I don’t know, but it’s not the food. Many dogs (especially seniors) cannot tolerate the side effects of Rimadyl, such as diarrhea. Sometimes all is fine until the dog has been on it for a while, like a couple of months.
If this is happening you should talk to the vet about changing the medication, there are alternative medications for pain and arthritis.
Rimadyl really can irritate the intestinal tract and colon after the dog has been on it for more than a few weeks. I don’t think changing the food would do anything significant to help..