Depending on your hay and your straw, good barley straw may be better for very young calves than hay which can be a bit scratchy.
All cattle should do well on good barley straw. I worked on an organic dairy farm where the housed Jerseys got only barley straw for forage over the winter.
BH is most particular about where and what barley straw he will buy however - he won't have it if the fields have been sprayed, he prefers it crimped if poss, and says that some green stems of grass in it are a good sign. (Not least that it hasn't been sprayed.)
I doubt if you need to encourage the appetites of your Shetland cattle...
Hillie has got us onto offering rock salt at all times; they seem to need less from the Crystalyx when they have access to good chunks of mineral-rich rock salt.