As Destructo said those rings shouldn't be a problem at all. If that were a stave instead of a board there would be a hump or knot on the back side. Consequently, the limb might tend to be a little stiff at that point and carefully need scrapped. If the back side shows that several rings have been cut through there you will want to back it with some type of material. The way the rings lay in the middle of the limb suggest you have a pretty good cut of wood. Where will this knot lay in relationship to distance from the handle area or towards the tip?