Fixed formula used to be what made a premium food premium. They used to guarantee that they wouldn’t change the formula without notice, which Science Diet did with the last change, so they are still considered a fixed formula food. I really don’t know who else might have the same policy. Most premium foods don’t switch around the formula with every batch. They make occasional adjustments, so they should be close enough to fixed formula for all but the most sensitive dogs. I don’t know what the issue with Merrick could be, but they shouldn’t be changing their formula that often. Changing the formula takes a lot of work and some expense. But I do wonder if they have a supplier that gets differing qualities of the same ingredient or something like that.
