what is the formula to calculate tdn in feed

Lussy04

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23-01-2025

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Understanding Total Digestible Nutrients (TDN) is crucial for anyone working with animal feed. TDN represents the total amount of energy available to an animal from a feedstuff after accounting for losses during digestion. This article will explore the formula and methods for calculating TDN in feed. Knowing how to calculate TDN allows for accurate feed formulation and better animal nutrition management.

Understanding the Components of TDN

Before diving into the formula, it's vital to understand the components that contribute to the TDN value. TDN is the sum of digestible organic matter, expressed as a percentage of the dry matter of the feed. This includes:

• Digestible Crude Protein (DCP): The amount of protein the animal can actually utilize.
• Digestible Carbohydrates: This includes digestible fiber (like cellulose and hemicellulose) and non-fiber carbohydrates (like starches and sugars).
• Digestible Fat: The amount of fat that the animal can digest and absorb.
• Digestible Ether Extract (DEE): Represents the fat content in the feed. Note that this is often expressed as a percentage, but the contribution to TDN calculations usually requires conversion to a digestible energy value.

The Formula for Calculating TDN

There isn't one single universal formula for calculating TDN. The calculation method varies based on the available data and the specific analytical approach used. However, a common approach is based on summing the digestible fractions:

TDN (%) = DCP (%) + Digestible Carbohydrates (%) + DEE (%) x 2.25

Important Considerations:

• Digestibility Coefficients: Accurate determination of TDN relies heavily on accurate digestibility coefficients. These coefficients represent the percentage of each nutrient (protein, carbohydrates, fat) that's digestible by the animal. These are often determined through digestion trials. Several databases exist offering digestibility coefficients for various feedstuffs and animal species.

• Conversion Factors: The factor of 2.25 used for DEE is an approximate energy conversion factor. The actual value may vary depending on the specific fatty acid composition of the feed and the animal species. Some calculation methods might use a different conversion factor, potentially closer to 2.25 for ruminants and slightly less for monogastrics.

• Analytical Methods: The accuracy of the TDN calculation depends heavily on the methods used to analyze the feed's nutrient composition. Different labs might use slightly different procedures, leading to variations in results.

• Feedstuff Variation: Keep in mind that the nutrient content, and thus the TDN value, of a feedstuff can vary considerably based on factors such as growing conditions, harvest time, and storage methods.

How to Obtain Digestibility Coefficients and Nutrient Data

Digestibility coefficients are often obtained through laboratory analysis of the feed using in-vivo or in-vitro methods. In-vivo methods involve feeding the feed to animals and measuring the nutrients that are absorbed. In-vitro methods are laboratory procedures that can estimate digestibility, but they are less precise than in-vivo methods.

You can often find digestibility data in:

• Published Research: Peer-reviewed scientific articles and publications often contain detailed nutrient composition and digestibility data for various feedstuffs.
• Feed Composition Databases: Numerous online databases (like those from agricultural universities or government agencies) provide information on nutrient content and digestibility for various feeds.
• Commercial Feed Analysis Laboratories: These labs can perform detailed analysis of your feed to determine its nutrient composition and TDN value.

Example Calculation

Let's assume we have the following digestible nutrient data (as percentages of dry matter) for a particular feed:

• DCP: 10%
• Digestible Carbohydrates: 60%
• DEE: 5%

Using the formula above:

TDN (%) = 10% + 60% + (5% x 2.25) = 70% + 11.25% = 81.25%

Therefore, the TDN of this feed is approximately 81.25% of dry matter.

Conclusion

Calculating TDN requires a detailed understanding of the feed's nutrient composition and the digestibility coefficients for each nutrient. While the formula provided offers a common approach, the specifics can vary based on factors like analytical methods and feed composition. Always consult reliable sources for digestibility data and consider potential variations due to feedstuff characteristics and analytical procedures. Accurate TDN determination plays a crucial role in effective animal nutrition management and optimizing feed formulation strategies.