Hum, I feed alfalfa, and while my horse’s horn quality is excellent, she has always been prone to an odd yeasty thrush.
Anyway, I have not heard of poor feet and alfalfa. The excess protein in alalfa can cause them to urinate more, which is not good if they are stalled and standing in it.
Googled a little, and all I could find was some references to an UK study which said:
Research published in the Vet Record (1987) demonstrated that alfalfa increases the quality and quantity of horn growth which was attributed to the highly available calcium and essential amino acids found naturally in alfalfa.
I would ask what degree your farrier has in equine nutrition. Alfalfa fed half and half with grass hay is the perfect mix! Some grass hay can have a higher sugar content than alfalfa..... Insulin resistant horses are often recommended alfalfa by vets to help improve hooves
Did he say your horse had wall separation? Thin walls? what?
I would get a second opinion on the work this guy is doing because more often than not, the farrier work is a high contributor to crappy hooves-along with feed, work, environment.
So is your farrier a nutritionist as well...Alfalfa is high in protein is better with calories and has great calcium to phosphorous balanced for bones...my horse 28 due exceptionally well when feed alfalfa.....doubt it has anything to do with thrush either...you horse genetically is pre disposed to crappy feet and doubt if any supplement or topically applied voodoo at his age is going to help him. I am a real stickler for good feet selling OTTB you gotta be careful with feet....
Instead of blaming the alfalfa, make sure everything is balanced as it should be. You'd be surprised at what you're missing/have too much of when you take a good look at the diet, and what exactly you are feeding! My mares hooves have improved immensely since I went on FeedXl and fiddled with all her feed...I don't have tested hay, either, and I still saw a huge difference!
"On the back of a horse I felt whole, complete, connected to that vital place in the center of me...and the chaos within me found balance."