Understanding Total Digestible Nutrients (TDN)
TDN, or Total Digestible Nutrients, is a measure that estimates the energy content available to an animal from its feed. It represents the sum of the digestible protein, digestible carbohydrates, and digestible fat in a feedstuff. Historically, the TDN system has been a widely used metric in feed formulation, especially for ruminant livestock. A higher TDN value indicates a higher percentage of the feed's nutrients are digestible and can be used for energy by the animal, supporting functions like growth, maintenance, and reproduction.
The Importance of TDN
- Energy Assessment: TDN offers a practical way to gauge the energy density of a feed, which is fundamental for meeting an animal's daily energy needs.
- Ration Balancing: It assists nutritionists and farmers in creating balanced diets that match the energy requirements of specific animal species and life stages.
- Feed Comparison: By providing a single energy value, TDN simplifies the process of comparing different feedstuffs to select the most cost-effective and nutrient-dense options.
The Traditional Proximate Analysis Method
One method for how to calculate TDN utilizes results from proximate analysis, which breaks down a feed sample into its components. The calculation sums the digestible parts of key energy nutrients, often determined via in-vivo digestion trials. {Link: Applied Animal Nutrition https://courses.ecampus.oregonstate.edu/ans312/one/feed_3_story.htm} The formula, typically a percentage of feed dry matter, is detailed in {Link: Applied Animal Nutrition https://courses.ecampus.oregonstate.edu/ans312/one/feed_3_story.htm}. A practical example using a feed sample is also outlined in the same source. {Link: Applied Animal Nutrition https://courses.ecampus.oregonstate.edu/ans312/one/feed_3_story.htm}
The Acid Detergent Fiber (ADF) Method for Forages
Forages often use a calculation based on Acid Detergent Fiber (ADF), which includes cellulose and lignin. Higher ADF levels correlate with lower digestibility and TDN.
A common formula for all forages is:
$TDN\,(\%) = 88.9 - (0.779 \times \%ADF)$
For a hay sample with 45% ADF on a dry matter basis, the calculation would result in approximately 53.85% TDN. While quick, this method is less accurate across varied feedstuffs or different production levels compared to more advanced energy systems.
Comparison of TDN Calculation Methods
| Feature | Traditional Proximate Analysis Method | ADF-based Method (for Forages) |
|---|---|---|
| Data Required | Digestibility coefficients for CP, CF, NFE, and EE | Acid Detergent Fiber (ADF) percentage |
| Primary Application | Applicable to a wider range of feedstuffs, including concentrates | Primarily used for forages (hay, pasture) |
| Accuracy | Generally considered a reliable estimate of energy, though less precise than net energy systems | Quick and practical, but less accurate for diverse feedstuffs or across production levels |
| Limitations | Digestibility coefficients can vary; requires extensive lab analysis for digestible components | Different plants have different fiber characteristics, so specific equations often exist; does not fully capture all digestibility variations |
Factors Affecting TDN Accuracy
Several factors can influence the accuracy of TDN estimates:
- Feed Maturity: As forage matures, fiber increases, decreasing TDN.
- Animal Factors: Species, age, and physiological state impact digestibility.
- Processing: Heat can reduce nutrient digestibility, lowering TDN.
- Feed Composition: High indigestible components like lignin or ash reduce TDN. Higher digestible fat increases TDN.
- Environmental Factors: Temperature can influence nutrient use.
Conclusion
Understanding how to calculate TDN is essential for livestock management and feed planning. TDN is a widely accepted measure for estimating feed energy content, even with modern energy systems available. Using proximate analysis or the ADF method for forages helps producers compare feeds and balance rations. Knowing calculation methods and influencing factors allows for better feeding decisions, improving animal performance and profitability. For further research on the physiological basis of digestion, resources like {Link: Oregon State Ecampus https://courses.ecampus.oregonstate.edu/ans312/one/feed_3_story.htm} are valuable.
