A split in your horse's hoof shouldn't be ignored. Here to explain all about it is farrier, Matthew Eastwood.
Q: I've noticed a split developing in my horse's hoof, starting from the coronet down to the floor. Why is this?
A: Your horse may have damaged his coronary band, and this split is now growing down the hoof wall.
An abscess could also have burst out of the top, resulting in a split.
Any other split could be the result of an imbalanced hoof capsule, or a conformation fault that hasn't previously been addressed by shoeing or trimming.
Your farrier will need to assess the crack and decide how much, if any, material to cut away.
He or she should discuss with you whether the back will be better off filled or left open.
If your horse is already shod, your farrier can trim the foot so that it lands level, with the hoof capsule landing under the centre of the descending weight.
Over time you'll start to see some improvement. If the foot isn't shod, an option is for your farrier to remove the wall around the crack and use remedial shoes to give your horse the correct support.