What is Digestible Energy (DE)?
Digestible energy is the energy absorbed by an animal after the digestion process. It is a measure that subtracts the energy lost in feces from the gross energy (GE) of the feed consumed. Gross energy is the total potential chemical energy in a feedstuff, measured through bomb calorimetry. The calculation for DE is straightforward: DE = Gross Energy - Fecal Energy. While a useful metric, DE does not represent the total energy available to the animal because it fails to account for other significant energy losses that occur after absorption. Fecal energy comprises not only undigested feed but also endogenous contributions like sloughed-off intestinal cells and digestive enzymes. Therefore, DE is often considered a less precise measure of truly available energy, particularly in species with complex digestive systems.
What is Metabolizable Energy (ME)?
Metabolizable energy is a more accurate measure of the energy truly available for an animal's metabolic processes. It goes a step further than DE by subtracting additional energy losses that occur during metabolism. Specifically, ME is calculated by subtracting energy lost in urine and gaseous products of fermentation (like methane) from the digestible energy. The formula for ME is: ME = Digestible Energy - Urinary Energy - Gaseous Energy.
- Urinary Energy (UE): Represents the energy lost through the excretion of nitrogenous compounds, like urea, which result from the metabolism of protein. A high-protein diet, therefore, can increase urinary energy loss.
- Gaseous Energy: This primarily includes methane, a byproduct of microbial fermentation in the gut. This loss is significantly higher in ruminants (e.g., cattle, sheep) than in monogastric animals (e.g., pigs, poultry), as they rely heavily on microbial fermentation in their rumen.
Why ME is Always Lower Than DE
ME is, by definition, always a smaller value than DE. Since ME is derived by removing further energy losses (urinary and gaseous) from the DE value, it represents a more refined and lower amount of usable energy. For ruminants, it is estimated that ME is approximately 81% of DE, with the remaining 19% lost as methane and urinary energy. The specific percentage can vary based on several factors, including the diet composition. For monogastric animals, the difference is smaller because gaseous energy losses are negligible.
Comparison Table: DE vs. ME
| Feature | Digestible Energy (DE) | Metabolizable Energy (ME) |
|---|---|---|
| Calculation | Gross Energy - Fecal Energy | Digestible Energy - Urinary Energy - Gaseous Energy |
| Considered Losses | Energy in feces only | Energy in feces, urine, and gases (e.g., methane) |
| Represents | Absorbed energy from the digestive tract | Energy actually available for cellular metabolism |
| Accuracy | Less accurate for metabolic use | More accurate for formulating diets based on net energy |
| Difference in Animals | Large differences in efficiency exist | Accounts for species-specific energy loss differences |
| Primary Use | Often used in equine and swine nutrition | Common in poultry nutrition, and increasingly in ruminants |
Factors Affecting the Difference Between ME and DE
The ratio of ME to DE is not constant and can be influenced by several dietary and animal-specific factors:
- Diet Type: Diets high in fiber, common in ruminants, increase fermentation and thus gaseous energy loss, widening the gap between DE and ME. High-concentrate diets can have a smaller difference.
- Dietary Protein Level: Higher protein intake can lead to increased urinary nitrogen excretion, which translates to a greater urinary energy loss.
- Dietary Fat Concentration: Adding fat to the diet can reduce methane production in ruminants, thereby increasing the ME:DE ratio.
- Animal Species: Ruminants experience significant gaseous energy loss, making the difference between ME and DE substantial. Monogastrics have minimal gaseous loss, so the difference is smaller.
Practical Implications for Animal Nutrition
Knowing the difference between DE and ME is essential for accurately formulating animal diets to meet specific production goals, such as milk yield, growth, or maintenance. Relying on DE alone can overestimate the energy truly available to the animal, potentially leading to underperformance or nutritional deficiencies. This is particularly critical in ruminant feeding programs where methane losses are substantial. For poultry, where feces and urine are voided together, ME is the standard measure. Therefore, selecting the appropriate energy system is a fundamental step in precision animal nutrition.
Conclusion
To conclude, metabolisable energy is not the same as digestible energy; rather, it is a refinement of DE that provides a more accurate picture of the energy available for an animal's use. While DE considers only fecal energy loss, ME additionally accounts for losses via urine and gas. The difference between these two measures is particularly significant in ruminants due to methane production, whereas it is less pronounced in monogastrics. A precise understanding of ME is crucial for accurately assessing the true energy value of feedstuffs and ensuring the optimal health and productivity of livestock. Ignoring the distinction could lead to underestimating an animal's energy requirements and failing to achieve production targets. The use of ME is therefore considered the more biologically accurate system for feed evaluation, especially when formulating modern diets.
For more advanced information on this topic, a comprehensive overview can be found in the Principles of Animal Nutrition by Guoyao Wu. [Link: https://open.oregonstate.education/animalnutrition/chapter/chapter-17/]
