Understanding the Concept of Digestible Energy
Digestible energy (DE) is a fundamental metric in animal nutrition, representing the amount of energy from a feedstuff that an animal's digestive system can actually absorb and utilize. It is distinct from gross energy (GE), which is the total energy content measured by burning a feed sample in a bomb calorimeter. High-fiber feeds, for example, may have a similar GE to high-starch feeds, but the fiber-rich diet will result in significantly lower DE because much of the energy is indigestible and passes through in the feces. The calculation of DE is therefore a crucial step in formulating efficient and balanced diets for various animal species.
The Fundamental Formula for Calculating DE
The most direct and fundamental method for calculating digestible energy involves a digestion trial. This procedure measures the total energy of the feed consumed and the total energy of the feces excreted over a specific period. The core formula is straightforward:
$DE = GE{intake} - FE{loss}$
In this formula:
- $DE$ is the Digestible Energy.
- $GE_{intake}$ is the Gross Energy of the feed consumed.
- $FE_{loss}$ is the Energy lost in the feces.
Practical Steps in Conducting a Digestion Trial
- Determine Gross Energy ($GE_{intake}$): The first step is to measure the gross energy content of a feed sample using a bomb calorimeter. This device incinerates the feed under controlled conditions and measures the heat released, which corresponds to the total potential energy.
- Conduct a Feeding Period: A controlled feeding trial is conducted with the animals. A known amount of feed is provided over a set period, and all feed refusals are collected and weighed. Special care must be taken to ensure the animal consumes only the test diet.
- Collect Fecal Samples: Over the same period, all feces are collected and weighed. This is often the most labor-intensive part of the process and requires meticulous control to prevent contamination.
- Analyze Fecal Energy ($FE_{loss}$): Fecal samples are also analyzed using a bomb calorimeter to determine their gross energy content. This step quantifies the amount of energy that the animal did not absorb and was instead excreted.
- Calculate DE: By plugging the measured values of $GE{intake}$ and $FE{loss}$ into the formula, the digestible energy can be calculated.
Alternative Predictive Equations for DE
Because conducting exhaustive digestion trials is expensive and time-consuming, especially for large animals, many nutritionists rely on predictive equations. These summative equations estimate DE based on the feed's proximate analysis, which breaks down the feed into its core components (crude protein, crude fiber, fat, etc.).
Example of a summative equation-based approach:
- Estimate digestible nutrient content by multiplying each proximate nutrient (crude protein, fat, carbohydrates) by its respective digestibility coefficient, often derived from previous research.
- Use the Atwater factors (5.65 kcal/g for protein, 9.40 kcal/g for fat, and 4.15 kcal/g for carbohydrates) to convert the digestible mass of each nutrient into its energy equivalent.
- Sum the energy contributions to get the total estimated DE. The Dairy NRC (2001) System, for example, uses a summative equation to estimate DE and eliminate the reliance on older total digestible nutrients (TDN) values.
Factors Influencing DE Calculation
The accuracy of DE calculations is not solely dependent on the formula; many variables affect an animal's ability to digest and absorb energy.
Key factors include:
- Feed Composition: The ratio of different nutrients significantly affects digestibility. Feeds with higher fiber content have lower digestibility due to the complex, harder-to-break-down nature of cellulose and lignin.
- Feed Processing: Grinding, pelleting, and cooking can alter feed structure, increasing surface area and making nutrients more accessible to digestive enzymes. This typically improves DE.
- Animal Species: Ruminants (like cows) have different digestive systems than monogastrics (like pigs or horses). Ruminants can digest fibrous material more efficiently due to microbial fermentation in the rumen.
- Age of the Animal: Young animals may have less developed digestive systems and lower enzymatic activity, which can affect digestibility compared to mature animals.
- Level of Feeding: Increased feeding rates can reduce digestibility because food moves through the digestive tract faster, allowing less time for complete digestion and absorption.
Comparison of Energy Systems: DE vs. ME
While Digestible Energy is a valuable metric, it's essential to understand its place within the broader spectrum of energy evaluation systems. Metabolizable Energy (ME) is another widely used measure that takes the calculation a step further.
| Feature | Digestible Energy (DE) | Metabolizable Energy (ME) |
|---|---|---|
| Calculation Basis | Gross Energy (GE) minus Fecal Energy (FE). | Digestible Energy (DE) minus Urinary Energy (UE) and Gaseous Energy (GE). |
| Energy Losses Accounted For | Fecal losses only. | Fecal, urinary, and gaseous losses (e.g., methane from fermentation in ruminants). |
| Accuracy | Generally less accurate for predicting usable energy, especially in ruminants due to significant methane losses. | Provides a more accurate representation of the energy available for cellular metabolism. |
| Common Use | Historically used for many farm animals, still widely used for equine nutrition. | The most common system for poultry due to combined fecal and urinary excretion; also widely used for ruminants. |
| Primary Limitation | Does not account for urinary energy loss and combustible gas losses. | Does not account for heat increment, the energy lost as heat during digestion and metabolism. |
Conclusion: The Importance of Accurate DE Calculation
Calculating digestible energy is a foundational practice in animal nutrition, providing a key indicator of a feedstuff's actual value to an animal. The most accurate method involves a controlled digestion trial to measure gross energy intake and fecal energy loss, but validated predictive equations offer a practical alternative for a variety of species. Understanding the numerous factors that can influence digestibility, from feed composition to animal species, is critical for interpreting and applying DE values correctly. By carefully calculating and considering DE, nutritionists can formulate diets that maximize performance and health in livestock and companion animals. This ensures not only the well-being of the animal but also the economic efficiency of production systems. For further in-depth reading, the National Institutes of Health offers extensive resources on the relationships between different energy metrics.
