Well I didn't put cabbage in there, but the giant grass stalks we have growing on the property seem to be doing the trick. It's possible my gosling was younger than I thought and just hadn't been growing in her head and neck feathers yet, but they're starting to come in now. It's the only place she didn't seem to lose the baby feathers, while the rest of the feathers came in, but now they're coming in, and the barren spots SEEM like they might be trying to regrow. I guess if the grass doesn't continue to work, then I'll try a cabbage or lettuce.
 
And is there any kind of fruit or veggie you CAN'T give a goose or duck, because of it being poisonous to them?