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Fiber digestion, time and distance

Woody Lane, Ph.D for Progressive Dairy Published on 12 December 2019

I recently came across a magazine article about fiber that included the acronym ANDFOM. Huh? What did that mean? ANDFOM: “antifoaming agent”? or “and for older men?” Then I realized what happened – some of those letters should have been in lowercase, and the magazine editor’s spell checker “corrected” them all to uppercase.

Progress. In any case, the correct version is aNDFom, which means: neutral detergent fiber (NDF) corrected for starch by the amylase procedure (a), expressed on an organic-matter basis (om). A few months ago (Oct. 1, 2019, issue of Progressive Dairy) I wrote an article describing these codes. Maybe it’s time we continued this discussion with some additional codes, before the spell-checker software tries to correct them for us.

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The first set of codes focuses on the rumen digestibility of the fiber – namely, how much of the fiber will rumen microbes ferment while the feedstuff remains in the rumen? This is an extremely important concept in ruminant nutrition because the extent and type of fermentation which occurs in the rumen determines a major part of the nutritional value of fibrous feedstuff.

It’s like that old Shakespearean phrase: fiber is not fiber is not fiber (or at least Shakespeare probably would have said it if Hamlet had been a ruminant nutritionist). Feedstuffs differ in the composition of their fiber (NDF), especially in the amount and proportion of lignin. Lignin has zero digestibility, and as plants mature, the lignin proportion in fiber increases and thus reduces fiber digestibility. Grasses generally have higher levels of NDF than legumes like alfalfa and clover, but grass NDF is generally more digestible than legume NDF, primarily because it contains a lower percentage of lignin. Of course, this digestibility declines considerably as the grasses mature. The amount of lignin is also affected by other factors, including forage species, the genetic lines (varieties) within species and also environmental factors like heat and day length.

Nutritionists measure rumen digestibility of fiber by a fairly straightforward assay: fill a beaker with rumen fluid (alive with active rumen microbes), put the fiber sample in the beaker, let the whole thing ferment for a period of time and then measure the remaining undigested amount. Then they add a number to the NDF code that represents the number of hours of fermentation. The full code looks something like uNDFxx where u means undigested and xx is the number of hours of fermentation.

One logical quirk to numbers like uNDF30 is they represent the results of disappearance (i.e., the amount of undigested fiber). Human beings like to think in terms of positive amounts rather than negative amounts because we generally associate higher numbers with greater value, like ballgame scores and test scores (let’s not go into the sociology of golf scores here). So nutritionists have changed that number to report fiber fermentation results in a positive light – in terms of digestibility rather than disappearance. The NDF code usually found in current forage reports looks like “NDFDxx” where xx is the number of hours of fiber fermentation and NDFD is the digestibility of NDF for that period. Higher numbers are indeed better.

Therefore, on forage reports, you may see any or all of three standard values for NDF digestibility: NDFD24, NDFD30 and NDFD48 based on fermentation times of 24, 30, and 48 hours, respectively.

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Why are these numbers important? First, the value for NDFD48 is used in the formula for relative forage quality (RFQ), which is quickly replacing the older relative feed value (RFV). Second, forage researchers and private seed companies are beginning to use fiber digestibility values to identify superior genetic lines and develop new cultivars that provide better nutrition for pastures and hay. And third, fiber digestibility numbers are needed in some ration-balancing software, especially for high-producing dairy cows. Forage is critically important in these exquisitely balanced grain-based rations, and subtle differences in fiber digestibility can make major economic differences in the amounts of milk produced by these cows and the efficiency of that production.

This may be all fine and good, but here’s an unsettling thought: All this work on fiber digestibility has been focused on rumen fermentation, but what about the lower tract (the large intestine and the cecum)? Doesn’t fiber fermentation also occur in the large intestine? Yes, it certainly does. What about horses? Or elephants? Or kangaroos? These animals don’t have a rumen, but they definitely live quite well on forages, and they probably would be greatly upset if you applied rumen-based digestion values to them.

There is, however, a potential solution – and this is the last NDF code in our docket. Since fiber fermentation takes place in the rumen and also in the lower tract, shouldn’t we have a single number that represents the whole ball of wax, the sum-total digestion of the fiber as it moves from the mouth to the manure? Actually, we do: total tract NDF digestion (TTNDFD).

This is a new concept that has been developed over the past ten years, primarily by dairy researchers in Wisconsin. But interestingly, it may also be an old concept, retrofitted with modern chemistry and new understandings.

TTNDFD is a single number that describes the digestibility of NDF over the entire gastrointestinal tract, from beginning to end, expressed as a percent of NDF. Simply put, TTNDFD represents the percentage of NDF that disappears as the fiber passes down the entire tract.

This is a very powerful concept, and it covers a lot of bases. TTNDFD essentially incorporates four main concepts:

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1. The percentage of the fiber which potentially can be digested as it moves down the tract

2. The rate at which the microbes digest this fiber, wherever those microbes live in the GI tract

3. The rate of passage of the fiber through the tract

4. The actual amounts of fiber fermentation that occurs in the GI tract, both in the rumen and in the lower tract

If we think about it, TTNDFD could apply to any plant-eating species, from dairy cattle to sheep to kangaroos.

So what are some typical TTNDFD values? So far, these numbers seem to average around 44%, which means that NDF is approximately 44% digestible for the entire GI tract. But this is just a raw average. Alfalfa TTNDFD can range from 20% to 60%, corn silage from 25% to 80% and cool-season grasses from 6% to 80%. Those are extremely wide ranges, and the devil is in the details.

Plant genetics, soil fertility, forage maturity, heat units, level of intake, rumen pH, other feedstuffs in the diet including feed additives, maybe even animal genetics which relate to subtle differences in anatomy – this list can get very long, which makes researchers rub their hands in excitement at all the possibilities for good research.

TTNDFD is still in its infancy. I suspect, though, that we’ll see a lot more of it in the future.

But I’ll come back to the concept of a new concept. Is TTNDFD truly a new concept? Well, on the surface, it is. But in reality, it’s the recycling of the very old concept of digestibility.

In the late 1800s, researchers made the first measurements of nutritional value by measuring nutrients that went into the mouth and then measuring the residue that came out the other end. They labeled the difference as the digestible portion of the feed. We now know those numbers were inaccurate because of nutrient recycling and fecal contaminants, so we call them “apparent digestibility” rather than “true digestibility.” (There were many laboratory limitations at that time. For example, the crude fiber assay they used was not a valid estimate of plant fiber like NDF.)

But here in the 21st Century, what are we doing? We are measuring the amount of NDF that goes into the mouth and then measuring the undigested amount that comes out the other end – for the entire gastrointestinal tract. We call it the very modern term TTNDFD. Sure, today we have more sophisticated laboratory assays than a hundred years ago and a better understanding of fiber. NDF is definitely a valid nutritional entity, and its presence in manure is not contaminated by recycled compounds.

But isn’t the concept of digestibility the same? In essence, TTNDFD is really the true digestibility of NDF. Hmmm. I can see some of those old nutritionists smiling magnanimously and knowingly nodding their heads. Sometimes, what goes around comes around.  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 Lane.

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