It is always recommended to put salt out when feeding protein tubs. Most if not all tubs are made without added salt because it messes up the manufacturing process.

So, salt is cheap, feed them the salt. I like salt blocks with tubs, but I don't know that it makes a big difference versus loose.

Check with your tub supplier, but most tubs are formulated to have complete levels of minerals, trace minerals and vitamins so you should be good on that.

It is an old wives tale that cows can choose what they need. Salt is the only mineral they can regulate themselves, and they will often consume more than what they need.