When a horse's shoes are pulled and the hooves are left unshod, the hooves generally break around the edges up to some extent, especially where weakened by the nail holes, until the hoof tissues can adapt to being barefoot. It's important that a barefoot horse be trimmed regularly and properly to keep the feet balanced. The edges of the hoof should be rounded to minimize chipping and cracking. The toe should be squared off somewhat and rounded as well. If your mare is comfortable being ridden, you can keep riding her barefoot, but minimize riding on sharp gravel or abrasive surfaces like concrete.
Exercise promotes blood flow in the hoof and is the best way to moisten hooves.
Often, hooves will crack not because they are too dry, but because they have been too wet. Horses in a consistently dry environment have better feet. To be sure your horse's hooves are growing at an optimum rate, make sure the horse is getting a balanced diet.