I know there was some discussion about this among my boxer breeder friends. It seems that there's been a change in thinking. Many now feel that back to back is better, for two litters - maximum three, but two seems to be the standard - and then spay. So that would put the dogs at over two before ever being bred and done and spayed by four or five. It was mentioned that the instance of pyometra was increased in intact females that weren't bred. Kind of a use it or lose it thing. I don't have literature to back that up, just what I remember being said. It was against everything I was ever told to breed back to back, but doing some reading I can see that it may not be the worst thing as long as you only do it for two litters and look at each dog individually. Breeding back to back for the life of the female is wrong, but the way my friend was discussing it I would be okay with.