Grass-cracks or something worse? Cracked hooves in horses explained

to see if the foot lands evenly on the ground or not. If one side of the hoof wall lands first it is likely that there is a mediolateral foot imbalance. Sand cracks are not always painful particularly if they are superficial. But they are a good indicator that there is a problem with foot balance. 
Horizontal cracks in the hoof wall are sometimes seen following the rupture of a foot abscess at the coronary band or trauma to the hoof wall. These will usually grow out without too much concern unless they are very large. 
What to do when you notice a crack
The first thing to do is to clean the foot well so that you can inspect the damage properly and to reduce the risk of further dirt reaching into the defect. Watch your horse walk to see if the crack widens as the horse bears weight on the affected leg. This is an indication that the crack is unstable and may worsen over time. Any signs of lameness or discharge from the crack may indicate the involvement of deeper structures. 
If possible move the horse to dry hard standing especially if the field is muddy. Cover the damage with a clean dry dressing. Call your vet or farrier as soon as possible to arrange a visit. While it is not always an emergency hoof wall damage should be assessed promptly before any further damage or complications arise.
All cracks may lead to instability of the hoof wall And as such even if the crack is not causing a lameness if left untreated they may lead to more serious complications. It is also important to determine the cause to treat the problem correctly. 
Treatment
Hoof cracks will not resolve without intervention the earlier the better. In addition to treating the defect itself removal or resolution of the underlying cause is essential both to aid healing and to prevent recurrence. 
Severe sand cracks may need to be stabilised with materials such as fibreglass or acrylic
Some farriers will use wire or metal plates to achieve this. The method used will depend on the shape of the hoof the depth of the crack and your farriers preference. It is important to remember that the treatment does not stop with stabilisation of the hoof since these cases will need frequent visits from the farrier to ensure that the foot remains balanced and stable with
 minor adjustments made if necessary. 
As mentioned earlier it takes on average 912 months for the hoof wall to grow from the coronary band at the top to the sole at the bottom so it will take a long time as well as regular farriery and a lot of patience for the fissure to disappear. 
Prevention
Whilst some horses are more prone to cracks than others the importance of hoof care cannot be emphasised enough. Feet should be checked at least once daily to monitor for early signs of damage. In winter this does mean hosing off any mud to inspect the feet. 
An area of hard ground to stand on while sometimes difficult to find is hugely beneficial in providing good hoof care. Horses that are kept on deep litter or in muddy conditions are more prone to developing diseases such as thrush or white line disease and standing for prolonged periods of time in damp conditions may lead to reduced horn quality. Although we cannot control the weather if at all possible during prolonged periods of wet weather horses should have access to hard standing or clean dry stabling.