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How much are you feeding? If you are feeding Orijen in the same amount you fed for your old food, it can cause digestive upset. You may need to feed much less, depending on what you were feeding before. Were you feeding grain free before? If not, that will also take some adjustment. I would spread the conversion over several weeks or a month.
FWIW I never feed my older dogs Senior foods. One dog ate Orijen Adult or Six Fish till the day she died at 13 1/2.