Post the link to that site, I wanna read it. My horses love Johnson grass and it's hard to even get hay around here that doesn't have it in there. I don't like it in hay because it gets too hard and dry when baled and is like just a bunch of sticks. I didn't know it could have adverse side effects.
Here it is:
http://www.horseresource.info/plants.cfm
I was suprised to see Johnson grass on there. The horses I used to have loved the stuff. I never knew it was supposed to be bad for them.