XVII. Bioenergetics

This chapter discusses energy metabolism in the animal body and the movement of energy from one form to another. As energy is the most important commodity in the animal diet, this section discusses units of measurements, distribution of energy in the whole animal, and disorders related to energy metabolism.


New Terms
Comparative slaughter technique
Digestible energy
Gross energy
Heat increment
Metabolizable energy
Net energy
Total digestible nutrients

Chapter Objectives

  • To introduce different energy terminology and discuss energy flow through an animal
  • To discuss the measurement of energy retention and disorders related to energy metabolism in livestock

Energy is not a NUTRIENT, but a property of some nutrients such as carbohydrates, fats, and proteins.





Why Study Bioenergetics in Animal Nutrition?

Bioenergetics is the study of the balance between energy intake and utilization by the animal for different life-sustaining processes (e.g., osmoregulation, digestion, locomotion, tissue synthesis). Energy intake in the animal is through feed and energy losses are through different sources such as heat, feces, urine, and other gaseous losses. Bioenergetics enables the nutritionist to formulate the ration per the energy need of the animal and helps in evaluating different feedstuffs accurately. As feed represents the major cost of raising livestock (>65%), formulating the right diet will cut down on feed costs and enhance animal productivity and health while minimizing nutrient loss to the environment.

Studying energy measurements and partitioning in animals is important for ration formulation and optimizing animal production.

In the US, the calorie is the unit usually used to express feed energy. In other countries and scientific journals, the joule is used as the unit to express energy. One calorie is the amount of heat required to raise the temperature of one gram of water by 1° C from 15.5° C to 16.5° C. One thousand (1,000) calorie is one kilocalorie (1 kcal) and 1 kcal is 4.184 joules (J). For practical purposes, kcal is commonly used in ration formulation and in expressing caloric value of feeds.



Energy Measurements: Different energy measurements and flows of energy in the animal are shown in Figure 17.1.


Gross energy (GE) is the total amount of chemical energy in the diet consumed. It is also known as the heat of combustion. GE content of feed is measured as heat liberated during complete burning (oxidation) of the feed sample and is determined by an apparatus called a bomb calorimeter. Briefly, the feed sample is burned in a combustion chamber (bomb) inserted in another chamber containing a known weight of water. The heat liberated during burning of the feed raises the temperature of the water. From the weight of the sample, the weight of the water, and the increase in temperature, the GE of the feed can be calculated. This measurement is easy, precise, and accurate. However, GE does not have much practical value, as it does not provide much information on the nutritional value of the feed and does not account for palatability, digestibility, or other animal physiological factors. High-protein and high-fat feeds will have more energy than high carbohydrate feeds, and feeds with high ash will have less energy than lower ash feeds.

Gross energy is the amount of heat liberated when a feed sample is completely burned into carbon dioxide and water and is determined by a bomb calorimeter.

To determine the fraction of the GE that animals utilize for different metabolic processes, animal bioassay studies or digestion trials should be conducted to assess different losses through feces, urine, gas, and heat.

Digestible energy (DE) is the energy remaining in the diet after fecal energy is subtracted. DE represents the indigestible components of the feed that will be excreted in the feces; however; they still contain energy that was not utilized by the animal. Fecal loss of energy is the major source of energy loss to the animal and depends on the nature of the feed. For example, diets containing high fiber may have less digestibility, and fecal loss will be higher than starch-based diets.

Energy Terms

  • Gross energy
  • Digestible energy
  • Metabolizable energy
  • Net energy

Because DE measurements need fecal energy loss, animal feeding trials need to be conducted. Digestibility trials are easy to conduct. Total feed intake and total fecal output are recorded, and GE is determined based on the diet and fecal matter using a bomb calorimeter. DE is calculated as GE − fecal energy (FE). Most DE values in feed tables are experimentally determined using live animal feeding trials. The carbohydrate fraction of digestibility is highly variable due to the presence of fiber (less digestible) and nonfiber (starch; highly digestible) components. DE is not a true value and is an “apparent” value as the gastrointestinal tract contributes to extra energy voided in feces. This “extra energy” originates from endogenous sources such as sloughed-off cells, unused enzymes, or other microbial contributions.

Digestible Energy = Gross Energy (GE) − Fecal Energy (FE)

Metabolizable Energy

Metabolizable energy (ME) is defined as the energy remaining after urinary loss and gaseous losses arising from the gastrointestinal tract are subtracted from DE. Values obtained reflect losses due to digestion, fermentation, and metabolism of the feed by the animal. Urinary loss is the major one and is the total energy lost in urine. Urinary losses are usually stable but can increase when high protein is included in the diet. Urine is the end product of metabolism, which contains energy in different compounds, such as urea. Gaseous products of digestion include combustible gases produced by the digestive tract during fermentation of food by microbes (methane, carbon monoxide, hydrogen). In ruminant, 4% to 8% of feed energy is lost from the rumen as methane.

Losses from gaseous loss are minor and are ignored in monogastric species. On average, combined energy losses in gases and urine are about 18% of DE in ruminant animals.

ME = DE −Urinary Energy (UE) − Gaseous Energy

To determine metabolizable energy, metabolic trials are conducted using live animals. Daily intake (feed) and losses (fecal, urinary, gaseous) of energy are documented. Breathing masks (or chambers) are used to assess gaseous losses. Bomb calorimetry is done on all the samples. The amount of methane expelled is documented and is multiplied by its energy concentration (13.2 Mcal /kg). Collection of urine and measuring gaseous losses are prone to errors, and thus ME is less accurate. Diets with high protein content can increase urea loss, and diets with fiber can increase methane and acetate loss. ME is usually conducted in poultry because urine and feces are voided together and gaseous loss is negligible in these species. ME is more difficult to determine than DE, and most of the tabular values for ME are calculated. ME can be used for two purposes: (a) maintenance and (b) production.

Net Energy

Net energy (NE) is ME minus the heat generated by the inefficiency of transforming energy from one form to another. Simply speaking, heat generated is heat lost during the energy transformation process and is called the heat increment (HI). No matter what purpose ME is used for, this is not at 100% efficiency. This inefficiency of the biological system is represented by HI.

NE = ME − Heat Increment (HI)

HI is a difficult concept, and it is very difficult to determine accurately. Live animals continually produce heat, and HI depends on analyzing fasting versus fed animals. Heat increment is therefore all the heat produced by the act of eating, chewing, and digesting the feed and absorbing the nutrients from the gut. When an animal is fasted, stored nutrients are used instead of absorbed nutrients. Heat increment represents the difference in the efficiency of using absorbed nutrients (fed animals) versus stored nutrients (fasted animal). The type of feed (e.g., fiber vs. starch) also can affect HI.

HI = heat loss of the eating animal − heat loss of the fasting animal.

NE is the remainder of the “useful” energy after all the losses “available” to the animal and could be used for both animal maintenance and production purposes (e.g., milk, eggs, meat, etc.)

NE = NE (maintenance) + NE (production)

NE represents the fraction of the total energy consumed that is utilized for production purposes.

NE represents the best scientifically designed energy system because NE is the actual amount of energy that is useful to the animals; it should be the best way to describe feed energy. Nonetheless, we seldom directly measure NE systems due to the cost and difficulty of determining the NE values. The NE values in feed tables are derived from total digestible nutrients (TDN), DE, body weights, and regression equations based on experiments depending on the species.

Figure 17.1. Energy flow chart

*This is negligible in monogastric animals.

Methods for Measuring Heat Production and Net Energy

To determine NE, HI has to be measured, and it is not an easy task. Measuring HI requires whole animal calorimeter (respiration chambers). This equipment is very expensive and is limited. Therefore, NE values are limited especially for large animals. Total heat production measured by direct or indirect calorimetry is often employed in NE calculation. Alternatively, NE can be determined by measuring the energy retention of the animal using the comparative slaughter technique. The different methods are briefly discussed below.

Calorimetry: Animals lose heat to the environment through conduction, convection, radiation, or evaporation. The latter loss is through the skin, respiratory tract, or excreta. Heat loss is measured directly using direct calorimetry or indirect calorimetry.

In direct calorimetry, sensible heat loss is measured as a rise in temperature of the medium (e.g., water) circulating outside the walls of the chamber. Evaporative loss can be determined by the increase in humidity of the ventilating air. The equipment used for these types of trials is costly and very limited.

Indirect calorimetry is based on the principle that metabolic heat production is the result of the oxidation of organic compounds. Thus heat production can be calculated from the amount of oxygen consumed and the amount of carbon dioxide produced. However, this measurement is not 100% accurate as nitrogenous compounds of protein oxidation, such as urea, and other anaerobic fermentation products, such as methane, are not accounted for.

Direct calorimetry measures heat production directly. Indirect calorimetry measures gas exchange as it is related to heat production from the oxidation of organic compounds.

Indirect calorimetry can be open or closed. In the open type, which is the most common, a mask or hood or animal chamber may be used. Air intake and carbon dioxide and methane output are precisely measured. Automated gas analysis and computer control make it much easier to handle air intake and CO2 output. These types of machines are common in use. But errors in measurement can affect the results obtained.

In the closed type, the animal is kept in a temperature-controlled chamber. Air in the chamber is continuously circulated through absorbent silica gel or KOH, which removes water and carbon dioxide. Air pressure is maintained using a constant supply of oxygen, and methane is allowed to accumulate within the chamber. Oxygen use is determined as the amount of oxygen supplied to maintain the pressure and carbon dioxide production is determined from the amount collected by the absorbent.

Comparative Slaughter Technique: In this test, live animal feeding trials are conducted by providing a common ration of known energy for a two week adaptation period. At the end of the adaptation period, a group of animals is slaughtered and body composition and gross energy are determined to get baseline information. The remaining are fed the same ration for a certain period of time and are then slaughtered and body composition is determined. Energy retention is then determined as the difference between body energy content in the initial baseline animals versus those from the animals at the end of the trial. Information derived simulates live animal feeding trials under normal conditions but requires a large number of animals and is time consuming, expensive, laborious, and destructive (animals can only be used once).

The comparative slaughter technique is a better estimate than calorimetry but needs a long time and is costly and the animals can be used only once.

Feeding Systems in Diet Formulation

Energy is the most important “commodity” of a diet and all feeding systems seek to match the nutrient need of the animals. As explained, GE is of no value to the animal as it does not give any information on digestibility or palatability. In the US, among the different energy systems, DE is commonly used in swine, and ME is commonly used in poultry. In ruminants, in addition to NE, TDN analysis is also used.

TDN is an old system of estimating the energy content of feeds. TDN is commonly used in ruminant animals and is carried out by conducting digestion trials. TDN is the summation of the digestible crude protein, digestible fiber, digestible nitrogen-free extract, and digestible ether extract and is expressed as a percentage of the total amount of feed. The additional energy value of fat is compared to carbohydrates by the inclusion of 2.25 as a multiplier. The digestible value of protein is given the same value as carbohydrates and thus indirectly corrects for urinary N loss.

In practice, TDN is in between DE and ME. The major weakness of TDN is that in ruminants, TDN overestimates the energy supply from high-fiber feed ingredients, such as straw and hay, compared to highly digestible low-fiber-containing cereal grains. TDN still is the most popular system used on farms as it is easy to understand and a large database is also readily available.

TDN = digestible crude protein + digestible crude fiber + digestible nitrogen-free extract + (digestible ether extract × 2.25)

Several factors can influence energy need, such as activity, body size, environment, physiological state (pregnancy, lactation), breed/strain, hide thickness, and coat condition.

Disorders Associated with Energy Intake

Excess or inadequate energy intake can lead to several disorders in food-producing animals.

Obesity: Obesity is considered a disorder associated with excess dietary energy intake and is more commonly diagnosed in companion animals (e.g., dogs) and equines. Obesity can decrease the quality and length of the animal’s life. The greater the deviation from optimum body weight and body condition score (BCS), the greater the incidence and severity of orthopedic disorders and cardiovascular diseases.

Obesity occurs when dietary energy exceeds energy expenditure by the body. Influenced by daily energy expenditure and other factors (genetics, diet composition [fat, fiber], neutering). Treatment of obesity is mainly through diet management (high fiber, low fat), and exercise.

Disorders such as bovine ketosis, ovine/caprine pregnancy toxemia, and fat cow syndrome are associated with inadequate energy intake. Ketosis is common a few days after calving. Low dry matter intake leads to negative energy balance. Due to the energy demands of early lactation and pregnancy (e.g., twins or triplets in ovine) and glucose shortages, depot fats are oxidized to acetyl CoA via β-oxidation. The tricarboxylic acid (TCA) cycle intermediates, particularly oxaloacetate, are limited due to less energy intake, resulting in ketone body formation and increased concentrations of ketone bodies (acetoacetic acid, β-hydroxybutyric acid, acetone) in body fluids. In affected animals, ketone bodies are excreted in urine and acetone is excreted through the lungs as “sweet breath.” Diet management (providing high-quality forages) is recommended at least two to three weeks before calving. Dextrose (50%) IV, propylene glycol (glucose precursor), and glucocorticoids are recommended as therapy.

Fatty liver syndromes occur in cattle and horses during negative energy balance. Depot fats are broken down, resulting in an increase in free fatty acids in the blood. Excessive free fatty acids are presented to the liver and result in fat accumulation in the liver, also called hepatic lipidosis.




Bioenergetics: Summary

  1. Bioenergetics is the topic of energy and its metabolism, or biochemical thermodynamics.
  2. Energy is a concept and not a nutrient. Energy is the property of some nutrients.
  3. The unit of energy is the calorie or kilocalorie. It is the amount of heat required to raise the temperature of one gram of water by 1° C.
  4. Physiological oxidation happens inside an animal’s body through various metabolic pathways. Physical oxidation takes place inside a bomb calorimeter, which converts feed energy to heat.
  5. Gross energy is determined in a bomb calorimeter. This provides a measurement of total energy in feed.
  6. Digestible energy (DE) is determined by subtracting energy loss in feces from the gross energy (GE) of feed. Digestion trials are needed to get this value. Not all DE is retained by the animal.
  7. Metabolizable energy (ME) represents retained energy.
  8. ME supports two different functions: tissue maintenance and production. Maintenance functions include all organ work (e.g., heart, lungs, kidneys) and ion balance and production include products (e.g., milk, meat, eggs).
  9. ME is most commonly used in poultry, as feces and urine are voided together and easy to measure, while DE is more commonly used in swine.
  10. Net energy (NE) accounts for all the losses and is theoretically more accurate.
  11. NE accounts for heat increment (HI), energy loss as heat. HI is heat production associated with nutrient digestion, absorption, and metabolism.
  12. Methods to measure energy retention and heat production include direct and indirect calorimetry and the comparative slaughter technique.
  13. Total digestible nutrients analysis uses digestibility and proximate analysis to provide an estimate of the energy content of a feed.
  14. Disorders of energy metabolism include ketosis and obesity.

Review Questions

  1. What is a calorie? How are the total calories of a feed determined?
  2. Draw the energy diagram from gross energy to net energy.
  3. What is digestible energy?
  4. What is metabolizable energy (ME)? List two factors affecting the ME content of a feed.
  5. What is heat increment (HI)? List the factors affecting HI?
  6. How do you define net energy (NE)?
  7. What are the two methods to measure energy retention?
  8. List the advantages and disadvantages of the different methods used for measuring energy retention?
  9. What is TDN?
  10. List two disorders associated with energy metabolism.


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A Guide to the Principles of Animal Nutrition Copyright © 2019 by Gita Cherian is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.