No dog in this fight, but I , too, breed and raise horses in central FL. I also have a forage-based program.
And I will back up every nutritionist that says “hay alone” is not enough. There is no hay out there that is perfectly balanced for a growing horse or lactating mare. I have fed the rich western alfalfa free choice. And used an appropriate ration balancer to complement that alfalfa hay. I’m currently feeding a gorgeous orchard/alfalfa mix free choice, along with beet pulp and a proper ration balancer. My horses eat an average of 25lbs of excellent hay per head per day. Their solid toplines, healthy guts, and shiny coats are a testament to a successful diet. Their protein and energy needs are met. My weanlings are at a healthy weight with cold joints; and as they grow up to go through the TB sales, the vast majority of them have clean xrays (even the big moose-y ones!).
I think it’s fine to feed unlimited alfalfa if you can properly balance the phosphorus, magnesium, and other necessary nutrients, and if the horses don’t get overweight from excess calories.
Photo of fall weanlings with their unlimited hay buffet. (The filly with the big pointed star went on to sell for six figures as a two year old.)