Animal Nutrition - Energy Systems and Their Importance

Animal Nutrition AVBS2004

Course Coordinator

  • Prof Alex Chaves

  • Energy Systems in Animal Nutrition: Focus on the transition from Gross Energy (GE) to Adenosine Triphosphate (ATP).

  • Last update: 12/08/2025, 8:02 AM

  • Institution: The University of Sydney

Learning Outcomes

  • Definitions:

    • Focus on key concepts in energy related to nutrition.

  • Energy forms in nutrition: Detailed discussion on how energy is categorized and utilized.

  • Energy partitioning: Outlining the process from GE to ATP through different stages:

    • Gross Energy (GE)

    • Digestible Energy (DE)

    • Metabolizable Energy (ME)

    • Net Energy (NE)

    • ATP

  • Comparison of Ruminants and Monogastrics: Understanding how these two types of animals process energy differently.

  • Importance of Metabolizable Energy (ME): Essential for formulating diets for animals.

  • Factors affecting ME and digestibility: Comprehension of key factors that influence energy absorption from feed.

  • Implications for diet formulation: Real-world applications in agriculture and animal husbandry concepts covered.

What is Energy in Animal Nutrition?

  • Energy Defined:

    • Energy is described as the ability to perform biological work.

  • Caloric Concept:

    • 1 calorie (cal) equals the energy needed to raise the temperature of 1 gram of water by 1°C at standard pressure.

    • Energy in animal nutrition is most commonly represented in kilocalories (kcal), where 1 kcal = 1,000 cal.

    • Conversion to SI units: 1 kcal ≈ 4.184 kilojoules (kJ).

  • Energy sources:

    • Nutrients that provide energy include carbohydrates, fats, and proteins.

    • Distinction in terminology: “200 Calories” (capital C) refers to 200 kcal.

  • Energy Requirements:

    • Energy fuels various biological processes:

    • Maintenance: Daily caloric intake to balance energy expenditure.

    • Growth

    • Reproduction

    • Lactation

    • Physical activity

The Energy Flow System: From Feed Intake to Useful Energy

  • Energy Terms Defined:

    • Gross Energy (GE): Total energy contained in feed, determined using a bomb calorimeter.

    • Digestible Energy (DE): Represents the Gross Energy minus fecal losses.

    • Metabolizable Energy (ME): Derived from DE minus losses from urine and gases.

    • Net Energy (NE): Calculated by subtracting heat loss from ME; this is the energy available for maintenance, growth, and reproduction.

The Bomb Calorimeter Explained

  • Description:

    • A bomb calorimeter is a strong metal chamber placed within an insulated water tank.

    • The heat released during the oxidation of the fuel sample is absorbed by the bomb itself and the surrounding water, providing a measurement of energy content.

Feed Constituents and Gross Energy

  • Energy Values of Various Feedstuffs:

    • Various feed components have specific GE values measured in MJ/kg of dry matter (DM). For example:

    • Glucose: GE = 15.6 MJ/kg DM

    • Starch: GE = 17.7 MJ/kg DM

    • Cellulose: GE = 17.5 MJ/kg DM

    • Casein: GE = 24.5 MJ/kg DM

    • Butterfat: GE = 38.5 MJ/kg DM

    • Fat (oilseeds): GE = 39.0 MJ/kg DM

Fermentation Products

  • Types of Fermentation Products:

    • The Gross Energy (GE) of various fermentation products is as follows:

    • Acetic Acid: GE = 14.6 MJ/kg DM

    • Propionic Acid: GE = 20.8 MJ/kg DM

    • Butyric Acid: GE = 24.9 MJ/kg DM

    • Lactic Acid: GE = 15.2 MJ/kg DM

    • Methane: GE = 55.0 MJ/kg DM

Energy Values of Animal Tissues

  • Animal Tissues (ash-free) have the following GE values:

    • Muscle: GE = 23.6 MJ/kg DM

    • Fat: GE = 39.3 MJ/kg DM

Comparison of Feeds and Milk

  • Gross Energy values for certain feeds:

    • Grass hay: GE = 18.5 MJ/kg DM

    • Maize grain: GE = 18.5 MJ/kg DM

    • Oat grain: GE = 19.6 MJ/kg DM

    • Oat straw: GE = 18.5 MJ/kg DM

    • Linseed-oil meal: GE = 21.4 MJ/kg DM

    • Milk (4% fat): GE = 18.9 MJ/kg DM

  • Reason for Similarity in GE:

    • Despite similar GE values, the difference lies in how efficiently animals can utilize energy from different food types.

Energy Conversion in Feeds

  • Energy values based on nutrient type:

    • GE values for carbohydrates (CHO): approximately 17.5 - 21 MJ/kg dry matter

    • GE values for proteins: approximately 21 - 23.6 MJ/kg dry matter

    • GE values for fats: approximately 39 MJ/kg dry matter

    • GE value for alcohol: approximately 29 MJ/kg dry matter

  • Average feed GE: About 18 MJ GE/kg DM

Conversion of Joules to Calories

  • Conversion Factors:

    • 1 cal = 4.184 J

    • 1 MJ = 1,000 kJ;

    • 1 Mcal = 1,000 kcal

    • 1 kg = 1,000 g

  • Energy Content Per Kg Inputs:

    • Carbohydrates: 18 MJ/kg = 4.3 kcal/g

    • Proteins: 18 MJ/kg = 4.3 kcal/g

    • Fats: 39 MJ/kg = 9.3 kcal/g

    • Alcohol: 29 MJ/kg = 7 kcal/g

Energy Availability to Animals

  • Loss of Energy:

    • Not all GE is usable by animals; losses happen through:

    • Solid, liquid, and gas excretions

    • Heat generated

    • Correcting for these losses yields a more accurate measurement of energy availability.

Ruminants vs Monogastrics

  • Energy Utilization Differences:

    • Digestive Process:

    • Ruminants: Microbial fermentation in the rumen

    • Monogastrics: Enzymatic digestion

    • Gaseous Losses:

    • Ruminants have higher methane (CH₄) and CO₂ production compared to monogastrics.

    • Metabolizable Energy (ME) to Digestible Energy (DE) Ratio:

    • Ruminants: Lower, 0.75–0.87

    • Monogastrics: Higher, 0.96–0.98

    • Gaseous losses can account for 6–12% of total GE in ruminants.

    • Use of NE System:

    • Ruminants use NE extensively for maintenance, gestation, lactation, and growth.

    • Monogastrics have limited NE system usage primarily in poultry and pigs.

Energy Values for Different Animal Meals

  • Example Energy Data:

    • Chickens: Wheat grain: GE = 18.1, Feces = 2.8, Urine = 4.9, Methane = 15.3, ME = 12.3

    • Pigs: Oat grain: GE = 19.4, Feces = 5.5, Urine = 0.6, Methane = 13.3

    • Sheep: Barley grain: GE = 18.5, Feces = 3.0, Urine = 0.6, Methane = 2.0, ME = 12.9

    • Cattle: Lucerne hay: GE = 18.3, Feces = 8.2, Urine = 1.0, Methane = 1.3, ME = 7.8

Importance of Metabolizable Energy (ME)

  • Significance in Diet Formulation:

    • ME is critical for balancing energy needs against the cost of feed.

    • ME patterns vary by species;

    • Ruminants focus primarily on ME to evaluate NE for maintenance, gestation, lactation, and growth.

Transition from ME to NE

  • Causes of Heat Increment:

    • Energy loss during nutrition involves metabolic inefficiencies:

    • Muscular activity for chewing, swallowing, and saliva secretion consumes about 3-6% of ME intake.

    • Ruminant fermentation processes contribute to heat from gut microorganisms, consuming around 7-8% of ME intake.

  • Nutrient Metabolism and Heat:

    • Different feeds induce varying heat increments based on the food type and metabolic processes.

Animal Calorimetry Methods

  • Types of calorimetry include:

    1. Direct Calorimetry: Measurement of heat production.

    2. Indirect Calorimetry: Use of open circuit respiration chambers to measure respiratory gases to estimate energy retention; commonly employed for carbon and nitrogen balance trials.

    3. Face Mask Measurements: For short time measurements in grazing animals.

    4. Radio-Labelled CO2 Infusion: Infusion of isotopes (e.g., 14C sodium bicarbonate) to track metabolic processes.

Brouwer Equation for Heat Production

  • Brouwer Equation: ext{Heat production (KJ)} = 16.18 imes ext{VO}2 + 5.16 imes ext{VCO}2 - 5.9 imes N - 2.42 imes ext{CH}_4

    • Where:

    • ext{VO}_2 = oxygen consumption (L)

    • ext{VCO}_2 = carbon dioxide production (L)

    • N = urinary nitrogen excretion (g)

    • ext{CH}_4 = methane production (L)

ME in Diet Formulation

  • Expression and Goals:

    • ME is usually expressed in MJ/kg DM or Mcal/kg DM.

    • Required for various physiological processes:

    • Maintenance

    • Growth

    • Lactation (milk production)

    • Gestation

    • Work or physical activity

    • Overall formulation objective: Maximize animal performance while keeping costs manageable.

Factors Influencing ME and Digestibility

  1. Feed Composition:

    • Includes fiber content (NDF, ADF), fat and starch levels, and protein quality.

  2. Ration Composition:

    • Associative effects among feeds can impact overall digestibility.

  3. Processing Methods/Feed Preparation:

    • Techniques such as grinding, pelleting, and heating improve digestibility.

  4. Anti-Nutritional Factors:

    • Elements like tannins and phytates can reduce nutrient availability.

  5. Enzyme Supplementation:

    • Can enhance digestibility and nutrient absorption.

  6. Animal Factors:

    • Species differentiation (ruminant versus monogastric), age, weight, physiological state, and microbiome influences (e.g., ruminants).

  7. Feeding Level:

    • The amount of food consumed can affect digestive passage rates and consequent digestibility.

  8. Environmental Conditions:

    • External factors like temperature and stress can increase maintenance energy requirements.

Energy from Respiration and Cellular Work

  • Conversion to ATP:

    • After digestion, energy is primarily converted through metabolic processes to Net Energy (NE), which is then utilized in cellular respiration for ATP production.

  • Functions of ATP:

    • ATP serves as the immediate energy currency for cellular processes, including:

    • Muscle contraction

    • Active transport of molecules

    • Biosynthesis of essential compounds (proteins, fats, etc.)

Energy to Work & Measuring It

  • Key Energy Determinants:

    • The energy required to perform tasks varies by:

    • Type of work involved

    • System efficiency (how effectively work processes convert energy)

VO₂ Max—Aerobic Capacity

  • Definition and Purpose:

    • ext{VO}_2 ext{ Max} is the highest rate of oxygen consumption during intense exercise, indicating aerobic metabolism's capability to generate ATP.

  • Higher VO₂ Max:

    • Indicates the potential to perform at increased power outputs before reliance on anaerobic pathways becomes significant.

  • Determinants of VO₂ Max:

    • Factors such as cardiovascular oxygen delivery, mitochondrial density, training, genetics, and species physiology contribute to variations in VO₂ max.

  • Importance of ME Supply:

    • A sufficient ME supply is crucial for maintaining ATP demands during physical exertion.

    • VO₂ max establishes limits on sustained aerobic ATP production, influencing overall performance.

Definition of Power

  • Power Defined:

    • Power is the rate at which work is done or energy is transferred.

    • Mathematically, power can be expressed as:
      ext{Power} = rac{ ext{Work Done}}{ ext{Time}}

    • Units:

    • Measured in Watts (W), such that 1 W = 1 joule/second.

    • Additional formula includes:
      ext{Power} = ext{Torque} imes ext{Angular Velocity}

    • Where:

    • au (Torque) = the force applied at a radius from the axis of rotation (in N·m)

    • ext{Angular Velocity} ( ext{ω}) = rotational speed (in radians/s)

Cycling Power

  • Cycling Power Dynamics:

    • Cycling power reflects the interaction of torque and angular velocity:

    • Torque derives from pedal force application.

    • Angular velocity is represented by crank rotation speed (cadence).

    • Higher torque or cadence leads to increased power output.

Power Output versus Cadence Graph

  • Graphical Representation:

    • Displays the relationship between power output (W) and cycling cadence (RPM) featuring peaks in power output at specific cadences.

  • Characteristics Presented:

    • Max Power: 1350 W at 135 RPM

    • Distinctions between force-dominant and frequency-dominant zones.

Functional Threshold Power (FTP)

  • Definition:

    • FTP is the maximum number of watts that can be sustained for one hour, indicating effort level.

    • Requires power meters for real tracking; for running, estimated based on weight and pace.

  • Power Zones:

    • Z1-Z2: Ideal for recovery.

    • Z2-Z3: Moderate endurance efforts.

    • Z4 and above: Intense exertion zones.

Power and Heart Rate Zones

  • Power and Heart Rate Correlation:

    • Reflects training zones based on heart rates related to maximum heart rate.

    • Illustrates various exercises based on power outputs and heart rate response:

    • Zone 2 (Moderate): Power output of 229 W

Advanced Heart Rate Zones Analysis

  • Zone 3 and Zone 4:

    • Identification of time in zones relative to heart rate:

      • Zone 3: 49.5% time spent in this category.

      • Zone 4: 32.5% time (intense tempo).

Power Over Time Metrics

  • Measurement Data Summary:

    • Displays insights into power output (W), heart rate (bpm), and cadence metrics over time such as average, maximum, and activity duration in various units.

Power or Pace Duration Curve

  • Energy Utilization:

    • Shows performance metrics of power or pace relative to time across different durations (10s, 1m, 6m, 40m, 1h, 3h).

Rate of Lactate Depletion / Accumulation

  • Lactate Concentration Curve:

    • Analyzes dynamic shifts in lactate production and removal linked to aerobic and anaerobic thresholds, measuring individual heart rates and lactate levels.

Take-Home Messages

  • Metabolizable Energy (ME): Links nutrient supply directly to animal performance.

  • Importance of Accurate ME Values:

    • Crucial for efficient diet formulation, optimizing growth/lactation, and minimizing environmental impact (e.g., methane emissions).

  • Understanding Energy Partitioning: Enhances insights into energy loss mechanisms throughout digestion and metabolism.

  • Digestibility Factors Impacting ME: Factors influencing digestibility inherently affect ME, highlighting the importance of accuracy in estimation for enhancing feed efficiency and profitability.

  • Respiration Contribution: NE translates into ATP during cellular respiration, providing energy necessary for biological functions such as growth and locomotion.

  • Efficiency and Losses at Each Step: Emphasizing the need for optimizing diets and environmental factors for enhanced energy conversion from GE to ATP and subsequently to work output.