Define metabolizable energy (ME) and explain why it's used in feed formulation.

• A. ME equals energy lost as feces and urine only.
• B. ME equals gross energy minus fecal energy and urine losses.
• C. ME equals gross energy minus fecal, urine, and gas losses; used to estimate energy available for maintenance, growth, and production.
• D. ME equals net energy.

Answer: (C)

Metabolizable energy is the portion of a feed’s gross energy that the animal can actually use after accounting for energy lost in feces, urine, and gases produced during digestion. In practical terms, ME = GE minus energy in feces minus energy in urine minus energy lost as gases. This makes ME a better predictor of what the animal can devote to maintenance, growth, and production than gross energy alone.

Why this matters in feed formulation is that different feeds vary in how much energy is digested and how much is lost in those pathways. Using ME allows nutritionists to compare feeds on a realistic basis of usable energy, guiding diets to meet energy needs without overestimating what’s available. In ruminants, gas losses from fermentation (like methane) are a meaningful part of the total energy loss, so they’re included in ME; in non-ruminants, these losses are smaller but the concept still holds—ME sits between gross energy and the more refined net energy by accounting for the main losses after digestion.