rosehip is great for iron, vitamin c, the lungs, kidney health and they also promote strong healthy hoof growth! I haven't personally tried it, but a friend has and even though the effects weren't immediate, the new growth of hoof looked a lot stronger and healthier. Just be prepared to be patient.
Anythings got to be worth a try, and with rosehip you can pick it yourself (well not this time of year) and let it dry out and then feed it to your horse, so it's a cheaper alternative to some of the other supplements out there.
I am currently usinga mixture of Seaweed and Rosehip and found it to be quite good and much more economical - my farrier recommended. Horse had very poor feet previously but, touch wood, I am very pleased with the results.
They are said to be good for feet and also very good for joints.
I feed black sunflower seeds and find they really did improve my beasties hooves - both are now iron hard and rarely have any splits or cracks and they are barefoot.