What is developmental orthopedic disease?

Basically - anything that causes problems in a growing horse's limbs or joints falls under this banner, including "growing pains" and some limb deformities.

Most of these problems are caused when people feed young horses too much - horses are meant to grow off of lots of low energy feed, so when people start giving foals grain it can cause all kinds of problems. The incidence is pretty high, and the cure is generally to feed the baby less.