Supplemental nutrition can be very important in certain times of the year. It is important to maintain cows in good body condition to avoid many of the diseases that we will commonly see. An animal with good nutrition is more likely to have a good immune system and more likely to recover from diseases without intervention.
If you are having troubles with animals not performing as you would like, you may want to consider the possibility of their being a trace mineral deficiency. We commonly see problems with deficiciency of Copper and Selenium and also sometimes other minerals like Phosphorus and Cobalt.