I have a Yellow Rockie in every field - it has magnesium, cobalt, iodine, selenium, sodium, iron, manganese and zinc. They cost £2.75 each. I also have rock salt, which I buy in a 20kg bag. I keep these in old sinks with drain holes, so that any rain drains out. I also have a Crystalyx Standard in with the ewes, tups and calves. Shortly, these will change to the Garlyx one to try to deter flies.
There are so many products - this is a bit belt and braces but I know from our soil tests that some of these nutrients are low in the soil, so certainly low in the grass as well.