Thanks for all the great replies all!
So, regarding nutrition, he gets much more than hay and hemp oil. As I said, he has a customized diet of supplements and minerals done to complete the nutritional value found in his hay after a complete hay analysis. Specifically, he gets biotin, zinc, copper, vitamin A, vitamin E and selenium (our soil is selenium deficient, but iron rich so he gets no additional iron in anything). I add slippery elm bark (for digestive issues and to help with runny stool in the winter) and hemp oil (for his joints, skin, etc.). And of course salt. These are fed twice a day with beet pulp (molasses free) and a few hay cubes (timothy only) for palatability. His hay and grass are a mix, but predominantly timothy. He is 20, and his metabolic profile suggested he is at risk for becoming IR (though he is not there yet), so I am avoiding anything with sugar or high NSC. He wears a grazing muzzle in the spring and on and off as required in the summer since he is an easy keeper.
I do have stalls, but my horses are mostly on full-time turnout year round. He has had respiratory issues in the past, so good air quality is a high priority. I can stall him, I just choose not to. He has access to his stall, however, and does tend to stay in there a lot when it’s muddy out. My pedding is wood pellets which absorb a lot of the excess moisture. While there is mud for a few weeks in the spring, I wouldn’t say that my paddock is anywhere near the worst I’ve seen. There are always dry areas where they can stand and since he is the dominant horse of the herd (and a bit of a wimp in bad weather), that’s usually where he is.
He is trimmed regularly, but yes, I am learning to rasp between trims so you’re right on the ball with this suggestion. His hoof growth accelerates tremendously this time of year, and he requires much more frequent trimming (last summer we got to the point where I was having the trimmer come very 2-3 weeks). This may contribute to his soreness, however, so it’s a delicate balance. I was timid about rasping between trims at first, but my trimmer has been guiding me, and I am doing a trimming workshop in early June so I will feel more confident about it. Currently, I try to rasp his toes once a week just to stay on top of it. I never take a lot off (no nippers), but I figure this will help keep things more consistently balanced.
Also, I took some time to research it, and was a bit discouraged by the fact that a lot of these products include things like alcohol and acetone as ingredients. I can’t see that doing anything but weakening the hoof. Others contain vaseline, which is also not recommended since it blocks pores.
The crack started at the toe and made its way up about 1/3 of the way. My trimmer did notch it, and it didn’t open up further. It has since completely grown out of course. I was just hoping to avoid a re-occurrence. It sounds like I’m doing all I can and that hoof ointments won’t help much. Maybe just keep a close eye on his hooves, and if I see a crack beginning, nip it in the bud. Any other suggestions?