I sure hope you can find what this is. and get it corrected. How is she doing this morning? Is she able to hold her head normal? That is one sign of a toxin "limber neck"
Might not be a bad idea to flush them all but only problem is figuring out who is actually drinking the activated charcoal...
7 acres is a lot of land to walk looking for what might be a toxin but if she will drink you could try a flush with the Activated charcoal.
Do you have any standing water on your property that maybe contaminated?
You might want to walk your property just to rule out they are getting into...