Are you talking about the other side so the horses don't stick their head through??
Hmmmmmm...... that's tricky..what about no chew, I know it is so horses stop chewing wood but you could try to spray it on the grass so the horses don't touch it
Yep Roundup works well. Actually we have found a product from Tractor Supply called Pronto Big n Tuff. In the 2.5 gal size it is about $20 less than Round-up mixes the same and works the same. Around here I call my wife the round up queen as she uses it to control weeds and grass around all her flower beds etc... The only precaution we take when spraying fence rows os to keep the horses away for a couple hours