Summer in the southern parts of the country can be worlds away from the northern parts, especially in more arid areas. During the hot summer months in these areas horses can stay cooler if fed less forage. Instead feed more high fat and soluble fiber diets. Hay forages have a higher heat increment or heat of fermentation than more soluble starch type feeds like grains or protein supplements. So in the summer months when the temperatures are reaching the 100-degree mark you don't want to create excess internal body heat so reduce the amount of dry forage that a horse eats and replace it with fats, oils or concentrates, such as the newer low starch, high fat and soluble fiber products, that create less internal heat upon digestion. Therefore the heat of fermentation is lower and the horse doesn't have to struggle to rid his body of excess internal heat. Overall, he will be able to regulate his body temperature better. You can take away several pounds of dry hay per day during the summer and replacing the calories with a low starch type feeds or mixtures of oils (usually an omega 3 fatty acid source) as a topical dressing.
In the ‘cooler’ parts of the country, where pastures are more standard, people can get fooled and assume the grass they see is full of nutrition, but it’s easy to get tricked by all that tall grass. Tall, mature grass is actually low in protein, energy, vitamins and all the good stuff horses need (apart from fiber) when it’s tall. The more mature the grass the more stem it has and the less nutritious it is. Two thirds of nutrition in grass is in the leaves so you want to have is a lot of leaves and less stem.
The best way to keep your pastures young is to mow it a couple times during the growing season, which will also keep the invasive weeds down and make the grass more palatable. On the other hand, if your horse gains weight easily, letting the grass grow taller may help keep their weight manageable. The problem here is that the horses will chew some areas short and leave the tall stuff. The parasite load increases because the horses will hang out there, and overgrazed plants will have damaged root growth and be susceptible to dying off.