I don't know his horse, So I'm speaking in very general terms.
But the high calcium content of alfalfa contributes to a heavy lathery type of sweat. It's one of the reasons endurance riders avoid feeding a diet mainly of alfalfa. It also causes the horse to pee more. Thus requiring the horse to drink more water to stay hydrated.
Now alfalfa is great during the endurance race when the horses need a recharge of calcium to help with the muscle trigger.
You can read more about equine nutrition for distance horses at: Susan E. Garlinghouse, DVM horse management articles and lectures