Somewhere along the way, someone said "protein is good"....and no one bothered to check to see if there is such a thing as "protein toxicity". THERE IS !! I will never ever again feed alfalfa. Been there, done that, and had WAY too many health issues. Since going to 100% grass hay, my horses have never been healthier.
Alfalfa can be linked to kidney failure in horses. It is also a primary culprit in such problems as cushings, founder, hypothyroidism, etc.
There is way too much protein in good alfalfa for a horse's kidneys to get rid of. If you see your horse has darker urine (it should be a VERY pale, almost clear yellow and have almost no ammonia smell), it is because there is too much protein in its diet.
I prefer bermuda, timothy or orchard grass. I have my hay tested. I do not want it any higher than 16% protein. Good alfalfa can run 25% or more protein!! That's a killer for horses. As far as I'm concerned, good alfalfa should be fed to dairy cows....and maybe some beef cows that are lactating. But I would never feed it to horses.
Here is a quote from only one of many on-line articles about the inadvisability of feeding alfalfa to equine (just "Google" the words "alfalfa" and "hypothyroidism" and you'll get a ton of hits):
"Feeding alfalfa can cause: hypothyroidism high calcium causes lower absorption of iodine which causes lower thyroid function. (Cresty necks) Tying up- feeding higher then what a horse needs in protein decreases the thyroid function. Too much calcium causes magnesium levels to decrease which leads to lactic acid build up in muscles. kidney problems - ammonia smell from urine is not good. excess protein from alfalfa can clog the kidneys over time. The calcium can turn into kidney stones. Lowers the immune system- too much protein and low in fiber diets lead stressed horses to get sick easier. Enteroliths- this is more common in CA and other western states where diets are high in alfalfa. Excessive protein in alfalfa causes ammonium, magnesium, and phosphate to come together to form enteroliths. Changes the development of bones- too much calcium throws off the absorption of copper and zinc. Copper and zinc are very important for healthy bones. Colic- alfalfa is too low in fiber to keep the digestive system happy. Arthritis- protein digestion causes acid. feeding alfalfa causes more acid in the stomach. The body pulls minerals from muscle and bones to help get the acid levels down. This causes build up of calcium on joints which leads to arthritis."