Your first recourse should be to try adding a spoonful of pure canned pumpkin to each meal. The added fiber will help firm things up. Second, you might want to start adding digestive enzymes to their kibble. Processing kills all the enzymes and some dogs don’t make enough of their own and/or their body doesn’t adjust enzyme production easily when they change foods. Third, no matter what you decide to do add some probiotics back into their system. Anytime you feed the same thing for a long time, their population of probiotics in the gut adjusts to the food supplied. If you change the food, the necessary probiotics to deal with the new food are not there, and may take some time to get re-established. Once you get your dogs settled on the new food, you should consider the next food you are going to switch them to, so they keep that healthy balance of probiotics in their gut. After a few switched, it becomes easy for a healthy dog to switch to any food at any time.
