We're back & forth between Washington & BC on a regular basis. If we're not sure about the admissibility of something, we simply phone the nearest border crossing. For the longest time, as long as the dog's food is in the original bag (open or not) and the ingredients are clearly marked and don't contain lamb or goat, we're good to go. We've never had the dog's food confiscated.
We often find our fellow Canadians take things for granted. They sometimes fail to realize they are entering a foreign country. We regularly meet people in RV parks whining about how they were "hassled at the border". If you're not sure, just phone and ask.