"Some horses do great on alfalfa; others, especially the EMS horse, don't."
Dr Eleanor Kellon
Here's a quick word on alfalfa, mainly sourced courtesy of Dr Kellon.
Millenia years of evolution put horses on dry desert/steppes/tundra lands with a food source of only coarse grass rough-forage fibre. Our modern-day equine gut system is still exactly the same as that wild horse from millions of years ago, and to this day the horse's gut should still only have that same low-nutrient roughage going through its gut system. Alfalfa is anything but this - for starters its a legume, not a grass, it's way too high in nutrients compared to grass forage that the horse doesn't need, and it's not fibrous enough, which is why it's renowned for causing gut sensitivities, which lead to skin and hoof issues.
It’s been implicated in enterolith formation in horses - enteroliths are stones in the intestinal tract composed of primarily magnesium, ammonia and phosphate, with the high ammonia (from protein metabolism by the microbiome) and magnesium coming from alfalfa.
It also has a very high calcium content - the excess is excreted in the urine, which can not only contribute to urinary tract sludge, but also throws the Ca:P ratio way off the ideal balance – it takes over 8g of phosphorus to balance just 1kg of alfalfa.
Equally, many metabolic horses do not tolerate alfalfa well and is said to be a source of ongoing hoof pain for them. Although it generally tests below 10% sugar/starch, the starch percentage is quite high, as are the protein and calcium levels, the latter upsetting the ca:mg:p ratios - it's definitely best avoided for our EMS horses. Agreed some horses have no issues with it, but as it’s an unknown, the very informative ECIR group (Equine Cushings Insulin Resistant) cautions against feeding it.
Finally, it also has a reputation for making some horses ‘hot’ or ‘stressy’ in a behavioural sense.. As if we need that on top of everything else ...