Total Digestible Nutrients (TDN): How Useful Is It?


There are many methods of evaluating ingredients for animal feeding.  This is important because the composition and characteristics of the ingredients that make up a complete diet affect animal performance, and diet costs are ~70% or more of total production costs.

One evaluation method is called, “total digestible nutrients”, or TDN, though it’s popularity is waning, it still sees usage, especially in the beef industry.  It’s important to note that different versions of TDN exist (see p. 13 and beyond), based on the type of ingredient in question, so it’s important to understand which equation is being used when looking at the values.

A TDN value is expressed as a percentage.  The general calculations, of at least a few versions, are explained well in practice here.  The main idea is that the compositions and properties of major categories of nutrients are added up to give a score.  The categories are crude protein, crude fiber (or a specific measurement of fiber), fats and oils and nitrogen-free extract, each of which is individually corrected for total tract digestibility, while fats and oils, which are energy-rich, receive an energy factor correction.  Essentially, as the total tract digestibility of any component increases, the TDN will increase.  This will happen quicker with fats and oils due to the energy factor.  A higher TDN means that more energy is available for growth, at least in theory.

The benefits of using TDN are the following:

  • Higher amounts of moisture and ash, neither of which supply energy, will lower a TDN value – TDN will detect diluted ingredients.
  • Total tract digestibility provides some idea of how useful the energy in ingredients is for productive purposes. Undigested energy cannot support growth.
  • Having a TDN value is better than having nothing at all.

However, there are serious drawbacks to using TDN.  Several of them follow:

  • The category “nitrogen-free extract” is as much of a generic catch-all term as it sounds. Every ingredient has a different composition of nitrogen-free extract, with differing total tract digestibilities.
  • Crude protein is a generic category. It is the amino acids in proteins that affect animal performance.  One digestibility value doesn’t work; each will have a different value.
    • In addition, amino acid or protein digestibilities, determined over the entire digestive tract (rather than at the end of the small intestine), are often inaccurate, overstated, and the amino acids remaining here contribute little, or nothing, to total body protein (see here, chapter 12).
  • Other energy determination methods better predict animal performance, as I’ve blogged about before.
  • In fact, a study that looked at diets with different TDN’s reported that beef average daily gain, dry matter intake, and carcass characteristics were not affected by TDN.

So, the value of having a TDN figure for an ingredient or complete diet is very limited.  This is especially true for concentrates that have been improved through processing, such as corn that has undergone high-shear dry extrusion, or for any specialty ingredient.

Considerations to test ingredient quality should be tested using animal feeding trials, as Insta-Pro has done for over 50 years.

Speak with us about how to best value your ingredients in formulations.


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