Horse hoof quality
The old adage "no hoof, no horse" sums it up ... Good quality hoof horn is dry, hard, tough and not brittle, soft or spongy. Nutrition, environmental conditions, drugs and trauma can all have an effect on hoof quality.
The hoof wall essentially consists of two layers; the outer insensitive layer that should be dense and tough, with a moisture content of 15-20%. In contrast, the inner sensitive layer has a moisture content of around 45%. Interlocking sheaves of cells hold the two layers together. Blood and lymph vessels provide essential moisture to the inner layer while moisture to the outer layer relies on the process of diffusion which means excellent circulation to the foot is critical.
A hoof that is too dry will be inflexible, have poor shock absorbing function and it may contract and tighten around sensitive tissues. Conversely, cracking and peeling hooves are the result of too much moisture. A horse kept in muddy conditions will have a softened external hoof wall and pressures from within the hoof capsule may cause the foot to flatten and spread out. The mud draws out moisture and oils from the hoof wall and, as it dries out, it attempts to bend and warp but it is prevented from doing so by the stronghold that the inner wall has on it. To release stress cracks develop. Cracks may become packed with dirt and over time they spread upwards. Excessive moisture also alters the natural varnish-like layer that helps control the moisture content of the hoof capsule. So, preventing horses from standing solely on muddy ground will go a long way to prevent excessive outer wall drying and cracking. If you do not have a well drained paddock, consider building up an area of a few square metres by using lime or similar.
Essential amino acids such as DL-methionine and biotin (and other nutrients) have been shown to improve horn growth rates and hardness. There are several products on the market that are useful when supplementation is necessary. Generally, if nutrition is adequate, poor management and genetics may be the cause of poor quality hoof wall.
Conformation is a critical aspect to address as a foot must be "in balance" in order to adequately support all the weight upon it. A well balanced hoof capsule will have a better chance to retain its integrity and will minimise the risk of painful conditions on the structures above it.
If the wall is beginning to crack lanolin, fish oil and other combinations can be applied to the hoof wall in order to restore pliability and to stabilise moisture content. There are a large array of hoof preparations that can help hoof wall integrity, but they are not all created equal, so be sure discuss potential suitable options with your vet.
The above is just a superficial overview of a very complex and important topic. For more information don't hesitate to call your vet.