Alfalfa is higher protein and higher Phosphorus than grass hay in general. Founder is a condition where a horse cannot tolerate high protein levels. This results in irregular hoof growth and laminitis. Alfalfa is very high protein and is not recommended for smaller horses which are prone to founder. Extremely green pastures can generate founder as can too much grain. Alfalfa is fertilized with phosphorus and potassium since it actually generates its own nitrogen. This means that alfalfa is high in P (phosphorus) which binds up calcium in the digestive track. Research has shown that feeding alfalfa to young horses stunts bone development and old horses develop osteoporosis as a result of alfalfa for constant feed. Horses evolved on grass and it is still the best option. If you have to feed alfalfa - add a calcium supplement. There are a number of good forage labs. We use Ward Labs in Nebraska (www.wardlab.com) but there are some closer to you.