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The NDF alphabet

Woody Lane, Ph.D. for Progressive Dairy Published on 27 November 2019

This month, we’ll talk about the new alphabet of fiber: a, pe, om and ICP. Don’t worry, you haven’t wandered into an academic spelling bee. But seriously, have you recently submitted a forage sample to a laboratory?

Many lab reports now include quite a few codes that were not there years ago, especially concerning the values for fiber. Let’s visit about what they mean. Hang on, this is a wild ride.

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The key fiber number, upon which all this is based, is the granddad of all the modern fiber values: NDF, short for neutral detergent fiber. To get this number, a feed sample is boiled in a specially formulated detergent solution that’s buffered at (you guessed it) the neutral pH of 7.0. Soluble compounds in the feed dissolve into the liquid. The residue at the bottom of the beaker is the fibrous NDF, which then can be filtered and analyzed. NDF contains most of the fiber substances in the plant, particularly the major fiber compounds of cellulose, hemicellulose and lignin, as well as some other non-digestible or inert substances like heat-damaged protein, cutin, silica, etc. Nutritionists agree NDF is a good representation of the fiber portion of a feed, and it has been used extensively in formulas to estimate feed intake and forage value.

But this raw NDF value is not perfect. NDF often contains substances that are not true plant fiber. These substances either do not ferment in the rumen or ferment differently than fiber, and they can skew the results of the ration-balancing programs that depend on NDF. Ideally, we would like NDF to represent only the true fiber portion of the feed which is potentially fermentable in the rumen. NDF helps us better understand the microbial environment in the rumen and the large intestine. It makes ration balancing more accurate and helps nutritionists and livestock managers better manage rumen fermentation and avoid low-fiber problems like acidosis and milkfat depression. But those pesky substances get in the way. They inflate the NDF value, which misleads us about the fermentation potential of the diet and impairs our estimates of rumen turnover times and fermentation rates. Two of the most important contaminants are dirt and starch. The good news is: Modern lab techniques can now correct for them.

NDFom

The first is dirt, which nutritionally is called ash (or minerals). Sure, all plants and other feedstuffs contain some minerals. We routinely see mineral values in lab reports and reference books. But extra minerals can also get into a forage in many ways, like during the harvest of hay or silage, or rain splash, or contaminants from heavy machinery, etc. Either way, we don’t want minerals in our NDF. We just want our NDF to be composed of organic matter, without the minerals. Laboratories correct for ash by cooking the NDF in an oven at 1,000ºF to 1,200ºF for two hours. This definitely destroys all the organic matter, leaving the ash. We can then obtain the amount of organic matter in the NDF by simple subtraction. Laboratories list this value on their reports as the “organic matter NDF” or “NDFom” for short.

aNDF

The second main contaminant is starch. Huh? You thought that starch is an easily available, easily digestible carbohydrate, more like sugar than fiber. That’s true, more or less. But starch is a complex compound. Some starches do not dissolve easily in neutral detergent solution, which means they end up on the bottom of the beaker just like true fiber and become part of the NDF number, even though they don’t have the same fermentation characteristics as true fiber. We can, however, successfully eliminate starch from NDF by including the enzyme “amylase” during the initial boiling procedure. Amylase breaks down starch during the NDF procedure so that no starch molecules end up in the residue. It took many years to identify a good commercial amylase product that was heat-stable and detergent-stable, but now the amylase correction is used by many laboratories. Lab reports list the value of amylase-corrected NDF as “aNDF” (the “a” is for amylase). As you’d expect, this correction is more important in feeds that contain a lot of starch, like grains and certain byproducts. But when a laboratory lists the term aNDF instead of just simple NDF, we can be assured the lab has made that starch correction.

Maillard

There is also a third substance we should identify. This is the indigestible dark-colored goo created in heat-damaged hay and silage. Technically, this material is a protein-carbohydrate polymer known as “Maillard product,” after a reaction by the same name. (The Maillard reaction is the cause of many barn fires.) This substance will not ferment in the rumen and is completely indigestible, but nonetheless its nitrogen is analyzed as part of the protein, and it also ends up as part of the NDF. But we can correct for Maillard products by testing the NDF for nitrogen and then expressing that nitrogen as the crude protein bound to the NDF. Still with me?

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In any case, feed reports will list this number as “NDICP” (neutral detergent insoluble crude protein). How useful is this? Very, because too much heat damage can severely reduce the nutritional value of the forage. By subtracting the NDICP number from the crude protein value, you can get the actual effective crude protein value available to the animals (which a laboratory report will usually list as “available crude protein”).

We’ve covered three main interfering substances: organic matter (NDFom), starch (aNDF) and Maillard products (NDICP). But before we leave this topic, we should describe one more NDF code that is important for understanding fiber and using NDF to balance rations.

peNDF

The famous writer Gertrude Stein once said, “A rose is a rose is a rose.” This may be a great turn of words for gardens and literary romances, but it’s not so accurate for fiber. Even when corrected for contaminants, “NDF is not NDF is not NDF.” Some forms of NDF are more effective than others at influencing rumen fermentation, and this is primarily related to particle size. Larger fiber particles will cause more chewing activity, which increases the amount of saliva. (Ruminants produce a large amount of saliva as part of their digestive process.) Saliva is very important to ruminants, as it contains buffers that stabilize rumen pH. Saliva also carries nitrogen and various minerals back into the rumen as part of the nutrient recycling system. Also, large fiber particles in the rumen slow down the rate of passage, which affects fermentation time and the digestibility of the diet.

How do we account for these differences? By identifying the amount of “physically effective fiber – peNDF.” We estimate this by measuring particle size. A feedstuff is passed through a series of sieves (screens) of various aperture sizes. Roughages with a high percentage of large particles are given full credit as effective fibers; feedstuffs with smaller particles are proportionally credited with lower scores. For example, a long-cut early bloom alfalfa hay may have a peNDF value of 40%. That same hay cut at medium length would have a peNDF of 36%. And finely chopped, its peNDF would only be 29% – a 28% drop in fiber effectiveness compared to long-cut hay.

PeNDF really acts like fiber in the rumen. The peNDF value in a forage test report gives nutritionists and farmers a more realistic picture of what occurs in the rumen.

This is not splitting hairs. We cut hay and silage at wildly different lengths, and peNDF gives us a tool to estimate the effects of these lengths. An extreme situation occurs when we put forage through a grinder. Grinding essentially destroys the physical effectiveness of fiber, even though the uncorrected NDF value remains high. Pelleting does the same thing because pelleting is really a two-stage process: First the feed is ground, and then it is pushed through a pellet die. Grinding results in a low peNDF value. A practical result is an increased risk of acidosis from a forage diet, even though that same forage in long form is completely safe.

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There are more codes on some forage reports, but let’s stop here for this month. Now the new alphabet makes a little more sense. So let’s use this knowledge. Let’s create a doozy code that we can use in emails to our friends: “apeNDFomICP.” I have no idea what this means, but it sure looks impressive.  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 through Woody's website.

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