Was doing some googling for a friend on the 'cafeteria-style' mineral supplementation deal... I know this is an OLD post, but worth revisiting. I remember discussions of the cafeteria-style stuff from my animal science courses back in the 1970s, and it looks like it's another bogus deal that comes back around every so often... cattle really don't 'know' what they are missing, and will not pick and choose to consume only the ones that they need.
Our soils here in KY are particularly deficient in copper and selenium. The red trace-mineralized salt blocks are red because of iron oxide (rust) added as a filler/coloring agent. While cattle cannot utilize iron in its oxide form (and iron deficiencey virtually never occurs here), FeO will effectively block copper absorption sites in the small intestine, essentially making copper deficiency WORSE. And, the amount of selenium in the typical TM salt block is so low that the cattle can't lick/consume enough to approach a reasonable supplementation level.
We always fed a balanced mineral mix, which included salt. During the winter, when we limit-fed hay & DDG, cattle consumed their entire day's ration inside of a 2-hr period, for the most part, and stood around for the next 21-22 hrs waiting to be fed, and would seriously over-consume mineral, just because they were bored and had nothing better to do. We ended up having to cut our mineral mix 1:1 with white stock salt just to keep mineral supplementation from breaking the bank.