Do not stress to much about cocci always being present. I have similar situation and most of my birds appear very resistant. With vulnerable lines I have delayed their exposure treating them with Corid using prescribed methods about time birds are exposed (released onto cocci infected ground). Corid treatment simply suppresses cocci long enough for birds' immune systems to ramp up and keep it under control. By time treatment ends birds will have some cocci in gut but will not be impacted greatly. I apply treatment via water starting a couple days prior to exposure. Birds with weaker immune systems are culled but usually that repressents on a very small percentage of infected. I am not so certain the more heavily infected birds do not catch up as some I am following after a heavy infection almost resulting in loss are exhibiting compensatory growth where they are still putting on size even after siblings filled out. Still those lagards are culled from breeding flock.