Animal Nutrition AVBS2004

Animal Nutrition Course Overview

• Course Coordinator: Prof Alex Chaves

• Course Code: AVBS2004

• Last Updated: 1st August 2025, 4:45 PM

• Institution: The University of Sydney

Learning Outcomes

• Define the concept of digestibility and explain its significance in animal nutrition.

• Describe the main methods for calculating or estimating digestibility, including their principles and applications.

• Identify and evaluate the primary uses of digestibility data in animal feeding and ration formulation.

• Compare and contrast different digestibility estimation methods, highlighting the advantages, limitations, and appropriate use cases for each method.

• Understand the factors that influence digestibility.

Digestibility Estimation Methods

Types of Methods

1. In Vivo

   • Total fecal collection

   • Indigestible markers

2. In Vitro

   • Lab fermentation with rumen fluid/enzyme

3. In Situ

   • Nylon bag technique (rumen incubation)

4. Lab Prediction

   • NIRS (Near-Infrared Reflectance Spectroscopy)

5. Mathematical

   • Prediction equations, software models (e.g., CNCPS, NRC, BCNRM 2016)

In Vivo Method

Overview

• Measurement of digestibility within the live animal, achievable through two techniques:

  • Total fecal collection

  • Indigestible markers

A. Total Fecal Collection

• Method: Collect all feces over a defined period to assess digestibility.

• Accuracy: Most accurate but labor-intensive.

• Example Calculation:

  • If an animal consumes 10 kg of feed DM and excretes 3 kg of feces DM, its dry matter digestibility (DMD) would be calculated as follows:

  • Formula:
    $DMD = rac{(10 ext{ kg} - 3 ext{ kg})}{10 ext{ kg}} imes 100 = 70\%$

B. Indigestible Marker Method

• Definition: Uses substances that pass undigested through the gastrointestinal tract to estimate digestibility indirectly.

• Types of Markers:

  • Internal Markers: Naturally present in feed (e.g., lignin, acid insoluble ash (AIA), alkanes).

  • External Markers: Added substances (e.g., chromic oxide, titanium dioxide, alkanes).

• Ideal Marker Characteristics:

  • Completely indigestible

  • Non-absorbable

  • Easily analyzed

  • Uniformly distributed in the diet

  • Inert (no digestional impact)

Pros & Cons of Total Fecal Collection vs. Indigestible Markers

Summary of In Vivo Method

Pros

• Most accurate and realistic assessments of digestibility.

• Takes into account animal-specific and microbial interactions.

Cons

• Time-consuming and costly.

• Ethical concerns and logistical complexities associated with animal handling.

• Requires specialized equipment like metabolism cages and skilled labor.

Common Uses

• Validation of other digestibility estimation methods.

• Research and regulatory purposes.

Nutrient Digestibility vs. Digestible Nutrient

• Nutrient Digestibility does not equate to Digestible Nutrient; understanding this distinction is crucial.

• Units:

  • Nutrient Digestibility is expressed as a percentage of the nutrient (digestion coefficient).

  • Digestible Nutrient is expressed as a percentage of the feed DM, reflecting the fraction of feed DM that is digestible nutrient.

• Terminology: Mertens recommended:

  • $dNut = ext{digestible nutrient} = ext{ extit{percentage of digested nutrient in feed DM}}$

  • $NutD = ext{nutrient digestibility} = ext{ extit{percentage of nutrient that is digested}}$

  • For dry matter, $dDM = DMD$ for clarity.

Example Calculation of Crude Protein Digestibility

• A cow eats 25 kg of feed containing 20% crude protein (CP) and excretes 7.5 kg of feces containing 15% CP.

• Calculations:

  • Feed CP: $(25 ext{ kg} imes 0.20 = 5 ext{ kg CP})$

  • Fecal CP: $(7.5 ext{ kg} imes 0.15 = 1.125 ext{ kg CP})$

  • Crude Protein Digestibility (CPD) formula:
    ext{%CPD} = 100 imes rac{(5 - 1.125)}{5} = 77.5\%

• Digestible CP expressed in percentage of DM can be calculated as:

  • $dCP = 100 imes rac{(5 - 1.125)}{25} = 15.5\%$

• Alternatively:

  • $dCP = ext{Feed CP} imes rac{CPD}{100} = 20 imes rac{77.5}{100} = 15.5\%$

In Vitro Method

Overview

• Utilizes lab simulations of digestion using rumen fluid and/or enzymes.

Pros

• Less expensive and quicker than in vivo methods.

• Allows for controlled and repeatable experiments.

• Facilitates high-throughput screenings of feed samples.

Cons

• May not fully replicate in-animal digestion conditions.

• Relies on donor animals for rumen fluid.

• Sensitive to variability in lab conditions.

Common Uses

• Comparative evaluations of feeds.

• Preliminary screenings for novel feed ingredients.

Gas Production Correlation

• Gas production from in vitro fermentation is correlated with feed digestibility, quantified by the equation:
  $y = 0.4365x + 30.669$
  where $R^2 = 0.6089$ indicates a moderate correlation.

Rumen Simulation Technique

• RUSITEC (Czerkawski and Breckenridge 1977)

  • Consists of eight fermentation vessels each with a volume of 920 mL.

  • Operates at a constant temperature of 39ºC and uses artificial saliva.

  • Daily feeding of substrate via nylon bags for real-time assessments of digestibility, methane production, volatile fatty acids, and pH.

Lab Prediction Using NIRS

How NIRS Works:

• Feed samples are ground and exposed to Near-Infrared light (700–2500 nm).

• Reflectance of the light is analyzed to determine nutrient composition (moisture, fiber, protein, starch, fat, IVDMD, etc.).

• The predicted digestibility is based on regression models using reference data derived from in vivo or in vitro trials.

Pros

• Fast and requires minimal sample preparation.

• Suitable for routine evaluations on farms.

• Enables high-throughput analysis.

Cons

• Necessitates expensive calibration models.

• Prediction accuracy is dependent on the quality/range of the reference database.

• Not effective for novel feeds without prior calibration.

• NIRS does not measure digestibility directly; it predicts it.

Comparison: NIRS vs In Vitro

Applications of NIRS

• Ideal for on-farm feed evaluations and quality control at feed mills.

• Provides rapid screenings for forage digestibility but should not replace in vivo/in vitro methods.

• Frequently integrated into precision feeding systems.

In Situ Method (Nylon Bag Technique)

Overview

• Feed is incubated in porous bags positioned in the rumen of fistulated animals.

Pros

• Captures microbial degradation under authentic rumen conditions.

• Useful for investigating kinetic studies regarding the rate of digestion.

Cons

• Semi-invasive and subject to ethical regulations.

• Limited to ruminal digestion without post-ruminal data.

• Variability in animal response and potential impact from bag materials/size.

Common Uses

• Study rumen degradability of DM and nutrients like proteins and fiber.

• Conduct preliminary assessments of new feed ingredients.

Digestion Kinetics Formula

• In situ digestion kinetics are typically expressed using the formula of Orskov and McDonald (1979): $P = A + B(1 - e^{-kt})$

  • Where:

    • $P$ = potential degradation

    • $A$ = soluble fraction (g/kg of DM at t=0)

    • $B$ = degradable insoluble fraction

    • $k$ = rate of degradation

Effective Degradability Calculation

• Effective degradability is calculated using: $ED = A + rac{B imes k}{k + k_p}$

  • Where:

    • $k_p$ = passage rate

Mathematical Methods (Prediction Equations/Software)

Overview

• Mathematical models estimate digestibility from feed composition (e.g., NDF, CP, ADF).

Pros

• No animal application required.

• Fast and scalable assessments.

• Useful for practical diet formulation.

Cons

• Less precise than biological methods.

• Relies on the accuracy of input data provided.

• Calibration and validation are often necessary.

Common Tools

• Examples include: CNCPS, NRC Models, BCNRM 2016, etc.

Summary Comparison of Methods

Conclusion

• Selecting the appropriate digestibility estimation method must consider:

  • Needs for accuracy

  • Available resources

  • Ethical and logistical constraints

• In Vivo remains the gold standard but is resource-intensive.

• In Vitro and In Situ serve as good research tools.

• NIRS is advantageous for on-farm applications and precision feeding systems.

• Mathematical equations/software are most suitable for practical, large-scale applications.

Recommended Reading

• Refer to item 10.4 "Factors affecting digestibility" in Chapter 10 of the book "Animal Nutrition" by McDonald et al. (2002, 6th ed.), Pearson Education Limited, Harlow UK.

• Additional resources can be found in Badham (Call number 636.0852 3 E) at The University of Sydney.