The Hierarchy of Energy Measurement
Measuring the energy in feed follows a tiered system that accounts for progressive energy loss as it moves through an animal. Gross Energy is the starting point, with other values calculated through animal trials.
Gross Energy (GE): The Total Energy Content
Gross Energy (GE) is the total potential chemical energy in feed, representing the heat released during complete combustion. It is determined using a bomb calorimeter. This device involves burning a dried feed sample in a high-pressure oxygen chamber submerged in water and measuring the water's temperature increase. GE is a total value and doesn't account for energy lost during digestion.
Digestible Energy (DE): What the Animal Absorbs
Digestible Energy (DE) is the energy absorbed by the animal's digestive system. It's calculated by subtracting the energy lost in feces from the Gross Energy (GE). This requires digestion trials where feed intake and fecal output are measured and their GE determined.
$DE = GE{feed} - GE{feces}$
Metabolizable Energy (ME): The Energy Retained
Metabolizable Energy (ME) considers further energy losses from urine and gases, subtracted from DE.
$ME = DE - UE - GE_{gases}$
ME is practical for poultry as feces and urine are excreted together. Ruminants have significant gaseous losses, requiring complex respiration chambers.
Net Energy (NE): The Most Accurate Measure
Net Energy (NE) is the usable energy for maintenance and production. It's ME minus the "heat increment," the energy lost as heat during digestion and metabolism. NE is the most accurate but also the most complex to determine.
Comparison of Different Energy Measures
Different energy measurement systems vary in accuracy and complexity, suited for specific uses:
| Feature | Gross Energy (GE) | Digestible Energy (DE) | Metabolizable Energy (ME) | Net Energy (NE) |
|---|---|---|---|---|
| Measurement | Bomb calorimeter | Digestion trials (GE minus fecal energy) | Metabolic trials (DE minus urinary and gaseous energy) | Respiration calorimetry (ME minus heat increment) |
| What it Represents | Total potential chemical energy | Energy absorbed by the animal | Energy retained after digestive losses | Energy actually used for maintenance and production |
| Accuracy | Least accurate for usable energy | More accurate than GE, especially for monogastrics | More accurate than DE; accounts for urine and gas losses | Most accurate; accounts for all losses, including heat increment |
| Complexity | Low (lab procedure) | Medium (animal trials) | High (animal trials with gas collection) | Very high (advanced calorimetry or comparative slaughter) |
| Practical Use | Not used for ration formulation; provides a baseline | Used for swine; easily measured | Common for poultry; accounts for major digestive losses | Most precise but too complex for routine commercial use |
The Atwater System and Proximate Analysis
Direct methods are often too costly for routine analysis. An alternative is using predictive equations based on chemical composition (proximate analysis), pioneered by Wilbur Atwater. Proximate analysis determines moisture, crude protein, crude fat, crude fiber, ash, and nitrogen-free extract. Each component has standardized caloric values (e.g., 4 kcal/g for protein, 9 kcal/g for fat) used in equations to estimate energy.
Example Calculation: A simplified gross energy formula using proximate analysis is:
$GE{kcal/100g} = (5.7 × g{protein}) + (9.4 × g{fat}) + (4.1 × [g{NFE} + g_{fiber}])$
This provides a reliable, cost-effective estimate for nutritionists, though it assumes average digestibility.
Conclusion
Measuring feed energy progresses from total potential energy (GE) to usable energy (NE) by accounting for losses. The method used depends on needed accuracy. Accurate energy understanding is vital for optimizing animal diets, health, and production costs.
Lists
Key steps in measuring metabolizable energy (ME):
- Perform a digestion trial with an animal to quantify feed intake and fecal output.
- Use a bomb calorimeter to determine the Gross Energy (GE) of the feed and the feces.
- Calculate Digestible Energy (DE) by subtracting fecal energy from GE.
- Collect urine and measure combustible gases to quantify Urinary Energy (UE) and Gaseous Energy (GEgases).
- Calculate ME by subtracting UE and GEgases from DE.
Factors that can affect the accuracy of feed energy measurements:
- Feed Composition: The ratio of protein, fat, and carbohydrates varies significantly between feeds, affecting the final energy value.
- Animal Species: The efficiency of digestion and metabolism differs greatly between species (e.g., ruminants vs. monogastrics).
- Methodology: The choice between laboratory methods (calorimetry, proximate analysis) and live animal trials affects accuracy and cost.
- Endogenous Losses: Fecal energy includes some endogenous energy from the animal itself (sloughed-off cells, enzymes), which can slightly skew DE results.
- Individual Variation: Factors like age, physiological state (e.g., gestation, lactation), and genetics can influence an animal's ability to extract energy from feed.
Commonly used energy units in animal nutrition:
- Joule (J): The standard SI unit of energy, often used in larger units like kilojoules (kJ) or megajoules (MJ).
- Calorie (cal): A non-SI unit, but still widely used, particularly in the US. A food calorie is actually a kilocalorie (kcal).
- Megacalorie (Mcal): Commonly used for large animals, with 1 Mcal equaling one million calories.
Outbound Link
For additional scientific information on the derivation of food energy conversion factors and the different energy systems, see the detailed report from the Food and Agriculture Organization of the United Nations.
