It's not just metabolic issues - I think that would probably take a while to surface from that sort of feeding. Horses are 'trickle feeders' and their GI tract is designed to have small amounts going through it near constantly. Unlike our stomach, for eg, which is built more for infrequent meals and releases stomach acids when we eat, gastric juices are flowing through the horse's stomach constantly. Without constant food to digest and without the 'buffer' of saliva from long fibrous feed that requires a fair bit of chewing, stomach acid is free to cause damage to gut lining, ulcers and colic. That is one reason racehorses and other intensively kept and fed horses so commonly suffer these problems... & windsucking that goes with them. Laminitis is another potential which can come from toxins in the blood that leak from the damaged GI tract.
Alfalfa is generally a pretty good feed for horses *as part of a balanced diet*. It is very high in calcium, protein & a range of other nutrients, which can lead to problems when fed too much. Upon nutritionists advice I wouldn't feed more than about 1/3 of a horse's ration in alfalfa/lucerne, to 2/3 hay.