Net Energy vs Gross Energy: A Comprehensive Guide for Physics Students

Net energy (NE) and gross energy (GE) are both measures of the energy content of a substance, but they differ in the way they account for the energy lost during digestion and metabolism. GE is the total amount of energy contained in a feed, while NE is the amount of energy that is actually available to the animal for use after accounting for energy losses during digestion and metabolism.

Understanding Gross Energy (GE)

Gross energy (GE) is the total amount of energy contained in a feed or food item. It represents the maximum potential energy that could be released if the substance were completely oxidized. GE is typically measured using a bomb calorimeter, which measures the heat of combustion of a sample.

The formula for calculating GE is:

GE = (Mass of sample × Calorific value of sample) / Mass of sample

Where:
– GE is the gross energy (in kcal/g or kJ/g)
– Mass of sample is the weight of the sample (in grams)
– Calorific value of sample is the heat of combustion of the sample (in kcal/g or kJ/g)

The calorific value of a substance depends on its chemical composition, with carbohydrates, proteins, and fats having different calorific values. For example, the calorific values of these macronutrients are:

• Carbohydrates: 4.1 kcal/g (17.2 kJ/g)
• Proteins: 5.7 kcal/g (23.8 kJ/g)
• Fats: 9.4 kcal/g (39.3 kJ/g)

GE is a useful measure for comparing the energy content of different feeds or foods, but it does not account for the energy lost during digestion and metabolism.

Understanding Net Energy (NE)

Net energy (NE) is the amount of energy that is actually available to the animal for use after accounting for energy losses during digestion and metabolism. NE is calculated by subtracting the energy lost in feces, urine, and heat production from the gross energy (GE) of the feed.

The formula for calculating NE is:

NE = GE - (Energy lost in feces + Energy lost in urine + Energy lost as heat)

Where:
– NE is the net energy (in kcal/g or kJ/g)
– GE is the gross energy (in kcal/g or kJ/g)
– Energy lost in feces is the energy lost through the excretion of undigested material (in kcal/g or kJ/g)
– Energy lost in urine is the energy lost through the excretion of metabolic waste products (in kcal/g or kJ/g)
– Energy lost as heat is the energy lost as heat during the digestive and metabolic processes (in kcal/g or kJ/g)

The energy lost as heat, also known as the “heat increment,” is particularly important for ingredients high in fat and starch, as these nutrients result in different quantities of heat increment.

Factors Affecting Net Energy (NE)

The NE system is considered to be the closest estimate of the energy value of feeds and diets, but it is dependent on several factors:

1. Environmental Conditions: Environmental factors, such as temperature and humidity, can affect the animal’s energy requirements for maintenance and the efficiency of nutrient utilization.

2. Variation Among Animals: Individual animals can vary in their ability to digest and utilize nutrients, which can affect their energy requirements and the NE value of the feed.

3. Growth Stage: The animal’s growth stage can also influence its energy requirements and the NE value of the feed. For example, growing animals have higher energy requirements for maintenance and growth compared to mature animals.

Comparison of Energy Systems in Animal Nutrition

In animal nutrition, the energy content of feed is typically expressed in terms of DE, ME, or NE:

1. Digestible Energy (DE): DE is the amount of energy in the feed minus the amount of energy lost in the feces.
2. Metabolizable Energy (ME): ME is the amount of energy in the feed minus the energy lost in the feces and urine.
3. Net Energy (NE): NE is the amount of energy in the feed minus the energy lost in the feces, urine, and in heat production through digestive and metabolic processes.

The NE system is considered to be the closest estimate of the energy value of feeds and diets because it takes into account the heat increment, which is the energy lost as heat during digestion and metabolism. This is particularly important for ingredients high in fat and starch, as these nutrients result in different quantities of heat increment.

Examples of NE Systems in Animal Nutrition

1. Dairy Cattle Nutrition: In dairy cattle nutrition, the NEL (Net Energy for Lactation) system is used to express energy requirements for maintenance and milk production. The NEL system is based on the yield of total energy in milk and does not account for many of the differences in metabolic transactions or the substrates required for synthesis of individual milk components.

2. Swine Nutrition: In swine nutrition, the NE system is recognized as the closest estimate of the energy values of ingredients and diets because it takes the heat increment from digestive process and metabolism of feeds into account. However, the NE system is dependent on environmental conditions, variation among pigs, and growth stage, as those factors influence the energy requirement for maintenance and the ability of pigs to digest and utilize nutrients.

Conclusion

In summary, NE and GE are both measures of energy content, but NE provides a more accurate estimate of the energy available to the animal for use after accounting for energy losses during digestion and metabolism. The NE system is considered to be the closest estimate of the energy value of feeds and diets, but it is dependent on environmental conditions, variation among animals, and growth stage.

References

1. Estimating Net Savings: Principles and Practices
2. Metabolizable Energy
3. Net Energy Systems for Ruminants
4. Energy Systems in Swine Nutrition
5. Comparison of Net Energy Systems for Ruminants