Enteroliths are primarily made up of ammonium, magnesium and phosphorus, which precipitates into mineral layers around a central matrix or nidus, which can be a bit of rock, hair, cloth, anything. The ammonium you get from feeding excess nitrogen, as in alfalfa, and the magnesium generally comes also from feeding Southwest-grown alfalfa, which is very high in Mg. Not all alfalfas are high in Mg, but a great deal of the alfalfa from SW is. The phosphorus comes from feeding grains high in phosphorus and one of the very highest sources is bran, either wheat or rice bran.
AND there is also increasing evidence that hard water IS also a factor---Cal Poly is currently arranging funding to do some studies on the mineral content of water`s influence on enterolith formation. The thought is that it isn`t so much the mineral content as it is the pH influence on the hindgut which in turn favors the precipitation of minerals into enteroliths.
Another factor to throw in is the interaction of the high calcium content of alfalfa---this is sort of a secondary point, but very high levels of cations (these are just atoms with a positive charge attached, as opposed to anions, with a negative charge) also favors the pH of the hindgut to remain alkaline, which in turn favors the formation of enteroliths.
Bottom line is, I can`t think of a finer let`s-grow-some-rocks ration than straight alfalfa and bran.
The suggestion of feeding grass hay is a much better one. First of all, grass hays don`t have the high protein-nitrogen-ammonium content, nor do they have the same high magnesium levels that SW alfalfas do. They don`t have sky-high calcium levels, so the chances of forming enteroliths is much, much smaller, regardless of the hardness of the water. Since enteroliths are ammonium + magnesium + phosphorus, and by replacing alfalfa with grass you`ve eliminated two out of the three...well, you get the picture.
There is still alot of question about whether you can dissolve enteroliths by feeding cider vinegar. Most of the research will agree that acidifying the gut to below 6.5 will favor the dissolving of enteroliths, but whether a cup or so of a weak vinegar will do that is still under question. You CAN, however, shift the hindgut pH by feeding grains instead of brans, and grass hays instead of alfalfa. I bought a horse a year ago that had been on straight alfalfa and wheat bran his whole life, and although he had never colicked, I suspected he had enteroliths. I put him onto an acidic diet of grass hay, beet pulp and grain, hoping whatever stones he might have would dissolve.
Unfortunately, in my particular case (and this was really a freak occurance), while the enteroliths were in the process of dissolving, one fell apart and a sharp edge lacerated the small colon and caused peritonitis. He was in the hospital for two weeks and unfortunately, we ended up losing him to renal failure, BUT this is in no ways an indication that you shouldn`t do everything possible to feed an acidic vs alkaline diet to try and dissolve whatever stones you may have brewing in there. Every surgeon we had consulting on the case agreed that it was just a freak thing that one of the enteroliths broke with a sharp edge and was in just the perfect position to injure the intestinal wall. We also extracted a number of other enteroliths and it was obvious all of the others were just dissolving away like a sugar cube. So putting a horse onto a grass hay and grain diet WILL favor enteroliths dissolution, and despite my own experience with enteroliths, I`d still much rather have lost this one horse than be feeding an alklaine ration and risk enteroliths in ALL of my horses.
One more point that might explain why your vet suggested alfalfa and bran, while another suggested grass hay. This is absolutely not a slam in any way on DVMs and their education, but the simple truth is that the amount of nutritional education vet students get in vet school is almost non-existent...usually just one semester to cover all species, and that just doesn`t do it. Again, no flames here, it`s just that there is SO much material that the students have to learn in four short years that the schools have really been forced to eliminate a great deal of management education, and if a student didn`t get that education elsewhere, such as in undergrad as an Animal Science major, then they just don`t get it. Some vets already have the knowledge through their own experience (like Heidi Smith), some get it post-grad, some vets (like Sarah Ralston) go on and do a residency in veterinary nutrition, and those DVMs are excellent sources of nutrition information. One more time, no slams on your vet, but it sounds like either he`s a little short on nutrition information, or at least on recent nutrition information, because alfalfa (assuming SW) and wheat bran, especially in a hard water area is by no stretch of the imagination a very, very, very bad idea.