i have been using salt w/trace minerals and dicalcium phophate for years with good success.
you can buy it at any feed store. total cost about $22.00 for 130 pounds.
mix 2 salt to one calcium. the calcium is bitter and won't work alone.
i drill holes in an old stump and pour the mixture on top. it will seep into the stump
after awhile. i've seen where they had the stump chewed down to nothing.
once they find it look out. they will have trails getting to it.
they come to it mostly in spring and summer. this is a powder mix which they can consume
faster than a block. hope this helps...