Recently, the USA Today showed a picture of a horse that had died due to the drought in one of the Western states. Horses still get some nutrition from dry forages if it is several inches tall. Dry, short pasture forages are a bigger problem. For one thing, weeds take over.
Some weeds are toxic to horses.
But, horses will eat them if they do not have enough forage. When pasture forages grow normally, horses do not eat toxic plants. Another problem with dry, short pastures is nutrient intake. Feeding hay to horses on dry, short pasture won't be a problem as long as you have an adequate supply for winter. If your hay supply is not sufficient, and your horses are in good body condition now, feed some grain and a small amount of hay.