You can build good bows from only sapwood, a combo of sap and heartwood or heartwood(should be backed). The sapwood is good in tension so leave some on the back of your stave if the stave was handled properly from the time the tree was cut. Leaving 1/4" or a little less sapwood should work fine.
For a stave rawhide is an excellent backing for ERC. If in board form use hickory.