You don’t mention whether these cracks are few in number, more severe and toe to coronet (more likely poor hoof form/trimming issue) OR relatively numerous and just an inch or so up from the ground (several causes). If your mare’s environment goes through continuous wet to dry cycles, this can trigger such cracking. I also see quite a bit of white line “disease” (aka WLD or seedy toe) in new clients, which can extend through the WL considerably past the toes and upward “behind” the walls. Basically this is WL (the vitally important laminar connection of coffin bone to hoof wall) being degraded by naturally-occurring microbes faster than than the hoof grows out. If your hoof pick easily dislodges the WL area, my advice is to remove whatever comes easily and then apply Kopertox (or any other thrush remedy containing copper napthenate as the active ingredient) up in there once or twice a week until the problem resolves. Putting a little in a syringe (needle off) or very small spray bottle works well, but still messy and it does stain!
Speaking as a barefoot trimmer with 12 years of experience, I agree with other posters that this is not a genetic problem, nor due to your horse having white hooves. Addressing diet can be important and for sure, get your mare off any and all sweet feed. Also good to make sure her mineral balance is right (I LOVE Hiland’s Big Sky dry minerals and have seen great results with many health issues, not limited to feet). The other thing to watch is trimming interval, which may vary for the same horse depending on seasonal fluctuations in growth/wear. Once exfoliating sole has been removed, make sure you don’t allow the length of the hoof walls to get much above the sole. When walls form a ridge around the hoof, they are much more prone to flaring and cracking, especially on hard dry ground. Gently rounding or beveling the walls (the “mustang roll” suggested by others) tends to support a tight laminar connection, but can also result in contracting a hoof if carried excessively into the quarters.
Just noticed how long ago this topic first appeared–how’s it going? Those cracks could potentially be history be now!