I've found regular mowing resolves most pasture weed issues the easiest and least harmful. We've had good luck eradicating Canadian thistle and most of our stinging nettles that way. We have a brush hog towed behind a tractor and mowing keeps my husband busy -- but I have also put the riding mower on highest blade and tied up the blade guard so it blows more freely and mowed that way as well.
Will your horse actually eat the nightshade? Our neighbor ran horses and had a huge problem with tansy ragwort, which is toxic to horses, but they wouldn't touch it. The new neighbors inherited a very pretty yellow hillside (the tansy in bloom), but they are diligently mowing so hopefully that will eventually stop it.
The thistle took several years of mowing as the seeds remain viable for a long time. We're pretty much thistle-free now, and also livestock free, so much more mowing needed to keep the fields looking healthy.
