Great question. I'm not organic - I lost 3 out of 25 in 10 weeks (week 1, week 6 and week 7).
That was my first batch of meaties.
I gradually increase the scratch / additional corn to 50% of their feed for the last 10 days. Got a nice layer of fat (which 1 person did not like)
Maybe there are other considerations to the losses (and your success) - are yours raised in a pen or tractor? Is there something you do to encourage activity?
May I should train my dogs to do this.
. (disclaimer - I would never really do that intentionally)
But honestly - "enrichment" might make a difference .
My broilers start out in a refrigerator box in the garage. I follow the program of decreasing temps each week, but probably err on the side of cooler than recommended - maybe 90 degrees max. the first week, 85 the second, and 75 to 80 the third week. Generally right at the 3-week mark I can no longer stand the stink and dust in there, and move them out to their mini-coop and run. They generally aren't "fully feathered" yet but they seem to appreciate the fresh air and cool earth to lie on.
I've got a fenced run about 10' X 20' which is a sparse grass lawn with sandy soil. They have a 4' X 4' by 4' high wooden coop which is built on a concrete slab beneath a covered carport (in the shade). The coop has ventilation around the upper edge, and a larger panel cut out and covered with hardware cloth, which is about 1' X 4'. I use pine needles for bedding, as they're fluffy, airy, and FREE!
They move around and pick at the grass all day but do not "scratch" with their feet like layers. The males spend half their days bumping chests and challenging one another! I use an old section of gutter as a feeder (nice and long, no fighting allowed!) and raise it up so they can't lie down and eat. Our weather here in northern Wisconsin at broiler-raising time is generally 60's and 70's in the daytime, with a few warmer days mixed in, and anywhere from 30's to 60's at night.
Due to the small size of my broiler coop, I can only do 24 or 25 at a time (depending on minimum order at a hatchery). I buy straight run, don't vaccinate the chicks for Marek's or coccidiosis, and they are not debeaked as I've never seen them peck one another like layers do. I've gotten chicks from numerous hatcheries. This year I tried Sunnyside Hatchery in Beaver Dam and got the MOST ACTIVE and robust chickens I've ever had.
One year I did lose 3 chicks during the second week when the garage overheated on a sunny 90 degree day. Had the heat lamp off, doors open, fans blowing, and frozen liter pop bottles in the box, but just couldn't cool them down fast enough. Because of that bad experience, I now start my broilers in late May and move them outside at the 3-week mark, feathers or no feathers! Maybe I have just been lucky, and one of these times will experience the dreaded "keeling over" syndrome.
I buy Nature's Grown organic feeds. First the chick starter, then switch to a 21% protein grower for weeks 3-6, then back off the protein a bit (down to 16% I think) for the last week or two. I mix a tiny amount of DE in their feed for fly control (it works!)
I don't give them ANY people food, scratch, or treats, just all the organic feed they can eat. This is probably the single biggest difference, and I'm sure the cooler temps help alot. I'm so used to the snow and cold, I feel like keeling over on a hot day myself!