If they eat some ditch weed, and it is just an incidental part of their diet, I would not worry. Disclaimer: we do not have it here, so I am only inferring that since it is in the hemp family that ingestion of nominal amounts does no harm.
Too much of even very good things can cause problems. Fresh greens are good to have.
Up here, I like that the ducks enjoy chickweed, smartweed, and some jewelweed.