Originally Posted by darkwillow
I will probably get pounded for this, but I have generally found that if a horse has sufficent food, they will leave the poisonous ones alone. Horses have evolved over many years and instinctively "know" which plants are not safe. That is not to say that they shouldn't be removed IMMEDIATELY, but just to tell you not to panic and call the vet if you do find a weed.
Also, please be careful which weed killer you get, as some of them can have harmful chemicals in them, which unfortunately the horse does not "know" they shouldn't eat.
I agree too. When I bought this property, it was so severely neglected and overgrown, and I just turned the horses out in the pasture. They happily ate what was "good" and left the "bad" stuff standing - which gave me a much better idea of where I needed to dig stuff up and get it out of there. After removing the stuff I knew was poisonous, I mowed several times over the summer, and will continue to do so this spring/summer, because on top of the wild hemlock I had growing in my pasture (all dug up and gone now), I also had massive amounts of cockleburrs and thistles - which to my understanding will take YEARS to conquer!