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

the coronary band. Either of these may be complete or incomplete. 
In general grass cracks occur when the hoof wall is overgrown and begins to flare at the sole
These are rarely involved in severe lameness unless infection is present. 
Sand cracks are present at the front of the hoof extending from the coronary band. Those that occupy the side of the hoof wall are usually referred to as quarter cracks. 
Sand cracks are most commonly caused by poor foot balance
Quarter cracks are usually associated with a mediolateral side to side foot imbalance or with sheared heels. If you watch your horse walking it may be possible 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