I don't think it is toxic weeds. Ducks have very delicate taste buds, and just don't eat toxic plants. I've never heard of a duck dying from eating some plants outside. If that would be the case every duck book would have a list of toxic plants in it. Haven't found a single book with that yet, not even a reference on a duck breeders website. I have plenty toxic weeds in my yard and they simply will not touch them. I've seen some sites listed here before, but I quickly realized that they listed what the plants did to humans and applied it to ducks. Absolutely wrong, because they have a complete different way of absorbing nutrients and a different metabolism.
You've said your duck was panting. Check its throat, perhaps something got stuck in it. Coins and and nails are very toxic to ducks. Is that possible? Man it will be hard to figure it out.