Overview of Animal Nutrition and Energy Metabolism

What is Anima Nutrition

• the science of how animals assimilate feeds, and how the nutrients in the feeds are utilized for growth, tissue repair and replacement, production of animal products

• multi-disciplinary science

what are the 6 reasons on why animal nutrition is important

1. product quality

2. disease resistance

3. reproductive performance

4. economic efficiency

5. quantity of production

6. environmental impact

product quality

• Nutrition directly influences the quality of animal products.

• Examples:

  • Beef cattle: Good nutrition improves marbling (intramuscular fat), enhancing meat quality.

  • Laying hens: Calcium deficiency can cause brittle eggshells and lead to bone diseases.

disease resistance

• Proper nutrition enhances an animal’s immune function, reducing susceptibility to disease.

• Malnutrition can weaken immune response, increasing the risk of illness and mortality.

reproductive performance

• Nutrient balance is critical for successful reproduction.

• Poor nutrition can lead to:

○ Reduced fertility

○ Delayed puberty

○ Low conception rates

○ Poor offspring survival

economic efficiency

• Nutrition is the largest single cost in animal production.

• Feed costs can account for up to 50% of total production expenses.

• Optimizing nutrition improves productivity and profitability.

quantity of production

• Adequate nutrition is essential for maximizing output.

• Poorly nourished animals produce less meat, milk, eggs, or offspring.

• Direct impact on overall production volume and performance.

environmental impact

• Animal nutrition affects the environmental footprint of production systems.

• Key concerns:

  • Habitat loss

  • Methane emissions (from ruminants)

  • Water use (especially in intensive dairy/swine systems)

  • Nutrient runoff (nitrogen & phosphorus) into surface water:

    • Leads to algal blooms (e.g., blue-green algae)

    • Nitrate leaching contaminates groundwater

    • Impairs water quality and aquatic life

what is a nutrient

any chemical element or compound in the diet that is required for normal body functions (reproduction, growth, lactation, maintenance of life processes)

what are the 6 major classes of nutrients

1. carbohydrates

2. protein

3. lipids

4. minerals

5. vitamins

6. water

essential vs non-essential nutrients

essential nutrients have to be provided in the diet because the body cannot synthesize it in adequate amounts

Food

• Definition: Any edible substance that provides nutrients when consumed.

• Use: Generally refers to what humans eat.

• Context: Humane food or human-grade consumption.

Feed

• Definition: Edible material provided to animals for nutrient intake.

• Use: Specifically for animals in agricultural or production settings.

• Context: Livestock, poultry, pets, etc.

Foodstuff/ Feedstuff

• Definition: Any ingredient or material used to make food (for humans) or feed (for animals).

• Use: Terms used interchangeably depending on context (human or animal nutrition).

• Examples: Corn, soybean meal, barley, etc.

Diet

• Definition: The total combination of feedstuffs/ingredients formulated to meet the nutritional requirements of an animal.

• Use: Scientifically designed; considers nutrient balance (protein, energy, vitamins, minerals, etc.).

• Note: is the recipe, not the amount.

Ration

• Definition: The specific amount of the diet given to an animal daily.

• Use: Refers to quantity, not content.

• Example: A cow receiving 20 kg of a formulated diet per day — 20 kg is the ___, the diet is what it's made of.

glandular digestion

mechanism: driven by enzymes secreted by animal’s own glandular organs

location: occurs primarily in the stomach and small intestine

function: enzymes rapidly break down simple nutrients such as proteins, fats and carbohydrates

speed: fast and efficient

Ex: enzyme pepsin in the stomach breaks down proteins

fermentative digestion

mechanism: facilitated by microorganisms through microbial fermentation

location: occurs in specialized compartments of the digestive system

function: microbes break down complex carbohydrates like cellulose, which the animal itself cannot digest

speed: slower than glandular

ex: ruminants use fermentative digestion to extract nutrients from fibrous plant material. Hindgut (horses)

mouth

• teeth and tongue: grasp feed, aid in mastication (mechanical breakdown, increases surface area)

• salivary glands: secrete saliva containing amylase

• Saliva moistens feed

esophagus

transports food from mouth to stomach

stomach

• glandular organ that secretes enzymes and acid

• protein digestion

small intestine

• duodenum, jejunum, ileum

• duodenum and jejunum: primary sites for digestion and absorption

• chyme from stomach enters duodenum, neutralized by pancreatic juices

• liver: produces bile

• pancreas: produces key enzymes for carbs, lipids and protein digestion

hindgut

• large intestine

• site of fermentative digestion by microbes

• absorbs water and certain nutrients

what is the crop in chickens

an organ for feed storage

proventriculus

• glandular organ

• equivalent to the stomach in monogastric

caeca

where fermentative digestion takes place

fore-stomachs

• rumen

• reticulum

• omasum

abomasum

equivalent to he true stomach of monogastric

what are the four groups of microorganisms in the rumen

1. bacteria

2. protozoa

3. fungi

4. archaea

Why is microbial attachment to feed particles important?

Because microbial enzymes are located on the surface of microbial cells (bacteria, protozoa)

How does microbial attachment facilitate digestion?

When microbes attach to feed, their surface enzymes are in direct contact with the feed → faster and more efficient breakdown.

What is the primary source of energy for ruminants?

Volatile Fatty Acids (VFAs).

Why are VFAs important in ruminants?

They provide the major portion of metabolizable energy.

what are the three volatile fatty acids

• acetate

• propionate

• butyrate

What is the main source of amino acids for ruminants?

Microbial protein

villi

• are tiny, finger-like projections that line the small intestine

• increase surface area for absorption

microvilli

• on the surface of the villi

• surface of the epithelial cells

• helps maximize absorption

epithelial cells in villi

• secrete brush border enzymes to complete carbohydrate, protein digestion

rumen papillae

• Papillae: finger-like projections (similar to villi, but larger and visible to the eye).

• Function: increase surface area for absorption of volatile fatty acids (VFAs) produced during carbohydrate fermentation.

• Diet effect on papillae: High-grain diet → larger, more developed papillae. High-forage diet → smaller, less dense papillae.

• Secretion: rumen itself does not secrete enzymes; its role is mainly in microbial fermentation and absorption.

diffusion

• Movement of nutrients from high concentration → low concentration.

• No energy or transport protein required.

• Can occur through tight junctions (spaces between closely linked cells), allowing nutrients to pass into the bloodstream.

facilitated diffusion

• Still moves down a concentration gradient (high → low).

• Requires a specific transport protein to help nutrients cross the cell membrane.

• No ATP required.

active transport

• Moves nutrients against the concentration gradient (low → high).

• Requires a transport protein.

• Energy (ATP) is required.

What is retention time?

The length of time feed stays in the digestive tract.

Why is retention time shorter in monogastrics?

Limited microbial fermentation; digestion is mostly enzymatic.

Key difference between monogastrics and ruminants in terms of retention time?

Ruminants digest more slowly for fermentation; monogastrics digest faster enzymatically.

Anabolic Processes (Building)

• Function: synthesize complex molecules from simpler ones.

• Examples:

  • Protein synthesis → uses amino acids absorbed from the small intestine.

  • Lipid synthesis → uses fatty acids from digestion and stored fat from adipose tissue.

• Energy requirement: depends on ATP produced by catabolic processes.

Catabolic Processes (Breaking Down)

• Function: break down molecules to release energy.

• Examples:

  • Glucose oxidation → produces ATP, CO₂, H₂O.

  • Metabolic pathways involved: glycolysis → TCA cycle (Krebs cycle).

• Purpose: provides the ATP needed for anabolic processes.

general functions of nutrients

• building and maintaining the body structure

• repairing worn-out and injured tissues

• provide energy of various functions

• regulation of body processes

• fetal development

• production of products for human use

sequence of events in a nutrient deficiency

nutritional deficiency will have a biochemical defect → reduction of performance → prolonged deficiency will result in death

what makes a good diet

A good diet must

• contain all essential nutrients in the correct amounts and proportions

• is palatable (animals will eat it),

• digestible (nutrients are absorbed),

• economical (cost-effective; ~50% of production cost),

• safe (free of toxins or nutrient inhibitors).

Is energy a nutrient, and how do animals obtain it?

Energy is not a nutrient; it is obtained when nutrients like carbohydrates, fats, and proteins are oxidized by cells.

What nutrients are the main sources of energy?

Carbohydrates and fats; protein is not sufficient as a primary energy source.

Do water, vitamins, or minerals supply energy?

No, but they are essential for energy production because:

• Water is required for metabolism.

• Vitamins and minerals act as cofactors in energy-producing reactions

why care about energy

• important to formulate a balanced diet that costs the least amount of money

• overconsumption of energy leads to obesity

what is the definition of energy

the potential to do work

• associated with eating, breathing, building body tissues

• maintenance and productive functions

is the fuel of metabolism

how do we measure energy

calorimetry

• using a bomb calorimeter

• oxygen has to be present with fuel

What are the major components of a bomb calorimeter and their functions?

• Thermometer: measures the temperature change of water surrounding the sample.

• Insulating jacket: acts as a thermal shield, preventing heat loss or gain for accurate measurements.

• Steel bomb: contains the feed sample (~1 g, pelletized), has an ignition wire, and uses high oxygen to combust the sample.

• Bucket (water container): holds ~2 L of water, submerges the bomb, and allows measurement of temperature change to calculate energy content.

What is the main principle of bomb calorimetry?

Measures the heat released during complete combustion of a sample in an oxygen-rich environment; the heat is absorbed by water, and the temperature change is used to calculate the sample’s energy content.

Why is benzoic acid used to standardize a bomb calorimeter?

The bomb is not perfectly insulated, so some heat is lost or absorbed by the calorimeter. Benzoic acid has a known caloric value (6,319 cal/g), allowing calibration and correction for accurate measurements of sample energy.

gross energy

the amount of heat released when a substance is completely oxidized in a bomb calorimeter

Which nutrient has the highest gross energy value?

Lipids/fats (~9.3 kcal/g), compared to carbohydrates (~4.2 kcal/g) and proteins (~5.65 kcal/g).

How much more energy do lipids/fats provide compared to carbs and proteins?

About 2.25 times more.

What are volatile fatty acids (VFAs) and why are they important?

Three major VFAs are produced in the rumen; they are the main source of energy for ruminants.

Why is methane considered an energy loss in ruminants?

Methane contains energy but is excreted as a waste product, also contributes to greenhouse gas emissions, representing a major energy loss.

why are fats more energy dense

they contain more carbon, requiring more oxygen to completely oxidize. This high C–H content makes them ~2.25 times more energy-dense than carbs or proteins.

How do corn grain and oat straw compare in energy for animals?

Both have similar gross energy, but corn grain is much more digestible, so animals can use more energy. Gross energy alone has limitations—like coal, it contains energy but isn’t digestible.

what is the largest loss of ingested energy

fecal energy

What is Metabolizable Energy (ME)?

ME = DE – (Urine Energy + Gas Energy).
It represents the energy actually available for metabolism

UE

urinary energy loss

GEL

gaseous energy loss

GILL

gill energy loss

What is Digestible Energy (DE)?

DE = GE – Fecal Energy.
It’s the portion of feed energy absorbed by the animal after fecal losses.

What is Net Energy (NE)?

NE = ME – Heat Increment (energy lost as heat during metabolism).
It’s the energy available for maintenance, growth, lactation, etc.

What is urinary energy?

energy lost in the urine, mainly urea and uric acid

Gaseous Energy Losses

Gaseous energy losses are the energy lost as combustible gases produced during fermentative digestion

What is the SF₆ tracer technique used for?

To measure methane (CH₄) emissions from ruminants under normal grazing or feeding conditions.

How does the SF₆ tracer technique work?

• A capsule in the rumen releases SF₆ gas at a constant rate.

• A capillary tube collects exhaled gases (breath + eructation).

• The ratio of CH₄ to SF₆ in the sample is measured in evacuated flask

Why are gaseous energy losses small in monogastrics?

• Monogastrics have limited microbial fermentation in the hindgut.

• Most nutrients are digested enzymatically before significant gas is produced.

Net Energy

accounts for energy lost as heat during digestion and metabolism

heat increment

increase in heat production following a meal when the animal is in a thermo-neutral environment

What are the three components of heat increment (HI)?

• Work of Digestion: Heat from mechanical/chemical processing of nutrients and absorption.

• Heat of Fermentation: Heat produced during microbial fermentation in ruminants, especially on high-forage diets.

• Heat of Nutrient Metabolism: Heat released during metabolic processing of absorbed nutrients.

What is the maintenance component of Net Energy (NEm)?

Energy required to keep the animal alive and functioning

• basal metabolism

• thermoregulation

• voluntary activity from maintenance

What is the production component of Net Energy (Ne g)

Energy used for productive functions above maintenance, including:

• growth

• fat deposition

• reproductive products

• milk

• eggs etc.

factors affecting heat increment

• diet digestibility

• level of feeding

• diet composition (forage vs. concentrates)

• nutrient utilization (catabolism vs. retention as animal products)

• amino acid balance (nutrient imbalance)

• frequency of feeding

direct calorimetry

• heat production measured directly

• Animal is placed in a insulated chamber that measures heat released to the environment.

What is closed-circuit indirect calorimetry?

• Animal breathes from a closed system with pure oxygen.

• Exhaled CO₂ and water are scrubbed.

• O₂ decline is measured to estimate heat production.

What is open-circuit indirect calorimetry?

• Animal breathes continuous ambient air.

• Exhaled air is analyzed for O₂ consumption and CO₂ production.

Advantages of net energy system

• Accounts for all energy losses (feces, urine, gases, and heat increment).

• Shows actual usable energy for maintenance and production.

disadvantages of net energy system

• expensive and time consuming to measure

How is the Net Energy (NE) system used in pigs

NE is commonly used; accounts for urine and

How is the Net Energy (NE) system used in poultry

NE is rarely used; Metabolizable Energy (ME) is standard because heat increment is small

How is the Net Energy (NE) system used in ruminants

NE is widely used; accounts for large heat losses from rumen fermentation

What is Total Digestible Nutrients (TDN)

• uses values from the proximate analysis to estimate energy content

• comparable to DE