Although geese prefer a plant-based diet of wheat, grass, or barley, they will eat animals such as slugs and snails.
Most of the time, consumption of slugs and snails is accidental if they are lingering on a goose’s main plant food source.
Slugs and snails are healthy for geese in moderation. They can provide a lot of essential nutrients and minerals that geese will not get from just consuming a plant-based diet.
....Snails and Slugs do contain zinc and copper which can be toxic to geese if they ingest too much.