Corn is a high energy/lower protein feed. When a good energy source is needed but not necessarily more protein, corn is a good choice (but not the only one).
http://cattletoday.com/archive/2011/December/CT2638.php
"Every living organism needs energy. Energy is like the gas in your car, it's needed to make many if not all of the body processes work properly. All activities of the body require energy, and all needs are met by the consumption of food containing energy in chemical form. The human diet comprises three main sources of energy including carbohydrates, proteins and fats. Of these three, carbohydrates most readily provide the kind of energy needed to activate muscles. Proteins work to build and restore body tissues. The cow's body transforms chemical energy derived from forages and feed by the process of metabolism, an activity that takes place in the individual cell. Molecules of the feed substances providing energy pass through the cell wall. Inside the cell, chemical reactions occur that produce the new forms of energy and yield by-products such as water and waste materials
such as heat and those which are later processed into feces and urine. What remains in the body is used for the production of tissue (such as muscle or fat) or milk.
When we limit the amount of energy needed, some of the processes are then limited. The cow's body prioritizes how it uses nutrients and energy is of particular concern.
When energy is limited a variety of undesirable affects occur:
1) Loss of body condition (fat) –while this is often a good idea for people, it is seldom a good thing in cattle.
2) Reduced milk production.
3) Reduce growth of calves (both prior to and after birth).
4) Reduced function of the immune system.
5) Reduced ability for the animal to keep itself warm or cool.
6) Reduced reproductive function.
7) Reduced ability to get out and travel..."
