Hi there, welcome to BYC!
So sorry for your trouble.
Sorry for asking.. but vets sometimes don't have all the answers.. and I'm not sure what YOUR version of "tried everything" is.. Have you tried treating your ground already? Have you reduced your
stock load.. I mean how many birds in how much space? What type of bedding? I have a hard time believing coccidiosis is THAT persistent.. and have to wonder if there's more to the story than meets the eye? Like a hidden disease such as Marek's weakening the immune system and allowing for the coccidia to be an issue.
What is fed on a regular basis including treats and supplements? Are there other animals such as dog, goat, etc that could also be carrying coccidia protozoa which aren't being treated when the chickens are? Were the younger birds new additions or were they hatched there? What type of water dish is being used? Maybe a few photos of your set up and we point out some easy fixes or where any issues might lie for further investigation?
My dead birds go to the dump in my trash can, legally.
Sorry for all my questions.. I highly encourage you not to give up yet since you found us.. and take baby steps.. towards your final outcome, whatever that may be.. But maybe with some solid support from folks with actual coop experience and not from a book will help to restore some of your energy and confidence to keep going!
If it were me, and I had to cull an entire flock.. I'd probably start at the 6 year old or non layers and work my way from least healthy to still strong.
Also.. if it's truly coccidiosis that's the issue.. consider re-homing the birds, with disclosure.. as it CAN be dealt with and then even though you still experience a type of loss it's maybe a little less traumatic than culling via the cone?? Note amprolium does not have a withdrawal time for meat.
Bare ground is a breeding habitat for bad bacteria.. lots of garden debri, like dry leaves, pine needles, grass clippings, large chunk bark, etc.. to cover any bare ground and make it more like the forest floor.. soft, well draining, good bacteria and bugs join the party and balance things out.
Your daughter is brave.. and she rocks! My teenage daughter also lead the way in teaching me how to harvest cockerels.. Very proud of their resilience and the strength of character it takes to not only KNOW that something dies for us to live but also the resolve to move forward knowing every animal in their care lived the best life possible and also dies the best death possible! That's humane.
Here to support you and your family, hope you get some answers!