Weak Hoof Horn -
How to Improve Cracked feet and Splits in Horses' Hooves
Horses and ponies with hooves that produce very little growth and a tendency to split, flake and crack have a weak horn which is usually caused by a vitamin imbalance.
A vitamin imbalance can lead to a fault in the horn growing cells that are present in the coronary band of the hoof. This in turn can lead to a failure in the production of normal strong horn.
Weak bonds between the horn and the normal stresses at this point cause this weakened horn to split and crack.
SUPPLEMENTS TO IMPROVE WEAK HOOVES
Biotin has been shown to aid the production of good quality horn in the horse's hoof. It has beneficial effects on weak flaking hoof horn.
Methionine also has a role to play in the maintenance of good hooves. When combined with biotin they have a synergistic beneficial effect on weak, splitting or cracking equine hoof horn.
To improve hoof growth look for an equine feed supplement which contains both biotin and methionine. Your vet may be able to recommend one.
It may take several months of feeding a supplement to your horse before you see the benefits - but it is well worth persevering.
TRIMMING THE HORSE'S FEET
Regular trimming of the hooves by a good farrier also reduces the chance of splitting and cracking of the new horn.
It can take 9 months for a horse's hoof to regrow so do continue with a feed supplement even if you don't see immediate results.
RETAIN MOISTURE IN THE HOOF WITH HOOF OIL OR HOOF DRESSING
Keeping as much moisture as possible in the horse's hoof will also help
to prevent cracks and flaking of the horn. 
Treating the horses feet with hoof oil or hoof dressing will act as a waterproof coating for the hooves.
These hoof dressings retain the moisture already present in the horn - but do not add to it.
It can be beneficial to soak the horses feet in water before applying hoof oil in hot dry weather as this will seal in the maximum amount of water within the feet.
Conversly in the winter when the ground is wet it is better to apply a hoof dressing before turning out as too much water can soften and weaken the hoof.
TREAT WITH HOOF HARDENING PRODUCTS
You can buy products such as Keratex which strengthen weak and cracked
hooves by actually improving the molecular structure of the horn. Using
Keratex the quality of existing hoof can be improved in just a few days.
Obviously, cracks will not just disappear overnight as it takes time to
regrow, but the horn around the cracks will be made stronger.
Your vet may also be able to supply a preparation that will harden hooves.
Racehorses are often treated with such preparations.