It could even be harmful. The hot, dry summer has created a lack of adequate pasture forage in some areas. Often in these conditions, weeds dominate. Normally, horses avoid weeds because they aren't as palatable. But when pasture forages are dried-up or overgrazed due to hot w eather, horses may eat weeds.
Some weeds are toxic to horses.
Beef and dairy cattle farmers often plant summer pastures for grazing when other pastures aren't productive in July and August. If it's good for cattle, it's okay for horses, too. Be cautious, especially if the forage is sorghum or a relative. The sorghums, sorghum-hybrids, sudangrass and johnson grasses aren't suited for horses. The reason is that these forages produce cyanide, which can poison a horse.