excessive gas is often a sign of incomplete digestion, it could be caused by several things:
- too much food at once (in which case feed smaller portions but more frequently)
- the dog is allergic to something in that particular food or unable to digest it (change the food or experiment with different types of food)
- the dog has some inherent or chronic digestive insufficiency (mine does), in which case adding digestive supplements to the food can help to break down the food so the body can absorb the nutrients more easily.
i'm sure I've missed out a lot too, but those are the only ones I can remember at the moment...
however all dogs (like humans) will have some amount of gas from time to time, you may want to check with your vet if your dog's level of 'gassiness' is considered normal