I am so sorry about the loss of your horses.
You also need to contact your extension agent and ask him about any plants/grasses/weeds that become toxic when the weather is in severe drought, (or rain too for that matter) as many growing things change their chemical makeup when the weather is far from normal.
Somewhere I read about just this, and believe it too was in Southwest? But the plants were not toxic when the weather was normal for that area, but during extreme prolonged drought became poisonous.
But the USDA extension office could tell you, AND call the best university in your state and ask to speak to the botany professor and describe what is happening. IF they are any good, they might be able to help. And the Vet department too at the school.
Make sure too, to contact the mill that feed came from as they could be running chicken feed through before horse feed, and that will colic and kill them too, if even a tiny bit of some of the ingredients gets put into the horse feed.
And report this to the CDC, as I think they are the ones who keep a rundown of pet food recalls and medications.
But type in search engine "animal food recalls" and try "drought makes plants deadly to livestock" or words to that effect and see what comes up.
I will try and find the book that I read about the plants killing livestock during drought, if I can. Older book, and will have to search through my horsebooks, but will look.