Best diet and care for barefoot backs?

[Kitei]

Obie currently only has shoes on his fronts. I had the backs removed for a couple of reasons, including that he does occasionally kick out at the cob if in a strop, and in general we aren't doing a huge amount of work currently. However, he is a little footy on loose gravel or bumpy surfaces, which means we can't do the nicer hacks in the village.

What would people recommend to 'harden' up his feet? Currently is out 24/7 and gets a medium feed of alfa a and chaff after a ride.