I don't know that I would use the tubs as a primary source of protein. If your hay is reasonable (7-9%?) they should be able to consume enough hay. Or there are lots of other sources of protein.
Where the tubs are extremely valuable, in my opinion, is getting vitamins and minerals into the cattle, especially calves and bred heifers & cows who will be calving in the next few months. I think Crystalyx Mineralyx while an expensive if used as a source of protein, is cheap when looked at as a vitamin source... a bit of copper or selenium or vitamin E etc can be worth its weight in gold if it gives you a healthy, easily born calf. And who knows if they are all present in your local soils and hay???
Between my hay and grazing corn my cattle get plenty of protein, energy, roughage and probably most other nutrients. But selenium and copper are known to be lacking in my area. Mineral (not protein) tubs are like buying insurance, to me.
In my opinion, mineral deficiencies are responsible for a lot of the troublesome health problems some folks see in their herds. I have put out dry mineral but it is just not as paletable as these tubs. I WANT them to consume the tubs, at a reasonable rate.
If they are consuming a lot of the tub, per head, that means the tubs are either like candy or they really need something in it. Cattle know what they need to be healthy. Most tubs have some molasses in them but the Mineralyx is not really being consumed as candy as far as I can see. Some of the stuff they need may just not taste very good in its raw form. Sort of like making kids vitamins taste good? Maybe not ideal philosophically but it gets the job done.
I put out dry mineral and several months later it was still there except as solid as a rock and definitely never going to be consumed.
jmho. Jim
Look at the ingredient list in Mineralyx:
http://www.crystalyx.com/beef/pdfs/Mineral-lyx.pdf