Common fat sources are tallow and calcium soaps (such as megalac). The addition of fat sources to a feed ration can increase the amount of energy accessible to an animal.

Tallow consists of rendered fats from animals and animal by-products. Calcium soaps are a form of protected fat that can bypass digestion in the rumen and end up in the small intestine, allowing for maximum utilization. These products can improve an animal's body score, and are commonly given to lactating cows who require high amounts of energy.

References:

Arm and Hammer Dairy, Megalac

Dairy Direct, Calcium Soaps