In 1970, when Keith Goering and Peter Van Soest published their USDA Handbook No. 379, “Forage Fiber Analyses (Apparatus, Reagents, Procedures and Some Applications),” they turned the nutritional world on its ear.
In one giant sweep, their new analytical procedures changed the way we look at fiber, forages and nutritional value, as well as rumen function, predictive equations and the entire array of nutritional aspects that guide our dietary decisions.
When laid out in a neat diagram, these procedures look like a logic tree with trunks and branches. Or, in current lingo, they look like a website diagram: main pages with links to subpages.
Many well-designed websites include a “site map” where visitors can view the entire structure all at once and see where information can be found. So let’s do the same thing with forage analysis: We’ll take a brief tour of the fiber site map.
First, some terminology: The key word is “fiber.” Although fiber is often a buzzword in the popular press, and lots of folks may think they are familiar with it, the reality is quite complicated.
Fiber is not fiber is not fiber. Plants contain different types of fiber, and each type functions and reacts differently. The main fiber types are cellulose, hemicellulose, lignin, pectin and cutin as well as a few other compounds that nutritionally act like indigestible fiber, such as silica and the heat-damaged protein-carbohydrate polymers called “Maillard products.”
Briefly, cellulose is a huge fibrous thread containing individual glucose molecules strung together in long chains – very long chains. Hemicellulose is a branched fibrous compound containing straight chains and also cross-linkages made of glucose molecules and other sugars.
Lignin is a radically different type of fiber – a stiff, rigid compound containing benzene rings, which are ultra-stable six-carbon circular structures with shared double bonds.
Benzene rings are so stable that, on one hand, they act as admirable “stiffening” components in fiber, but on the other hand, an animal’s digestive tract contains no intestinal enzymes strong enough to break these rings apart. This makes lignin virtually indigestible.
Two other common types of fiber are pectin, a complex fibrous compound found in high concentrations in some fruits and seedhulls, and cutin, a waxy indigestible substance that nutritionally acts like a fiber. Pectin is actually quite digestible under some conditions, while cutin is essentially indigestible.
One thing to remember: I’m keeping these characterizations relatively simple. In reality, fiber molecules are incredibly complex and variable, and these basic types of fibers are really families of compounds. But let’s reserve that complexity for the large textbooks. In this article, I’ll give you a general overview of fiber – something to hang your hat on.
Now for the analytical procedures. The ancient, popularized method of determining fiber is called crude fiber (CF), and it is an analysis of disappearance. A forage sample is first boiled in a weak acid solution (dilute sulfuric acid) and then boiled again in a weak alkali solution (dilute sodium hydroxide).
The residue, which contains fiber and minerals, is weighed and then burned in a furnace, destroying the fiber and leaving the minerals. The material that disappears during the burning is called crude fiber.
The main problem with crude fiber is that, nutritionally, it is a mess. The CF number does not represent the total amount of fiber in a feed or forage. The CF procedure systematically misses some of the hemicellulose and most of the lignin.
Since hemicellulose is a major component of most fiber, and the amount of lignin directly affects the digestibility of the total fiber, the CF value does not accurately portray the nutritional characteristics of a forage. In other words, crude fiber is too crude to be useful.
In contrast, most of the fiber procedures of Goering and Van Soest are analyses of detection, not disappearance. At each step, the material remaining in the container is weighed. Part of their brilliance in designing this system was that these researchers relied on special detergents to dissolve the components they didn’t want to measure.
They chose two different detergent solutions – one with a neutral pH (called neutral detergent) and one with an acidic pH (called acid detergent). In a sense, this detergent system of fiber analysis is kind of like the instructions for creating a stone sculpture of a horse: You start with a large rock and then chip away everything that doesn’t look like a horse.
Let’s begin with the most basic separation method: boiling the sample in a neutral detergent solution. The resulting residue is a fibrous mass logically called neutral detergent fiber (NDF). This is the NDF number you see on forage test reports.
This NDF value represents most of the true fiber in a sample. It includes all the cellulose, hemicellulose, lignin and cutin, as well as some other nutritionally indigestible compounds like Maillard products and silica. (Maillard products are the gooey black caramel polymers that occur in heat-damaged hay and silage. Silica is, well, the mineral in sand.
Many plants, like rice straw, naturally accumulate silica in their cell walls to improve fiber strength.
But silica has zero nutritional value, so it’s good to quantify it in the fiber portion of the analysis). NDF, in fact, is the most accurate measurement of the amount of cell wall in a sample.
Cell wall, as you may guess, contains nearly all the fiber in a plant. True, NDF does not include pectin, but pectin is a highly digestible fiber while NDF represents the types of fiber that can have great variation in digestibility or are not digestible at all.
NDF is a good number to know because of all the nutritional values in a test report, NDF is the number most highly correlated with forage intake.
This number is correlated in a negative way, of course – higher NDF numbers are associated with lower forage intakes. This makes sense when you consider that high fiber levels cause a sense of rumen “fill” and that an animal with a rumen full of fiber would be less willing to consume more forage than an animal with a rumen containing less fiber.
If NDF is the residue of the first separation procedure, what about the flip side of this analysis? What about the stuff that dissolves into the neutral detergent solution? This fraction is also very important nutritionally and represents another strength of the detergent fiber analysis system, so let’s discuss it now.
If NDF represents the cell wall of plant cells, then the portion soluble in neutral detergent represents the cell contents. This soluble fraction, commonly known as neutral detergent solubles, contains the sugars, starches, fats, pectin, soluble proteins and nonprotein nitrogen in the feedstuff, as well as some minor items like vitamins and secondary plant compounds.
In practice, everything in this soluble fraction is nearly 100 percent digestible, and the calculation is easy – just subtract the NDF number from 100. For example, if a forage contains 61.2 percent NDF, then its cell contents is 38.8 percent (= 100 – 61.2).
Perhaps you’ve heard of something called non-structural carbohydrates (NSC) or non-fiber carbohydrates (NFC)? These terms are part of the neutral detergent solubles and represent the amount of sugar plus starch in the forage.
Sometimes NSC and NFC are used interchangeably, but there are really slight differences that are debated among nutritionists. In essence, NSC is the result of the direct analysis of starches and sugars, while NFC is a calculated value based on a formula. Let’s not go into that controversy here.
In either case, NSC is important because it represents the carbohydrates that may ferment relatively quickly in the rumen, and this has implications on acidosis, the metabolism of nonprotein nitrogen in forages and the new high-sugar grasses. But more on this later.
Hmmm. We’re out of space. Well, in the next article, we’ll take apart NDF and its cousin, ADF, and show what these analyses can reveal about forage nutritional value. PD
Woody Lane 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 Lane Livestock Services.
ILLUSTRATION: By Corey Lewis.
- Ruminant Nutritionist and Forage Specialist
- Lane Livestock Services
- Email Woody Lane