What is the difference between net energy and gross energy?

December 21, 2025 •

4 min read

In animal nutrition, over 40% of the gross energy in some fibrous feeds is lost in feces alone, highlighting a critical distinction. Understanding this key difference between net energy and gross energy is essential for accurately evaluating a food source's true nutritional value for an organism.

Quick Summary

Explores the definitions and measurement of gross energy (total potential energy) and net energy (usable energy after digestive and metabolic losses). Compares the two to illustrate why net energy provides a more accurate nutritional assessment.

Key Points

Gross Energy: Represents the total potential energy in a feed, measured by complete combustion in a bomb calorimeter.
Net Energy: Is the final, usable energy available to the animal after all losses from digestion, metabolism, and excretion.
Energy Losses: The primary difference arises from accounting for energy lost in feces, urine, gas, and as heat, in a process known as energy partitioning.
Accuracy: Net energy is a more accurate measure of a feed's true nutritional value compared to gross energy because it accounts for metabolic inefficiencies.
Metabolic Cost: The conversion from gross to net energy involves a significant metabolic cost, known as the heat increment, which is subtracted from metabolizable energy to find net energy.
Application: Different energy systems (DE, ME, NE) are used depending on the animal species, with NE being the most precise for predicting animal performance.

The Fundamental Concept of Energy Partitioning

All life requires energy, which is derived from the carbohydrates, fats, and proteins consumed in a diet. However, the total potential energy in a feed is not the same as the energy an animal can actually use. The process of digestion and metabolism is inefficient, with energy lost at each stage. This concept of energy partitioning is the foundation for distinguishing between gross energy and net energy. Gross energy represents the total chemical energy in a feed, while net energy represents the fraction of that energy that is actually available for an animal to use for maintenance and productive purposes. Understanding this hierarchy of energy values is crucial for formulating balanced diets for livestock, pets, and even for understanding human metabolism.

Gross Energy: The Starting Point

Gross energy (GE), also known as the heat of combustion, is the total amount of energy released as heat when a feed sample is completely burned. This measurement is typically performed using a piece of equipment called a bomb calorimeter. The feed sample is placed in a high-pressure oxygen chamber within the calorimeter and ignited. The heat produced by this combustion process is measured by the temperature change of the surrounding water, allowing for a precise calculation of the feed's total caloric content.

While this measurement is accurate, it provides very little information about a feed's nutritional value to a living organism. This is because it doesn't account for the fact that animals cannot fully digest and absorb all the components of their food. For instance, sawdust has a high gross energy value because it combusts easily, but an animal cannot extract any useful energy from it. Therefore, gross energy serves as a starting point, but other measures are needed to determine the true energy available.

From Digestible to Metabolizable Energy

As the feed is processed by an animal, a significant amount of energy is lost. The system for measuring available energy accounts for these losses in a step-by-step manner:

Digestible Energy (DE): Not all of the gross energy consumed is digested and absorbed by the animal. The first major loss occurs in the feces. Digestible energy is calculated by subtracting the energy lost in the feces from the gross energy of the feed. The proportion of energy lost in feces can vary greatly depending on the feed's composition; for example, high-fiber diets result in higher fecal energy loss.
Metabolizable Energy (ME): After accounting for fecal energy, further losses occur through the urine and gaseous products of digestion (primarily methane, especially in ruminants). Metabolizable energy is defined as the digestible energy minus these urinary and gaseous losses. ME is commonly used in poultry nutrition, where feces and urine are excreted together, making ME easier to measure than DE.

Net Energy: The Usable Value

Even after accounting for the energy lost in feces, urine, and gas, the body's metabolic processes are not 100% efficient. The act of eating, digesting, absorbing, and metabolizing nutrients requires energy and produces heat. This energy expenditure is known as the heat increment (HI) or thermic effect of feeding.

Net energy (NE) is the metabolizable energy minus the heat increment. It represents the final, usable energy that the animal can put towards productive purposes, such as maintenance, growth, lactation, and reproduction. Because it accounts for all the major energy losses and inefficiencies, net energy is considered the most accurate measure of a feed's true energy value.

Why Net Energy is a Superior Measure

The net energy system offers several advantages over simpler measures like digestible or metabolizable energy, especially in animal feed formulation:

Accounts for Heat Increment: The heat increment varies depending on the type of nutrient being metabolized. For example, metabolizing fat generates less heat increment than metabolizing protein, meaning more of the fat's ME is converted to NE. NE systems therefore provide a more accurate prediction of animal performance, especially for diets with varying protein and fat levels.
Improves Ingredient Ranking: When evaluating different feed ingredients, the ranking can change significantly when using an NE system instead of a DE or ME system. Ingredients high in fat are valued more accurately with NE, while high-fiber feeds may be rated lower, reflecting their true efficiency of use.
Better Predicts Animal Performance: By considering all major energy losses, the NE system is the most scientifically advanced approach for predicting animal performance, carcass quality, and overall health.

A Comparison of Energy Measures


Feature	Gross Energy (GE)	Digestible Energy (DE)	Metabolizable Energy (ME)	Net Energy (NE)
Definition	Total potential chemical energy in a feed.	GE minus fecal energy loss.	DE minus urinary and gaseous energy loss.	ME minus heat increment.
Measurement Method	Bomb calorimeter.	Digestion trials (measuring intake and fecal output).	Digestion trials (measuring intake, feces, urine, and gas).	Live animal calorimetry or comparative slaughter technique.
Practical Application	Limited use; serves as a starting point for calculation.	Common in swine nutrition.	Common in poultry nutrition.	Most accurate for predicting animal performance, especially for ruminants and swine.
Key Consideration	Does not account for digestibility.	Does not account for urinary, gaseous, or heat losses.	Does not account for heat increment.	Most accurate but most complex and costly to determine.

Conclusion

While gross energy provides a foundational measure of the total energy locked within a food source, it is an incomplete picture of nutritional value. The progression from gross to digestible, metabolizable, and finally net energy reveals the significant energy losses that occur during digestion and metabolism. The key difference between net energy and gross energy is the accounting for all these inefficiencies. Net energy represents the true energy available for an animal's body, making it the most precise and reliable measure for nutritional science, particularly in the accurate formulation of animal feed. Using net energy helps nutritionists optimize diets more effectively, leading to better animal performance and health outcomes.

Learn more about the principles of animal bioenergetics here: XVII. Bioenergetics – A Guide to the Principles of Animal ....

Frequently Asked Questions

Gross energy is measured in a bomb calorimeter, an instrument that burns a feed sample in a high-pressure oxygen environment and measures the total heat released.

Net Energy (NE) is calculated by taking the Gross Energy (GE) and subtracting the energy lost in feces, urine, gas, and as the heat increment.

Gross energy is unreliable because it doesn't account for the portion of feed that is indigestible or the energy lost during the metabolic process, which can vary significantly between different feed types.

Metabolizable energy (ME) is the energy available after subtracting fecal, urinary, and gaseous losses from gross energy. Net energy (NE) is more precise, as it further subtracts the heat increment—the energy lost as heat during nutrient metabolism.

For accurate feed formulation, particularly in modern animal production, net energy (NE) is superior because it provides the best estimate of the energy actually used by the animal, accounting for metabolic heat loss.

The heat increment (HI) is the energy lost as heat during the processes of ingestion, digestion, and metabolism of a feed. This represents an inefficiency in the animal's utilization of absorbed nutrients.

The concepts of gross versus net energy apply broadly to biological systems, including humans. However, in human nutrition, simplified values (like the Atwater system) are commonly used to estimate metabolizable energy, though the principle of energy loss during metabolism remains valid.

An animal's net energy requirement is influenced by factors such as body size, level of physical activity, environmental conditions, and physiological state (e.g., growth, lactation, pregnancy).