Total Digestible Nutrients (TDN) is a system for estimating the energy content of feeds for livestock, particularly ruminants. It quantifies the sum of all digestible energy-providing components: protein, fiber, nitrogen-free extract (carbohydrates), and ether extract (fat). While it has been largely superseded by more precise net energy systems in advanced nutrition, TDN remains widely used on farms for its simplicity. Measuring TDN can be approached through several methods, each with trade-offs in accuracy, cost, and time.
In Vivo Digestion Trials: The Traditional Gold Standard
In vivo trials are the most accurate, though also the most expensive and time-consuming, method for measuring TDN. They involve feeding an animal a specific diet and then collecting and analyzing its feces to determine what was not digested.
Total Fecal Collection Method
This method requires animals to be housed individually for a specific period, typically 7 to 10 days, during which all feed intake is precisely measured and all fecal output is collected.
- Process: A known quantity of feed is given to the animal. After a preliminary adjustment period, the feces are collected entirely, weighed, and sampled daily. The samples are then analyzed chemically to determine the percentage of each nutrient (protein, fat, fiber) present in the feces. The difference between the nutrient intake and the nutrient output represents the digestible portion.
- Calculation: TDN is calculated by summing the digestible percentages of crude protein (dCP), crude fiber (dCF), nitrogen-free extract (dNFE), and multiplying the digestible ether extract (dEE) by 2.25 to account for its higher energy density.
Indicator or Marker Method
The indicator method provides an alternative to collecting all feces by using an inert marker that is consumed and excreted at a consistent rate.
- Method: An indigestible substance, such as chromic oxide, is added to the feed. By measuring the concentration of the marker in both the feed and a fecal sample, the total fecal output can be estimated. This reduces the labor associated with total collection. Natural markers, like lignin or acid-insoluble ash, can also be used.
Laboratory Analysis: Estimating TDN from Chemical Composition
Since in vivo trials are impractical for routine analysis, commercial laboratories use chemical analyses to predict TDN values. Two primary systems are used.
The Proximate Analysis System
Developed over a century ago, this system divides a feed sample into six components: water, crude protein (CP), ether extract (EE), crude fiber (CF), ash, and nitrogen-free extract (NFE).
- Component Measurement:
- Crude Protein: Measured by determining the nitrogen content via the Kjeldahl method and multiplying by 6.25.
- Ether Extract: Fat content is determined by dissolving the lipid compounds in a solvent like ether.
- Crude Fiber: Less digestible carbohydrates are measured by boiling the sample in acid and then alkali.
- Nitrogen-Free Extract: This represents soluble carbohydrates like sugars and starches and is calculated by subtracting all other components from 100.
- TDN Calculation: Standard TDN formulas use the percentages of these components combined with average digestibility coefficients. For example:
TDN, % = (0.75 x %CP) + (2.25 x 0.90 x %EE) + (0.50 x %CF) + (0.90 x %NFE).
The Van Soest Fiber Analysis System
This newer system was developed to provide a more accurate fiber evaluation, especially for ruminants, for whom the older crude fiber method was found to be less useful. It quantifies Neutral Detergent Fiber (NDF) and Acid Detergent Fiber (ADF).
- ADF to TDN Conversion: In this method, the TDN of a forage is predicted directly from its ADF content using a regression equation, as higher ADF is correlated with lower digestibility. A common formula for all forages is:
TDN, % = 88.9 – (0.779 x %ADF). Specific equations are often used for different types of feed, such as corn silage.
In Vitro and In Sacco Methods
These laboratory-based techniques simulate the animal's digestive process to predict digestibility, offering a balance between the accuracy of in vivo trials and the efficiency of chemical analysis.
- In Vitro (Test Tube) Digestion: The two-stage Tilley and Terry method involves incubating a feed sample with rumen fluid from a donor animal, followed by an acid-pepsin digestion phase. The undigested residue is then measured to determine digestibility. The Van Soest variation uses a neutral detergent wash to measure true digestibility.
- In Sacco (Nylon Bag) Digestion: The nylon bag technique places ground feed samples into mesh bags, which are then incubated inside the rumen of a fistulated animal. After a set time, the bags are retrieved, washed, dried, and weighed to determine the percentage of material that disappeared. This provides data on the rate and extent of digestion.
Comparison of TDN Measurement Methods
| Method | Accuracy | Cost | Time | Resource Requirements |
|---|---|---|---|---|
| In Vivo Digestion Trial | Very High | Very High | Long | Live animals, specialized facilities |
| Laboratory Analysis (Predictive) | Moderate to High | Low | Short | Standard laboratory equipment |
| In Vitro Technique | High | Moderate | Moderate | Rumen fluid from donor animal, lab setup |
| In Sacco (Nylon Bag) | High | Moderate | Moderate | Fistulated animal, lab and animal facilities |
Conclusion
While TDN has been a long-standing method for estimating feed energy, understanding how is TDN measured is key to appreciating its strengths and weaknesses. The most accurate method remains the direct in vivo digestion trial, but for routine and practical applications, laboratory methods using predictive equations based on proximate or fiber analysis are the most common. Modern in vitro and in sacco techniques provide a robust compromise, simulating animal digestion more closely than chemical equations alone. Users of TDN must be aware of its limitations, especially its tendency to overestimate the energy of high-fiber roughages and its inability to account for all energy losses, which more modern systems like Net Energy address. By understanding the methodologies behind TDN, livestock managers can better interpret feed analyses and make more informed decisions about animal nutrition.
