Hi Kate,
In theory, the Grandma Lucy’s pre-mix (once meat is added) should be a complete and balanced meal for your dog. You shouldn’t need to worry about other supplements though adding krill oil or fish oil can be beneficial. You can also add kefir, yogurt, or raw goats milk for probiotics.
Primal’s recommendation of feeding 2-3% of body weight is for raw food which is less calorically dense because of the amount of water in raw food. I don’t think the 2% rule carries over to dehydrated food like GL’s. I would feed the amount of food that keeps your dog in the right weight range. Feeding guidelines are just that…guidelines. Some dogs need more or less than what is suggested. It’s going to be trial and error to see what the right amount for your dog is.
I know finding a food your dog will eat has probably been very frustrating but I would keep at it. It’s best to rotate whatever food you are feeding, in this case the pre-mix. I would suggest rotating between different pre-mixes to make sure all of your bases are covered with vitamins/minerals. Some other pre-mixes you could look into include: The Honest Kitchen Preference, Urban Wolf, See Spot Live Longer Dinner Mix, and Sojos (though I’m not a fan of that one). Also, I would limit the amount of tuna you feed as it can have high mercury levels. Canned sardines in water with no added salt would be a good fish to use and it is naturally high in omega 3s. Canned salmon is also good.
I hope this helps.
