I had absolutely no idea I couldn't travel with pet food. They didn't mention it at my nexus interview either.
OK I looked into it and only certain meats are not allowed into the US and everything is allowed into canada.
Pet foods of Canadian origin (fresh, frozen or chilled, cooked, canned or otherwise processed products) that contain beef, veal, bison, and cervid (e.g. deer, elk, moose, caribou etc.) are now permitted when crossing the border from Canada. However, products containing sheep, lamb, or goat will not be allowed entry. The traveller must provide proof of the origin of beef, pork, poultry, cervid meat, and pet food in order to bring them into the United States. Examples of proof of origin include the grocery store receipt where the product was purchased or the label on the product indicating the province in which it was packaged.
Pet foods that do NOT have these ingredients can be taken across the U.S./Canada border as long as they are in their original packages and the ingredient list can be read clearly. Any pet food in non-labeled packages may be confiscated. These regulations apply to both canned and dry pet foods.
When returning to Canada, you may bring your pet’s food (limit of 20 kg) into Canada, if the import meets all of the following requirements:
The pet food or product must be of United States or Canadian origin and be commercially packaged (i.e. not re-packaged into plastic bags or other containers). The pet food or product must be in the possession of the traveller at the time of entry from the U.S. The pet that will eat the imported product must accompany the traveller at the time of entry. The imported product is fed only to the pet that accompanied the traveller into Canada