I have been feeding my 5 year old mare alfalfa pellets with free choice coastal hay and it has done WONDERS for her. She has put on weight (which is great for her) and is not "hot" at all.
Actually previously she was super impatient, wouldnt stand for more than 5 seconds without pawing or moving around. Now, she is very calm, but not lacking energy.
I was also worried that it would affect her negatively, but so far she has been awesome!