Simply put, alfalfa hay has higher levels of protein and minerals than most horses need. An average horse, even one who works hard, needs only about 8 to 10 percent protein in his diet. Alfalfa, depending on its age at harvest, contains at least 15 percent and as much as 25 to 30 percent protein. Super-rich alfalfa is usually reserved for dairy cows, which need the extra nutrition to produce milk.
Likewise, alfalfa is a good supplement for growing equine youngsters and pregnant or lactating mares, who need more protein to fuel their higher energy needs.
But giving the average horse too much of this rich forage is asking for trouble. Besides taking in too much protein, which leads to smelly, ammonia-heavy urine, a horse's biochemistry may be adversely affected by alfalfa's high mineral content.
In particular, alfalfa contains high levels of calcium, which can lead to metabolic problems like synchronous diaphragmatic flutter ("thumps"), and magnesium, which in excess has been associated with the production of enteroliths (intestinal "stones").
Given all of this, you don't need to pass up a convenient source of quality alfalfa, but you'll want to exercise caution in incorporating this rich forage into your horse's diet. Making alfalfa a limited part of his overall nutritional mix and balancing it with another type of forage will help keep your horse healthy.