The sweating your horse exhibits is probably related more to his temperament and anxiety associated with performance. Some horses that get very anxious before or during a competition will sweat more than others. He may not physically show you his anxiety in any other way than sweating. Physiologically, it will not hurt him as long as he is fed a balanced diet that has been properly formulated for electrolytes and salt. Also make sure he drinks plenty of fresh, clean water.
