That's a trim for the shoe. When the shoe came off, he came down on his bars, which are excessive. That's all bar on each side of the frog and what you are seeing is constant pressure on them, which they don't like and causes damage to the lateral cartilages and on down the path to sidebone. He needs either another shoe or a barefoot trim and padded boots for a bit.
Clean up the hoof well with a scrub brush and find the edge of the sole going around the hoof. The white line is right outside of the sole line. It should be creamy yellow colored and 1/8" wide all the way around the sole.
Hope this helps....
Last edited by missyclare; 01-15-2012 at 05:04 PM.