As the other posters have said - let the other horses in there. If the grass is tall and you have the equipment to cut it down I would do that first then put your horses in it. They will eat most of the grass but they'll trample some of the long stuff down while they're doing it so that will probably 'pop up' later. Depending on the size of the area and your budget you could follow this up with an environmentally and animal friendly weed killer. Around my place, we use a killer we make ourselves (vinegar, salt and a dab of dish soap - the vinegar kills the plant above ground, the salt makes the soil temporarily alkaline and kills the root and the dish soap (just a dab) acts as an adherent for the vinegar); it's not as potent as the real chemical stuff but works well for killing off unwanted plants in the short term.
Last edited by Chevaux; 06-19-2013 at 12:24 AM.