I just think its odd for an animal to get diarrhea from a food 3 weeks into the diet. Its expected to have GI upset immediately, but why so much later???
Hmm. I know you probably won't like this because it smacks a
lot of holistic hocus-pocus, but diarrhea and other odd things coming out of the blue right at twenty-one days sounds a lot like what they call "de-tox." You wouldn't believe what I've seen in dogs right at that point, especially dogs fed diets very heavily dependent on low-quality grains. For crazy bad problems, I usually just rest the GI for a day, then gradually re-introduce the food over a bland diet (something like boiled chicken and rice). I've always had good results and never discovered any true intolerance (besides my own Ben - just lucky there, I guess [8|])
I did have one kitty that was allergic to chicken and went off her food completely when I was trying to feed her a diet of raw chicken and balanced premix [

]. That poor kitty should have been a lesson in a dermatological pathology course - she even had lesions on the inside of her throat. I think her yeast infections were growing yeast infections before we figured out what was wrong with her finally. [

]
Sorry, WAY off topic. Anyway, you can tell whether you are dealing with detox versus a tolerance issue or an overfeeding problem, because (and this is going to sound reaaaallly dumb), detox goes away on its own and the animal suddenly "blooms." Because a lot of times detox almost makes the animal look sick (dull coat, bad body odor or breath, gi problems), the difference is almost invariably startling.
Some you hardly notice this phase, some young animals or ones that are only "stepping up" a teeny bit - they may not even do this at all. I think as many grocery store brands get with the program and offer foods that are less dependent on the grain fractions, that I don't see the severe "detox" as often.
I'll give you an example to show what this can look like at its most extreme. I took in a twelve year old dog and switched him over to raw - I was doing BARF style at the time, so he was going from feed store brand to raw meaty bones and a balanced meal of ground meat and veggies, fruits, and whole eggs with shell, plus various supplements, fish, and yogurt. I started him on cooked oatmeat and rice, with the eggs scrambled, over his kibble, and followed a careful program my mentor laid out for me, in weaning him off kibble entirely, and getting him up to a couple chicken wings, a couple of tablespoons of the veggie mixture, and some yogurt, plus the rice or oatmeal.
It took two weeks to go off the kibble completely, and about three weeks after that I noticed something funny about Greg. He'd been looking pretty good for a twelve year old dog, but suddenly he looked horribly
old. His coat looked ratty, he stank so bad you could smell him across the room, and he even had tear stains. I checked temp, gum color, urine color, etc to make sure we didn't have an emergency on our hands! He was bright eyed and bushy tailed so I hung tough even though I could hardly stand to be in the same room with him, poor guy.
After two days of this, we went to bed but I had trouble sleeping because Greg was making a ton of noise in his crate right outside our door. Suddenly there was a burst of unmistakeable sounds, followed by - horrors - splashing. Ah yes, the sound foster parents love to hear! I rushed out to find that about half the hallway was coated with barf, pee, and you-know-what else, all with that horrible stink. Instead of being half-dead, as you might think, Greg actually looked rather pleased with himself once I got him out of the mess, so I focused on getting the hall and crate cleaned up. I put Greg in the isolation kennel, figuring I'd clean him up in the morning.
The next morning I went out to give Greg his bath, but he came out of the kennel sparkling clean
already. His coat, which had always had that "kennel dog" brittle yellow look, was so white it was almost irridescent, and the black in his coat had a mirror finish, practically. His hair was fluffy, soft, and supple. Before, he had a kennel dog smell to him, until the last couple days when he smelled like lettuce that went bad in the fridge. Now he had NO odor. He bounced out the door and leaped around like a young dog and in fact I was able a few months later to take him to a couple of small sheepdog competitions, and he won! And he never, ever had another problem with the food.
I don't know why this is true, but I can't deny what I've seen over and over in dogs I've brought here (and three cats). I
don't think the theories the holistic experts put forward on this subject, are on the right track, but then, I'm not a vet and some of them are! I think it has less to do with "toxins" and more to do with cells refreshing, possibly the system adjusting to the new probiotic profile in the GI - but again, I'm not a trained professional, just an amateur who feeds a lot of dogs under very demanding conditions. [

]