It's a natural question. Sick animals are culled from the herd. They may be either processed or sold to another producer who is willing to medicate, but my program is focused on building resistant genetics. You can look into "South poll cattle" [1] for an example of this type of breeding program. I don't raise cattle at this time, but I will be soon, and I'll be using an aggressive culling system like what was used for that breed with good results. For more information about this methodology, check out the Lasater Philosophy of Cattle Raising [2]. In my case, I try to apply the same basic ideas to sheep (and eventually cattle).

[1] http://www.southpoll.com/History.html

[2] http://www.amazon.com/The-Lasater-Philosophy-Cattle-Raising/...