Here there are native flowers that can kill a human.
Your state poison control can tell you what native plants are toxic.
It is a long shot since only one breed was affected.
My personal experience was a hen that got into a small fairy garden and ate blue star. It turns out every portion of that plant is super toxic.
Yours recently getting to be outside are likely doing some mowing of their own.
I am really sorry for your losses. I can only also suggest contacting your states agriculture department about having a necropsy done.
http://vetmed.illinois.edu/vet-resources/veterinary-diagnostic-laboratory/anatomic-pathology/
Looks like this is the place.
Your state poison control can tell you what native plants are toxic.
It is a long shot since only one breed was affected.
My personal experience was a hen that got into a small fairy garden and ate blue star. It turns out every portion of that plant is super toxic.
Yours recently getting to be outside are likely doing some mowing of their own.
I am really sorry for your losses. I can only also suggest contacting your states agriculture department about having a necropsy done.
http://vetmed.illinois.edu/vet-resources/veterinary-diagnostic-laboratory/anatomic-pathology/
Looks like this is the place.