Three things to investigate with toe cracks.
1) Is the crack in the outer wall only or does it include the white line?
2) Has the hoof wall been thinned or had any kind of oil/hoof dressing applied
Five broad categories
Stretched white line - The bulk of toe cracks I deal with are an end symptom of a stretched white line which of course is a symptom of damaged laminae which is of course a symptom of hind gut issues of some sort.
When the white line is compromised opportunistic bacteria invade, (it's lovely and moist and bacteria friendly in there, especially under shoes. They chomp away and eventually the whole lot becomes compromised sufficiently for the hoof to split. This can be fixed. It takes time - because you have to grow the crack out and a bit of hard work, because you need to ensure the foot has the stimulation it needs to put out the growth and you may need to treat the bacterial infection.
Hoof wall length - LOTD (lack of trimming disease). The hooves have simply got too long and the cracking, splitting and chipping is the hoof trying to self trim.
Hoof dressings - CED - (I'll let you work that out), where OTC (over the counter) products available at any feed store have damaged the hoof wall - chemically or by disturbing the moisture balance, either over drying the wall or keeping it so moist that bacteria set up home and break down the keratin and it starts to crumble.
Trimming - Less than perfect mustang roll - if your horse tends to develop 'corners' and these are left too long (both in time and length of the 'pointy' section) the leverage can create a stress crack - usually mid way between the two corners Doesn't always happen. These close up nicely when the stress is removed.
Or it could be that your hoof care provider is getting too handy with the rasp and has been thinning the hoof wall. This damages it - ask them not to do it and if they insist find yourself another trimmer. Usually this is done when the hoof is scooting forwards and the trimmer doesn't know how to deal with it any other way.
Horn quality - If you have a perpetual short crack in the toe which is hard to resolve, but doesn't seem to penetrate to the white line you may have an issue with horn quality. Usually cutting right back on sugars/simple starches and checking mineral balances is enough. The horse may also have long term low level undiagnosed hind gut issues. Try probiotics such as Yea Sacc 1026 and/or charcoal*
*I am not a charcoal expert, but be careful of your source - there is some CED going on and poor quality charcoal is being sold to the unsuspecting. You want a hardwood charcoal that has been properly treated.
For peeps in the UK check out Fine Fettle Feeds. So far they haven't managed to make the UGP (Universal Guinea Pig) ill which is a start - I've only just started the test on Grace so I'll let you know how it goes in due course.