Losing chickens, please help!

I'm hoping someone can help us figure out what is going on with our flock. We began losing a bird every 3 days or so about a month and a half ago. These birds are approx. 4 months old. At first we thought it was the weeds all around our hen house as we had just started letting them free range. We were unaware that most weeds around the farm (nettles, burdock, pig weed, etc) were toxic. We have never had the issue with our older hens. So we sprayed all the weeds and kept them locked in for almost a week. We hadn't lost one in a while so we thought we were in the clear, until we lost a few more, a few days apart. We have now removed them from the hen house and put them in a chicken tractor to somewhat isolate them from other factors we'd been considering including algae in the water. Well, we lost another one today after being in the chicken tractor for 3 days. We have not lost any of our 1 and 2 year old hens, just the younger ones. There are really no symptoms until they begin to act lethargic, their legs don't seem to work, feet are curled under and breathing becomes slower until they die which happens fairly quickly after the symptoms are noticed. The only thing I have noticed in this whole time was this morning there was what looked like one of them was passing blood. And after seeing that this morning, one of the barred rocks is on her way out. Any ideas??

 

Thank you for your responses and dosage suggestions! That was the next thing I was going to try. I'll keep you all posted!