Have been reading back on this thread a little. Interesting. Here at my place, if I raise chicks artificially, they will get coccidia. Dirt or no dirt, they will get it, and untreated, some will die. The odd thing is, as virulent as it is, if I natural raise them, I will have zero with coccidia.
I learned the value of using hens instead of incubators and brooders. I can make a mobile pen, and get a hen out past the reach of extension cords. Every place within reach of extension cords there have been chicks raised before, infected with and treated for coccidia. Getting them out on the clean ground and exposed to hens seems to help build up the beneficial bacteria and let them build a slow resistance to the bad bacteria. I think being in groups of twenty or less helps, too.
As an Asil fancier, I have the perfect broody hens for raising all of my egg and meat chickens. There is no comparison when it comes to raising chicks. My hens run around four and a half to five pounds, they can cover a decent amount of eggs. They are bare breasted, they make skin contact directly with the eggs, no insulating layer of feathers keeping their heat away from their eggs. They are long legged, graceful, with a long neck, no clumsy hens breaking eggs. They have tons of instinct, don't foul nests, very good with chicks. While no match for large predators, they are very protective, not an easy meal. (I had one that beat up a skunk last year that dug into the pen with her and her six week old chicks.) They are reliably broody, if you have one that hits six to eight months old, and it's between March and September, she is going to go broody. If it's early, she will go more than once. My best raise three broods a year.
The downside is, that they don't play well with others. Fortunately they are very happy kept in breeding pairs, or alone. They are extremely friendly toward humans, the males are big pets, very uncommon to see human aggression. Females are friendly, too, but when they have chicks they can be a little more hands off. Generally you can turn one loose to range and walk up and pick them up with just a little handling. You don't need many to keep you in chickens, and they have a very long productive life, (not uncommon for ten year olds to still be laying eggs and raising broods.)
I just though I would throw this out there, seems that some people have bad luck trying to get something to go broody, or maybe less than ideal performance out of the ones that do. I have tried most of those others, and usually was glad that I had an incubator for backup. Now my incubator collects dust. And, I have a constant supply of egg layers of staggered age, nice and healthy.