I've been buying feed since 1985 and the ONLY reason that people try feed OTHER than Purina starts with the cost. Purina is ALWAYS more expensive. I've heard complaints about Purina, but many of them prove to be unfounded. For INSTANCE, I was told that Purina dog chow rips up your dog's intestines, but my Vet told me that they have a delicate digestive system and any quick food change will produce digestive upsets and sometimes blood in the stool.
Regarding Purina Equine Senior I saw results on "Corporal," (1982-2009, RIP), and "Ro Go Bar, (1982-2009, RIP) in their old age that the other senior feed products did NOT produce.
Nutrena is probably okay, but they compete directly with Purina, as well as other lesser known feeds. And, it's hard to find it on the road. Purina is available everywhere. Just mho, for what it's worth. =D