put the mineral tubs near by and the will use them. have had a couple for better than 30 years around here, diesel fuel with I believe, co-ral
in it.
every time I go by it, I generally give it a few spins to get the juice to start wetting the cylinder.
When the cows figure out the rub gives them some fly relief.....they will use it.  
make sure you stake it down.