You said you sprayed your weeds with a weedkiller 2-4-D. That could have poisoned them. But since your new chickens are not dropping, they could have brought in a disease. Sevin should not have bothered them.
Two things I would do: take in some combined droppings in to your vet to test for possible coccidiosis, and any chicken that dies next should be refrigerated in a plastic bag, and sent in for a necropsy. Sorry for your loss. Here is a list of state vets and poultry labs, along with some info on sending a bird:
http://www.metzerfarms.com/PoultryLabs.cfm
Two things I would do: take in some combined droppings in to your vet to test for possible coccidiosis, and any chicken that dies next should be refrigerated in a plastic bag, and sent in for a necropsy. Sorry for your loss. Here is a list of state vets and poultry labs, along with some info on sending a bird:
http://www.metzerfarms.com/PoultryLabs.cfm