Never once been asked by the CBSA officers about our dog food. Going into the US used to be a real problem for us because dog food had to be unopened and made in the US and not contain lamb or pork. They now allow opened bags of dog food if still in original package which really helps. Do NOT put a small amount in a baggie for a short trip!
The best thing to do is always go to the Canadian & US gov't websites for the current info. on pet foods. The rules are always subject to change. The current info. from the Canadian Food Inspection Agency says:
Travellers may bring into Canada a personal import of pet food (limit of 20 kg), if the import meets all of the following requirements:
The pet food or product must be of United States origin and be commercially packaged.
The pet food or product must be in the possession of the traveller at the time of entry from the U.S.
The animal that will eat the imported product must accompany the traveller at the time of entry.
The imported product is fed only to the animal that accompanied the traveller into Canada.