1 tablespoon of corn oil isn't going to do any good. Also, corn oil is high in Omega 6 fatty acids, which don't improve skin, coat, and feed like Omega 3's do. I would nix the corn oil and add flax meal instead. Horsetech.com has a product called BioFlax-20 that is high in flax and hoof building goodies. You can even custom blend a full vitamin/min supplement in it if you want added nutrients. Or go with their Glanzen product, which has added vitamins and minerals.
Vita Crumbles are the "flinstone knock-off" vitamins of the horse world. They are pretty worthless. If you haven't opened it, take them back.
I would have the filly on a 4-week trimming schedule, really beveling the toe to take ALL of the pressure off of it when she walks. Those cracks won't grow out, ever, unless you do. My mare had a similar crack and it finally started growing out when we switched to a Ramey-style "natural" barefoot trim. Now her feet NEVER chip or crack!
Here are a couple of good examples. Print these out and show them to your farrier. If he/she won't trim like this, find someone who will. New Page 11 Healthy Hoof - Solutions for Barefoot Performance barefoot_Breakover
A related article (print this out too, though don't hurt yourself trying to understand it, lol. I do my own feet and this one is a bit "deep" for me even...) BREAKOVER******* 2-15-05
Another related article (warning, cadaver feet pictured) New Page 6
Good article on nutrition: New Page 14
Foal trimming recommendations: More Topics