Poison hemlock, cocklebur, Johnsongrass, and common ragweed can all be poisonous to horses under the right conditions. Dr. William Witt of the University of Kentucky discusses these weeds' habitats and how to get rid of them.
Apache is in his prime: Paint Saddlebred gelding; 15 hands about 900#; easy keeper; very spirited; trail/western ridden. No formal training. vax'd June 2013; Regular hoof trimming; current Coggins.