Oh wow, I didn't know that - Actually, i'd been feeding the local Greylags grapes based on comments from this very forum that they were good for them and they seemed to love them! Looks like it's a tricky road for anyone finding the perfect diet for geese!
Treats should always be treats. Mine get the few weird grapes but not a bunch. My roommate likes eating sweet peppers for snacks and she offers the pepper tops to Golly who takes them. She occasionally offers him dried apple slices and other "healthy" treats too. Georgia is too shy to get any before he's hogged them all unless you toss them right at her feet.