I hand-cut some long blades of forage grass (very green and juicy), and tried it out. While my ducklings didn't enjoy it as much as spinach leavess or cut up Romaine, they still ate the grass with gusto.
As always, I am the worry wart, but I do know that grass clippings ferment VERY quickly. Being a horse owner, I have heard many stories about people giving their horse grass clippings and having major issues. I, personally, would not do it. I think if you pick grass and feed it to them, ok, but large piles of grass clippings from mowings make me nervous. Do some googling, maybe it is not correct, but horses weigh 1000 lbs, ducks are a bit smaller, stands to reason in my mind that if something can hurt a horse, it can't be great for a duck. JMO. Fermented is fermented, mold is mold, and I, personally choose not to feed things that are questionable to any of my pets. Not worth it to me.
Bugflipper, they DO eat things that are deadly to them. I know of horses that ate maple leaves, and coliced and died. I, personally, suffered heartbreak when two of my ducks ate deadly nightshade plants. My ducks have food and water 24/7. They get peas, greens, treats, etc. Yet, they ate this poisonous plant that was growing on my property. I lost two beautiful girls, at two different times to this horrible plant. They were both rushed to the vet, first one was a mystery,and I did not know what to look for, but with the second one I saw a better vet, and had a necropsy done. I was recovering from surgery and the plant was growing in an area that I could not walk to. But they found it. One ate the leaves and lingered, the other at the berries and went downhill fast. Both were euthanized and I felt horrible. So, they will eat things that are not healthy.