If you are not feeding pellets, it is definitely a good idea to give them a trace mineral salt block. You can buy a five pound block of the reddish-brown type intended for general livestock for about $3 or so. Just knock chunks off it with a hammer if your rabbits are in individual cages. You might want to serve it in small ceramic crocks to save your cage wire from the salt. I get the straight-sided ramekins at the dollar store, usually 2/$1.