When I googled for excessive urination, all I found was Cushings... and she's way to young for that.
As for the breastfeeding thread.... my pleasure ;~D
I had to research some plants the other day. One in particular comes to mind. Do you have johnson grass around where your horses can get to it?
This is a clip from a site I looked at:
Johnsongrass Tremors: frequent urination or defecation, gasping, convulsions