Are you saying nature determines which dogs live and breed as well as which dogs die (don't breed) in your program? If 90% of your dogs are killed by nature before they are a year old, personally....I see something wrong there. Either poor negligent care or unhealthy animals. Now, if you are saying you cull 90%, well...that isn't natural selection, but artificial selection.
However, I have to ask...are you breeding for guarding (true bandog) abilities or are you breeding for the ability to survive on a farm? A bandog is supposed to be a guard dog. Maybe it is your translation with English, but it sounded as if you are saying you let nature select your stock and I hope that isn't the case...because if so, then you are not really breeding guard dogs or PP dogs.
Do you test them in guarding or PP applications? Do you actually see which dogs will not only bark, but will also bite if needed...and stay in a battle to protect their pack and territory? Or do you just see which ones survive without giving them basic husbandry?