Consider reducing the fiber content of the diet. Be careful, because too little fiber causes digestive problems and lowers fat test. Use very high quality forages. High quality forages are more palatable and will help maintain intake during hot weather. Consider a dietary buffer such as sodium bicarbonate.
Buffers help to maintain intake as well as fat test, especially in low fiber diets.
Add fat to the diet. Fat has a high energy density and will help maintain energy intake during reduced feed intake. Whole cottonseed, soybeans and commercial fat products are options. However, high moisture feeds spoil rapidly, so clean feed bunks daily. Feed cows frequently to stimulate intake and minimize heat load from digestion of a large meal.