Protein tubs have been gaining popularity here lately with producers of all sizes. People that use them like the convenience of having a supplement that is designed to be self-fed in the pasture. They can deliver supplemented nutrients that does not require extra labor.
In an ideal world cattle would get all of their nutrition from the forages and/or the concentrated feeds that they are given. But unfortunately as you are aware we do not live in an ideal world. The quality of the forages can vary in large degree from year to year. Another factor is that each animal will have different nutritional needs throughout the year.
With the difference between nutritional needs and forage quality there is a need to “fill in the gaps”. As you have guessed this is where protein tubs come in.