There are rats out there with allergies to certain food products, and having too high of protein can cause skin problems, and internal organ problems, and I think urinary tract problems.
I don't know the specifics, but it's best to get dog food that has no more than 18% protein for nursing mothers. I think 21% is too high, and your better off mimicking Mazuri or Harlan for percentages of protein, fiber, and ingredients.

I could be massively wrong about this, perhaps Becky will chime in on the specifics.