A little while back I put out a new-to-me cafeteria style mineral feeder for the cattle and sheep. Each mineral is in it's own compartment, not mixed into a single blend. The animals can then choose what they need and leave the rest, so the thinking goes.
I just had to refill the first two compartments that were empty. Guesses to what they were? I would have guessed selenium and I'd have been wrong. The first two to run out were copper and silicon. The selenium was pretty depleted, but not quite to the point where I need to refill it.
Of course this doesn't prove anything. There are many variables at work and I have no idea if the cows and sheep just like the copper tinged salt better than the manganese tinged salt... but it seems promising all the same that they've clearly chosen some compartments over others and may be addressing their deficiencies. The silicon is particularly interesting because it tastes like gross, mealy sand to me. Yes, I did taste the minerals and each has a distinct flavor. For example, the boron is downright sweet in a sickly stomach turning way. I would not want more than a little pinch of it, which is good because it's toxic in large volumes.