What Are DCP and TDN?
Digestible Crude Protein (DCP) and Total Digestible Nutrients (TDN) are two fundamental metrics used in animal nutrition to assess the value of feedstuffs. Understanding these values is essential for formulating a balanced diet that meets an animal's specific needs for maintenance, growth, gestation, and lactation.
Digestible Crude Protein (DCP)
DCP represents the portion of the feed's total crude protein (CP) that an animal can actually digest and absorb. The crude protein value itself is an estimate based on the feed's nitrogen content, but not all of this protein is available for the animal to use. The DCP value provides a more accurate measure of the protein available for tissue repair, growth, and milk production.
Total Digestible Nutrients (TDN)
TDN is a measure of the energy content in a feedstuff. It is the sum of all digestible energy-providing components: protein, fiber, nitrogen-free extract (carbohydrates like starches and sugars), and a factor for fat, which provides more energy per gram. TDN is often expressed as a percentage of the feed's total weight. It serves as a straightforward estimate of the feed's energy value, though modern net energy systems can offer higher precision.
How to Calculate DCP (Digestible Crude Protein)
Calculating DCP requires a feed analysis to determine the crude protein (CP) percentage and the feedstuff's protein digestibility coefficient. The general formula is:
$DCP\% = CP\% \times Digestibility\ Coefficient$
If you have access to a lab report providing digestion coefficients, this calculation is very simple. For example, if a feed has a 16% CP and a digestibility coefficient of 75% (or 0.75), the DCP would be:
$DCP\% = 16\% \times 0.75 = 12\%$
Alternatively, many reference tables based on extensive animal trials provide average DCP values for common feedstuffs. For a more complex approach in specific research settings, formulas involving metabolic fecal nitrogen can be used, but these are generally for more specialized purposes.
How to Calculate TDN (Total Digestible Nutrients)
Calculating TDN is a summative process that combines the digestible portions of the major nutrient groups. The classic formula based on proximate analysis is as follows:
$TDN\% = DCP\% + Digestible\ Crude\ Fiber\% + Digestible\ Nitrogen-Free\ Extract\% + (Digestible\ Ether\ Extract\% \times 2.25)$
Example calculation using an estimated proximate analysis:
- Obtain Proximate Analysis Data: Assume a lab analysis provides percentages for Crude Protein (CP), Crude Fiber (CF), Ether Extract (EE), and Ash. Nitrogen-Free Extract (NFE) is calculated by difference: $100\% - (Water\% + CP\% + CF\% + EE\% + Ash\%)$.
- Apply Average Digestibility Coefficients: Use standard average digestibility coefficients for each component (e.g., CP: 75%, CF: 50%, EE: 90%, NFE: 90%).
- Sum the Digestible Parts:
- Digestible CP = CP% x 0.75
- Digestible CF = CF% x 0.50
- Digestible EE = EE% x 0.90
- Digestible NFE = NFE% x 0.90
- Calculate TDN:
- $TDN\% = (Digestible\ CP) + (Digestible\ CF) + (Digestible\ NFE) + (Digestible\ EE \times 2.25)$
A Simplified Calculation for Forages (using ADF)
For forages, a simplified method can estimate TDN using the Acid Detergent Fiber (ADF) percentage, which is a common lab measurement. The formula is:
$TDN\% = 88.9 - (0.779 \times ADF\%)$
This method is a reliable shortcut for estimating the energy value of roughages, though it's less accurate for complex concentrate mixtures.
Practical Application in Ration Balancing
Calculating DCP and TDN is the first step toward building a balanced ration. The next steps involve matching these values with the animal's nutritional requirements, which vary based on its body weight, age, and stage of production (e.g., maintenance, growth, lactation).
Steps for Feed Formulation
- Determine Dry Matter (DM) Intake: Estimate the total DM an animal will consume. This is typically a percentage of its body weight.
- Determine Animal's Nutrient Requirements: Consult feeding standards (like those from the NRC) to find the required DCP and TDN for the animal's specific status.
- Analyze Available Feeds: Obtain lab analysis or use reference tables to find the DCP and TDN values for your feedstuffs.
- Calculate Nutrient Contribution: Multiply the weight of each feed by its DM percentage to find DM intake. Then multiply the DM intake of each feed by its DCP and TDN percentages to find the total nutrient contribution.
- Balance the Ration: Adjust the mix of different feeds (roughages and concentrates) until the total DCP and TDN provided by the ration meet the animal's requirements.
Comparison of TDN Calculation Methods
| Feature | Proximate Analysis Method | ADF-Based Method (for Forages) |
|---|---|---|
| Basis | Sum of digestible crude protein, crude fiber, NFE, and fats | Single correlation with Acid Detergent Fiber (ADF) |
| Inputs Required | Full proximate analysis (CP, CF, EE, Ash, Water) and digestibility coefficients | Acid Detergent Fiber (ADF) analysis |
| Typical Use | More comprehensive for a wide range of feedstuffs, including complex concentrates. | Quick, cost-effective estimation primarily for roughages like hay and silage. |
| Accuracy | Generally more accurate for mixed rations and diverse feeds | Can be less accurate for feeds with high concentrates or specific nutrient variations. |
| Labor/Cost | Higher, as it requires a full proximate analysis and digestibility data | Lower, as ADF is a standard and less intensive analysis. |
Conclusion
Knowing how to calculate dcp and tdn is a foundational skill for anyone involved in livestock production. These metrics provide a practical and effective way to evaluate feed quality and formulate nutritionally balanced rations that support animal health, performance, and overall productivity. Whether using detailed proximate analysis or a simpler ADF-based estimation for forages, understanding these values is key to sound feeding management. By applying these calculation methods, producers can make informed decisions, optimize feed resources, and achieve desired production goals.
For further reading on nutrient requirements and feeding standards, consult authoritative sources such as the National Research Council (NRC) publications. A deeper understanding of these standards, combined with accurate feed analysis, ensures a more precise approach to animal nutrition.
