When i bring in a mustang to train i keep him incarcerated in a run-in barn w/corral. Invariably, within a month said horse will get ‘that smell’. At this point in their life, they usually are not very hoof-treatable and it will take me a couple/three weeks to get to the hooves.
What i use on their feet (like OP, almost always the front feet) is 50% iodine 10% solution and 50% turpentine. I use a plastic catsup squeeze bottle.
I’ve felt that my environment has waaay more bacteria than the desert from which they came. When they’re in pasture it almost never happens…but on dry lot (damp lot here) it sure gets to a lot of them.