I give corn as a treat during nice weather and when it is cold and grass is thin I mix some with their feed as a supplement. Usually the mix is about a half cup to a pound of feed.
First of all I love your avatar picture. Now, congratulations on being smitten by geese. It is something happens and, I believe, makes us better for it.
Geese are, by nature, primarily grazers. They will get their primary nutrition by grazing so long as there is grass available to them...