My dogs have always been on a rotation diet . 
Actually my Vet thought it was a good idea . 
Of course this is NOT ideal for all dogs but my dogs do great with it .
(Cats ,not so much . They are super picky . Like to eat the same food 

)
I usually rotate  my dogs food every 1 to 3 months . Most brands I have tried they do well on but a couple  have made them itchy . Those are the ones that only last 1 month . 
I research each food and company . I contact by email and if they will not give me the answers I want in writing  I will not buy the food . 
I keep all my research in a file on my laptop. 
I think am going to choose 3 brands that I like best and rotate between those .. 
Currently they are eating SD but it is making them a little itchy so I am trying Annamaet next . Good food and great company ..