We have some pretty great hay in my area and the horses are often also given grains and supplements. They are not short on vitamins or minerals, but it is the ground and the weather and often also their genes which leave some horses' hooves looking not-too-hot.
My mare has good, hard feet...sometimes too hard! They become brittle when the ground dries up after the rains and a little while after. During this time of year I apply plain and simple pine tar, once a week or so, as needed. I've tried so many different hoof oils and moisturizers and none have compared to this...even the ones with pine tar bases! The good news is this stuff is usually even on the cheaper end. Who knew?
It is very messy and smelly, but it's worth it. Softens them right up, just enough. They don't crack anymore.
If your horse is eating well or you do not control the situation, I recommend that and potentially also a hoof supplement. Farrier's Formula is the best of all by a mile, but it can be a bank breaker. It does tend to be worth it, though. Just remember the supplements take a longer amount of time to show a difference, because they only affect new hoof growth. I believe it takes roughly a year for a hoof to grow out?