Horses eating Ceader logs?

Today I noticed my two horses were both nawing on two ceader logs that are in their pasture. They aren't cribbing but they are eating bits of rotten wood, and the moss on the logs.

What would be causing them to do this??

It sounds like they are deficient in some kind of nutrient that is driving them to eat that. Maybe you should put some mineral licks out there and that'll get them to quit.