Accurately measuring the energy content of feed is a cornerstone of modern animal nutrition, enabling precise diet formulation for optimal health, growth, and production. The process is not a single measurement but a cascading series of determinations, each level revealing more about the energy actually available to the animal. This is because not all energy in a feed is useful; a significant portion is lost during digestion and metabolism.
The Energy Partitioning System
To understand the true energy value of feed, nutritionists use a hierarchical system that quantifies energy at different stages of utilization by the animal. This partitioning accounts for various energy losses that occur between consumption and absorption.
Gross Energy (GE)
Gross energy represents the total chemical energy in a feedstuff, measured by burning a sample in a bomb calorimeter. While accurate, GE doesn't reflect the energy an animal can actually use because it doesn't account for digestibility. For example, materials like wood sawdust have high GE but provide little usable energy to an animal compared to corn. The bomb calorimeter method involves burning a feed pellet in a sealed chamber within a water bath and measuring the temperature increase to calculate the energy released.
Digestible Energy (DE)
Digestible energy is the gross energy minus the energy lost in feces. This is determined through digestibility trials with live animals. DE is more useful than GE as it indicates absorbed energy but doesn't account for all metabolic losses.
- Formula: $DE = GE{\text{feed}} - GE{\text{feces}}$
Metabolizable Energy (ME)
Metabolizable energy is derived from DE by subtracting energy losses in urine and combustible gases. This provides an estimate of energy available for maintenance and production. The formula is $ME = DE - GE{\text{urine}} - GE{\text{gases}}$
Net Energy (NE)
Net energy is considered an accurate measure of usable energy, calculated by subtracting the 'heat increment' (energy lost as heat during digestion and metabolism) from ME. The NE system differentiates energy for maintenance ($NE_m$) from energy for production ($NE_p$). The formula is $NE = ME - HI$
Practical Methods for Estimating Energy
Live animal trials are costly and time-consuming, leading to the use of more practical laboratory methods and prediction equations. These include in vitro digestibility techniques that simulate digestion and Near-Infrared Reflectance Spectroscopy (NIRS), which uses light reflection to predict composition.
Total Digestible Nutrients (TDN)
TDN is an older system, still used, especially for ruminants, that sums the digestible fractions of protein, fiber, fat (adjusted for energy density), and carbohydrates. A drawback is its tendency to overestimate the energy value of high-fiber feeds for ruminants. The TDN Calculation is: TDN (%) = (digestible crude protein) + (digestible crude fiber) + (digestible nitrogen-free extract) + (digestible ether extract x 2.25).
Comparison of Energy Measurement Systems
A detailed comparison of these systems, including measurement methods, energy losses considered, and typical use, is provided in the table below. More information can also be found at {Link: The Pig Site https://www.thepigsite.com/articles/using-the-net-energy-concept-in-commercial-pork-production}.
| Characteristic | Gross Energy (GE) | Digestible Energy (DE) | Metabolizable Energy (ME) | Net Energy (NE) | |
|---|---|---|---|---|---|
| Measurement Method | Bomb Calorimeter | Digestion Trials (Live Animals) | Metabolic Trials (Live Animals) | Indirect Calorimetry / Comparative Slaughter (Live Animals) | |
| Energy Losses Considered | None | Fecal Energy (FE) | FE, Urinary Energy (UE), Gaseous Energy | FE, UE, Gaseous Energy, Heat Increment (HI) | |
| Primary Use | Foundation for other systems | Swine nutrition | Poultry nutrition | Most accurate, becoming standard for monogastrics | |
| Accuracy | Least accurate for animal | Better than GE | Better than DE | Most accurate reflection of useful energy | |
| Cost/Time | Low cost, fast | Moderate cost, time-consuming | High cost, time-consuming | Very high cost, time-consuming | |
| Key Consideration | Doesn't account for digestibility | Reflects absorption efficiency | Accounts for urinary and gas losses | Accounts for all major energy losses to get true usable energy |
Conclusion
Measuring feed energy involves progressing from Gross Energy to more refined systems like Digestible, Metabolizable, and Net Energy, which accounts for all significant energy losses. While live animal trials offer precision, practical methods like in vitro analysis and NIRS provide efficient estimations.
How can feed ingredients affect energy value?
Feed ingredients' chemical composition directly impacts energy value. High-fat ingredients are energy-dense, while high-fiber ones are less digestible. Digestibility varies based on nutrient source, and processing methods can alter available energy.
