I've been reading a lot of posts on both here and the duck boards where people are feeding their geese/ducks spinach. Please stop! A better green for them to eat is kale!
Spinach has something in it called oxalic acid. It depletes the calcium in bones of all birds. Feeding it in moderation is okay but unfortunately moderation to a goose or duck really doesn't apply because they like to gobble down greens.
Kale is a much better substitute to feed. I feed all my birds...parrots, ducks, geese, chickens, and guineas...kale on an almost daily basis.
I grow white clover in 6 pack soda flats (think the cardboard under a case of soda) for my ducks and geese. They absolutely love it. I always make sure that I have some growing and put a flat of it into each pen daily. You can do the same with kale as well.
Its really easy to provide fresh greens to your waterfowl if you think about it!
So please, lay off the spinach and think kale! BTW. Walmart sells 3lb bags of cut up kale for $3.86. No muss, no fuss, its already cut up for you.