Name two major energy systems used to classify feed energy (common in nutrition).

• A. Gross energy and net energy.
• B. Digestible energy and metabolizable energy.
• C. Metabolic energy and substrate energy.
• D. Physiologic energy and behavioral energy.

Answer: (B)

Energy in feed is tracked as it moves from total chemical energy to what the animal can actually use, subtracting losses along the way. Digestible energy reflects what remains after fecal losses, so it shows the energy the animal can absorb from the gut. Metabolizable energy goes a step further by subtracting urinary and gaseous losses, representing the energy available for metabolism and activity. These two stages—digestible energy and metabolizable energy—are the practical systems used to classify feed energy in nutrition, which is why they are the best pair to name.

To put it in context, gross energy is the total energy present in the feed, but not all of that energy is usable by the animal. Net energy would be the energy available after the heat produced during digestion, but in many nutrition contexts DE and ME are the primary labeling systems because they align with how animals actually utilize energy after different loss pathways.

For example, if a feed has a gross energy of 4,000 kcal per kg and fecal losses are 1,000 kcal, the digestible energy is 3,000 kcal. If urinary and gaseous losses total 300 kcal, the metabolizable energy is 2,700 kcal. Other pairings like metabolic energy with substrate energy or physiologic/behavioral energy aren’t standard two-system classifications in feed energy.