I have heard this rumor, and while I categorically say "No" I can understand where it came from. Alfalfa does have very high levels of protein and calcium, but if your horse’s kidneys are functioning normally this is not a problem. The only situation where it might present a problem is in horses with kidney dysfunction. Because of its high protein content, when alfalfa is fed to mature horses, the stall may smell strongly of ammonia. The ammonia is the extra protein that the horse did not need and which it had to get rid of in the urine. The other characteristic of the urine of horse being fed alfalfa is a reddy-whitish color. This is the extra calcium that is being eliminated. These conditions can occur with any feed that is poorly cured or carbohydrate rich. However, alfalfa is a nutrient rich feed and is the hay of choice for young growing horses and lactating mares.