How is metabolizable energy (ME) derived from digestible energy (DE)?

• A. ME = DE − energy losses in feces.
• B. ME = DE − energy losses in urine and gases (primarily methane); losses vary by species and diet.
• C. ME = GE − energy losses in urine and gases.
• D. ME = DE − energy losses in urine and gases.

Answer: (B)

Metabolizable energy is the portion of energy that is actually available to the animal after accounting for energy lost in urine and in gases produced during digestion and fermentation. Digestible energy already removes fecal energy from gross energy, so to obtain ME you subtract the urinary energy and the energy lost as gases from the digestible energy. The gaseous losses are mainly methane, and the amount of these losses varies by species and diet. For example, ruminants tend to lose more energy as methane due to rumen fermentation, and high-protein diets can increase urinary energy losses. Because ME reflects the energy the animal can use for maintenance and production after these losses, the formulation ME = DE − energy losses in urine and gases (primarily methane), with variation by species and diet, is the most accurate description.