I would stay away from dog food if you can, at all cost. When I used to breed rats I had the same situation with multiple random deaths occurring weekly until my whole colony was dead. I had a world renowned veterinary pathologist perform a necropsy and the majority of the dead animals had massive organ failure, including liver and kidneys.

Rodent block that is used in laboratories is a much better option, and does not cost anymore than dog food. All of the rats in our facility thrive on it, breeding prolifically.