Regarding heat are you talking about the temp?
Horses are hind gut digesters. They use bacterial fermentation, which generates heat. Hence why you always feed more hay in the winter.
Alfalfa is a low Heat Increment feed (HI). When you have the same amount of volume in grass hay and Alfalfa hay, Alfalfa will have more digestible energy than grass. Meaning, you can feed less of it. So yes, Alfalfa is a good high temp climate feed.
Corn is also a good high climate feed. It also has a low HI and high digestible energy.
Hope this helps.