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Forage fiber may not be a dinner table topic, but it’s important to understand

Woody Lane, Ph.D. for Progressive Dairyman Published on 12 September 2016

In an article earlier this year (“Fiber site map: Something to hang your hat on”), I outlined the general system of the modern fiber analysis and described two main values from that analysis: neutral detergent fiber (NDF) and non-fiber carbohydrates.

This month, let’s look at the other main fiber value and also an array of useful numbers we can derive from it.

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The basic procedure for analyzing a forage sample for ADF is to dry it, grind it and then boil it in a beaker containing special detergent solution that has been carefully adjusted to an acid pH. This solution is logically called “acid detergent,” and the fibrous substance that remains after the boiling process is called “acid detergent fiber” (ADF). This is the ADF value you see on forage test reports.

When you study a forage test report, one thing should be immediately obvious: ADF is always smaller than NDF. The reason is that ADF contains fewer components than NDF. If you remember from my previous article, NDF represents the entire cell wall of the plant and consists primarily of cellulose, hemicellulose, lignin, cutin and some minor fibrous compounds.

In contrast, the acid in the ADF procedure dissolves the hemicellulose, leaving nearly all the other fibrous components. Which means that, with minor exceptions, ADF contains everything in NDF except the hemicellulose. This also means we can easily calculate the amount of hemicellulose in a forage by subtracting ADF from NDF.

For example, a forage with 62.5 percent NDF and 32.8 percent ADF contains 29.7 percent hemicellulose (= 62.5 - 32.8). Well, OK, perhaps this number won’t become a hot topic at your family supper table, but it can be handy when you need it.

One of the beauties of the detergent system is: We can use it to run sequential procedures. Which means we can take a fiber residue like ADF from one part of the system and then run procedure X or Y or Z on it, and each of these procedures will give us additional information about the nutritional makeup of the forage.

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So let’s start with the ADF residue – the fibrous mass at the bottom of the beaker – and test that residue for nitrogen. This means running the classic Kjeldahl procedure (pronounced “Kell-dall”) involving lots of glassware and boiling liquids (although modern labs now use automated equipment for this assay). The resulting number is the amount of nitrogen in the sample.

Which means exactly what? If this nitrogen value were derived from the original forage sample, we would multiply it by 6.25 to give us the amount of crude protein in the entire forage. But remember, we started with ADF, not the entire forage.

Since we don’t expect much true protein in ADF, any nitrogen here is probably bound into compounds with little nutritional value. Therefore, if we want to know the real amount of crude protein available to an animal, we should subtract the ADF nitrogen from the total nitrogen of the feed. Read on.

Nitrogen in ADF generally occurs in one of two forms, which are both indigestible: Maillard products and leather. Leather? Well, not very much. Even if some forages contain lots of tannins, we really don’t expect plants to produce much leather (unless, of course, they are leather plants ... get it?).

On the other hand, Maillard products are quite common. Maillard products are the gooey, indigestible polymers formed from carbohydrates and proteins during heat damage – as in wet hay or poorly made silage. The nitrogen in ADF is called, logically, “acid detergent insoluble nitrogen” (ADIN). Multiplying this nitrogen value by 6.25 gives us a number that appears in forage reports as “heat-damaged protein.”

If this number is large compared to the total crude protein, then you know the forage experienced a significant amount of heat damage, and you should reduce the value of total protein by the amount of heat-damaged protein.

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For example, if a hay sample contains 13 percent total crude protein with 2.5 percent heat-damaged protein, then you should use 10.5 percent in your ration-balancing calculations, not 13 percent.

Here is another sequential procedure with ADF: We can burn our ADF residue in an oven and then bathe the resulting ash in a strong solution of hydrogen bromide. (Note: do not try this at home.) Hydrogen bromide is an extremely strong acid that dissolves all the minerals except silica. Silica is abundant in the soil, of course, and some plants incorporate it into their fiber to improve rigidity.

Although silica is not digestible, it can still influence nutrient digestibility, so the amount of silica in the forage is a good thing to know. But silica can also come from soil contamination, so the silica number should be viewed with a grain of salt.

Now consider a matched pair of procedures that analyze for three important types of fiber: cutin, cellulose and lignin. Cutin is a complex, waxy, indigestible substance found in many leaves and stems. Cellulose is a very large molecule that is a polymer chain of glucose units, strung together in long fibrous strands.

Lignin, on the other hand, is composed of rigid ring structures that are exceptionally stable and strong – so strong, in fact, that lignin cannot be digested by mammals because there are no digestive enzymes in the rumen or lower tract that can break it apart. But in the laboratory, we can use some chemicals that are not exactly found in gastrointestinal tracts.

One such compound is sulfuric acid (H2SO4). A concentrated solution of 72 percent sulfuric acid will dissolve cellulose. If you don’t believe me, consider this: What happens when you are working on a car and spill battery acid on your jeans? Your jeans now become holey. Why? Because strong acids dissolve cellulose, jeans are made of cotton, and cotton is cellulose. (That’s why you should wear wool when you work with car batteries.)

The other compound is potassium permanganate (KMnO4), an extremely strong oxidizing agent, which means it can break up those ring structures of lignin and therefore dissolve it.

Let’s use these compounds in two different sequential procedures. In both procedures, we begin with the ADF residue and then run an NDF analysis on it. This NDF step removes some interfering factors and gives us a residue that is analytically cleaner for our purposes than either ADF or NDF alone. Trust me on this.

Now for sequential procedure one: We first treat this two-step residue with 72 percent sulfuric acid, which removes the cellulose and leaves a substance containing lignin and cutin, which we weigh. Then we treat this residue with potassium permanganate, which dissolves away the lignin. Now all we have left is the cutin.

But think for a moment – we have just derived the values for two types of fiber. We know the amount of cutin because we can weigh it in the beaker, but we also know the amount of lignin. How? By subtraction. The pre-permanganate residue contained both lignin and cutin, but the final residue contained only cutin. Therefore, the substance that dissolved into the permanganate was the lignin.

Still following me?

Here’s sequential procedure two: We do the same thing as outlined in the previous paragraph, but in reverse order – first, the potassium permanganate, then the sulfuric acid. As before, the final compound in the beaker is cutin, but this time the subtraction gives us the amount of cellulose. OK, I’ll let you diagram the details by yourself.

Fiber analysis for livestock nutrition has indeed come a long way from the crude old days of crude fiber.  end mark

Woody Lane, Ph.D., is a livestock nutritionist and forage specialist in Roseburg, Oregon. He operates an independent consulting business and teaches workshops across the U.S. and Canada. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available online. Email Woody Lane

Woody Lane, Ph.D.
  • Woody Lane, Ph.D.

  • Lane Livestock Services
  • Roseburg, Oregon
  • Email Woody Lane, Ph.D.

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