Back in the day, when I was working for Moorman's, the Nutritionists told us you should never, ever use a salt block, because it would cause the cattle to consume too little Mineral.
I just sat in on a seminar with a professor from Ohio State University about nutrition in brood cows and he made a statement that I won't forget and that was if your using mineral blocks make sure you buy enough of them so that you can build a wind break with them because thats all there good for. Use a good trace mineral @ 4 oz. per day per head. Even the best mineral should still cost less than $60.00 per head per year. We use Vita- Ferm