Nutrition: TMR Corner
Published on Tue, 07/05/2016 - 2:30pm
By Dr. Alan S. Vaage Ph.D.
Most people understand that grains have more digestible energy than forages, and that fat or vegetable oil are even more concentrated sources of energy. But it is often unclear to many how “fiber” affects the energy content of feeds, especially with forages. The following is an attempt to clarify some important concepts and terminology when it comes to understanding fiber, and how it affects feed, and especially forage energy content.
Though it is often the most important and sought out number on a feed analysis, reported as TDN (Total Digestible Nutrients. %), DE and ME (Digestible or Metabolizable Energy, Mcal/lb.), or NEm and NEg (Net Energy of Maintenance (m) or Gain (g), Mcal/lb.,), there is no actual laboratory test that measures feed energy
per se. Virtually all estimates of feed energy are mathematical predictions based on the concentration of one or more of the analyzed components within the feed.
Essentially only three components contribute to feed energy: carbohydrates, proteins and fats. Carbohydrates and proteins contribute about the same amount of energy per unit digested, while fat contributes about 2.25 times that amount. In its simplest form, TDN is simply the sum of the amount of each component multiplied its digestion coefficient (digestibility) for that feed. Other energy values are based on mathematical relationships (regression) determined in animal feeding trials between the amount of one or more chemical components of the feed and actual energy production and utilization in the animal. For ruminants, the carbohydrate fraction known as fiber is often the most important and variable fractions in these equations, especially for forages.
Plant fiber composition
Plant fiber essentially comes from the rigid walls that surround each plant cell. Together, these cell walls give the plant its rigidity, ability to stand, and capability to carry out specific functions, such as movement of water and products of photosynthesis between the roots, stem and leaves. As with mammals, plants are comprised of different types of cells that are organized into specific tissues that vary in type and number depending on the species (e.g. monocots such as grasses, versus dicots such as alfalfa), as well as stage of maturity.
In general, plant cell walls are made up of three chemical components: cellulose, hemicellulose and lignin.
Cellulose: The main structure of the plant cell wall is made up of a crystalline matrix of cellulose fibrils that are comprised entirely of long chains of glucose, like starch, but the bond between the individual molecules is different, and requires a different enzyme, called cellulase, to break it apart during digestion. This enzyme, which is not present in mammals, is present in the microbial population that inhabits the rumen.
Hemicellulose: Hemicellulose is comprised of branching chains of various sugar molecules, and especially 5-carbon sugars. These chains intertwine between the cellulose fibrils to “glue” or cement the cell wall structure together. It is therefore necessary for an array of enzymes to be present to digest the hemicellulose, before the cellulase can access the cellulose and digest it. This slows the overall rate of cellulose digestion. Interestingly, cell walls of grasses contain about twice the amount of hemicellulose as do legumes.
Lignin: Lignin is a special class of carbohydrates classified as complex organic polymers that create crosslinking between the cellulose and hemicellulose in the cell wall in specific tissues. For example, the largest proportion of lignin is used to form a tissue called xylem that is part of the vascular bundles and responsible for the transport of water within the plant. It is also added to the cell walls in the stem and older leaves to add rigidity and help keep the plant upright. Lignin, however, is essentially indigestible by both mammalian and microbial enzymes in the rumen, and thus is detrimental to cell wall digestion. It is also distributed differently in monocots (grasses) and dicots (legumes like alfalfa).
Alfalfas versus grasses
Alfalfa: In alfalfa, lignin is mostly associated with larger organized vascular bundles in the stem, which branch out and become smaller and less intrusive as they travel into the leaves, in a similar way they do in trees. The remainder of the tissues in the plant generally contain little lignin and are highly digestible, especially the leaves.
Grasses: In grasses, the plant is predominantly comprised of leaves with numerous parallel rows of smaller vascular bundles that run from the tip of each leaf to its base, and into the root. This, along with the tendency for the older (lower) cells to increase in lignin content and undergo thickening, provides the rigidity for the plant to stand.
Thus, alfalfa cell walls will generally have a faster rate of digestion than grass cell walls, while grass cell walls will generally exhibit a greater extent of digestion.
Measuring plant fiber
Today, plant fiber is mainly measured by the detergent system first developed by Peter Van Soest in the 1960s. In its simplest form, this laboratory method breaks down and measures the plant cell wall components into the fractions you frequently see on a laboratory analysis: NDF: Neutral Detergent Fiber, ADF: Detergent Fiber, and Lignin.
Neutral Detergent Fiber: the NDF procedure removes the cell contents and leaves behind the plant whole cell wall.
Acid Detergent Fiber: The ADF procedure removes the cell contents as well as the hemi-cellulose fraction. The difference between the NDF and ADF values, is thus a measure of the amount of hemi-cellulose.
Lignin: The lignin procedure is designed to remove all organic material and acid soluble ash, and measure the amount of lignin associated with the cell wall. The difference between ADF and lignin is thus a measure of cellulose content. Lignin is often reported as a proportion of NDF content, as a measure of its inhibitory effect on cell wall or fiber digestion.
Utilizing fiber measurements
As mentioned earlier, laboratories use specific feed components to estimate feed energy values. Forage TDN values, for example, are generally calculated with a simple equation using an ADF value. In this case the equation should be for the specific type of forage (e.g. grass vs. legume), and should have been developed from research using representative forage for that area. The reason for this is it does not take into consideration the effect of factors such as degree of lignification. Unfortunately this is not the case with many laboratories and one can get predicted energy values that are of questionable value.
Another way to evaluate the quality and energy content of your forage is as follows:
• Evaluate the components: Identify the proportions of protein, fat, NDF and ash in the forage on a dry matter basis, then subtract them from 100 to get NFC: Non-Fiber Carbohydrate; NFC is a rough measure of the amount of sugars and starches together. As ash goes up, energy content must go down, as it contains no energy. As NDF goes up, energy must go down as it has a lower digestibility than NFC or protein.
• Look at lignin content: calculate lignin content as a proportion of NDF (NDFL: lignin DM/NDF DM *100%). As lignin concentration in the NDF fraction increases, within a forage type, digestibility of NDF and forage energy content will go down.
• Assess ADF content, adjusted for degree of cell wall lignification: Research has generally shown that ADF content is most closely related to forage digestibility, or energy content, while NDF content is more related to dry matter intake. I have found that forages with the same ADF content, and a “normal” lignin content for their type, will have roughly the same energy content regardless of type. That said, excellent quality forages that alone will meet the requirements of calving cows will be pre bloom and have an ADF content around 30-32%, good quality forage will have about 35% ADF, while fair to average (full bloom) quality forage will contain 40% or more ADF.
With an understanding of forage fiber and its effect on energy content, one can use ADF content, and the degree of cell wall lignification as a quality control measure to evaluate and improve their forage management, and thereby improve their TMR feeding program — Because Nutrition Matters.™
Dr. Alan Vaage is a Ruminant Nutritionist with over 30 years of experience in the beef industry, and currently provides technical support for Jaylor, in Orton, Ontario. Dr. Vaage can be contacted by email: firstname.lastname@example.org.