters what my experience will be, but if my chickens do NOT eat it, I will not.
I will try to let you know how it went though.
I also suggest your consulting with your local county extension office to know local ordinances regarding the plant as in many locations it is classed as a "noxious weed".
