Exposing pets to a wide range of foods actually makes life more difficult if they ever develop allergies. Allergies can develop to any antigen that the pet has been exposed to... and treatment of food allergies requires feeding a novel diet. If a pet has eaten everything under the sun, it's really hard to find a completely novel diet. A dog who has been eating the same food it's whole life? Super-easy to find a food with different protein sources.
I have no way of knowing this, perhaps you have a better idea, but what is the likelihood of a dog developing an allergy to every food it's ever been exposed to, when it's been exposed to a huge variety of food, verses a dog developing a sensitivity/allergy when it's been overexposed to the same thing it's entire life? I honestly don't know where to find information regarding this, but I would lean towards overexposure causing a more likely problem than rotation.
Of course, a person or pet can be/or develop allergies to anything at anytime, but I find the idea that I should have my pet on one food their entire lives, just in case they might possibly develop an allergy at some point in their life far reaching. Perhaps my logic is false, but I do my best based on research available to me. I'm always glad to learn new things and share information.
I can see how my manner of feeding, protein rotation and a variety of foods could make it difficult to track if any of the food had issues/recalled/source of a problem should there be one. I've definitely given that some thought. Weighed pros and cons.
It's always nice to hear from a veterinarian. There's been a lot of negativity spread about in regards to veterinary nutrition training and the link with vet schools nutrition classes being funded and even taught by Science diet/Royal Canin from text books they themselves wrote/endorsed and give to the students. Can you weigh in on this? Your experience? You never hear about it from the vets themselves, just the researchers on the outside and the media. And we all know how that goes.
I agree with you on the byproducts. My only issue with that is when the protein source is not indicated. I hate those really cheap foods and their "meat meals" and unidentified, "animal fats." I think the problem is that often you find byproduct meal in foods with other less desirable ingredients. I do definitely take issue with the advertising Blue Buffalo and ProPlan have done in regards to "real chicken being the first ingredient," so misleading!
And my experience is definitely anecdotal regarding my dog's seizures. But for him changing his diet to reduced grain, keeping it slightly higher fat, resulted in a dramatic reduction in seizures for him, but it's hardly a scientific study. No more Zonisamide.
I have seen that the grain free can also be higher in carbs (you ever notice a ton of foods don't even put the carb percentage on the bag? You have to calculate it yourself!). I discovered this was due to the extrusion process. They have to get the dough to a certain consistency to use the extruders. However some foods are dehydrated/freeze dried/ or baked and can have higher amounts of meat, and be grain free without using giant piles of potatoes/peas but most grain free kibbles are higher in carbs. It's amazing the amount of foods available now, the labeling, the assertions of the foods.
Thank you for weighing in and sharing your knowledge!