by: Stephen B. Blezinger
Ph.D., PAS

Part 3

Forages, especially forages of good quality are the main asset of any cattle operation. Forages affect individual animal performance regardless of age, sex, type, class, etc. As discussed in Part 2 of this series, the available nutrients that a forage carries affects individual animal production (i.e. gain per animal) while the amount of forage produced affects total production per acre. All forages possess a mixture of chemical, physical, and structural characteristics that determine the quality of that pasture or the accessibility of nutrients to that animal. Quantifying these characteristics helps us determine overall forage quality when considered in combination with the animal's performance. The following discussion will complete this series and is adapted in large part from a paper by Newman et al, Texas A&M University.

What is Meant by Forage Quality?

In grazing operations, where forages are the main diet component, forage quality of a pasture, hay or silage is determined by animal product (reproductive performance, milk production, pounds of beef, performance in a horse, etc.). Assuming the animal has the genetic potential, animal production of forage-based diets depends on the nutritive value of forage consumed. In other words, the crude protein concentration, available energy, minerals available in the forage tissue, but equally important, animal performance depends on the intake of the forage. Intake is conversely dependent on forage quality.

In instances where grazing management decisions, intentional or unintentional, result in overgrazed pastures (usually, prolonged high grazing density), the opportunity to select plant species or plant parts of higher nutritive value decreases. As a result, forage intake by grazing animals declines. When pastures are under-stocked, the initial nutritive value of the pastures can be adequate and even in excess of animal requirements; however, under high stocking rates the animal's ability to select forages diminishes over time and the amount of forage available also decreases. In overgrazed situations, management creates scarce forage by stocking too many animals; thereby, causing consumption per animal to decrease because the forage resource is in short supply. This is also an effect noted during periods of drought where plant growth does not keep up with consumption. In either case, the resulting effect is that fewer nutrients are consumed per animal.

What Drives Changes in Forage Quality?

Not every plant in a given pasture or field will have the same nutritive value. Even in pastures that are developed, having a very consistent plant population and species type there is variation in soils and growing conditions that can affect nutritive value. Additionally, there are different plant characteristics that directly or indirectly affect forage quality. The main factors affecting quality of a stand are maturity and weathered conditions. Maturity or stage of growth is the principal factor responsible for declining forage nutritive value. As the plant advances in growth beyond the first couple of weeks (where protein and digestibility are at its highest), stem growth develops as well as accumulation of various fiber components at the plant cell level. As the plant ages, one of the main “chemicals” deposited internally in the plant cell walls is lignin. Lignin is a component of fiber that is essentially indigestible, accumulates mostly at maturity, and strongly resists degradation by rumen microbes. The microbial population in the rumen is responsible for degrading the forage fiber and making it available to the animal. If the forage is too mature, less digestible fiber components increase and digestibility of the forage declines. This is also true of crude protein (CP) in the forage tissue. This decline is more pronounced and sudden in warm-season perennial grasses older than 35-40 days. Table 1 illustrates the sharp decline in digestibility and crude protein of Coastal Bermudagrass after week 5 (35 days) along with an increase in fiber (ADF and lignin).

Yet another major factor affecting forage quality is weather. Poor storage and harvest conditions can also lead to losses of soluble carbohydrates (sugars) due to weathered forage. This can include excessive or over-drying of hays or silages, prolonged exposure of hays to the elements after baling, etc. When a plant is cut, it does not immediately stop functioning. Forages that are harvested and not properly dried continue to respire, and with respiration a decrease in soluble sugars occurs.

What is in a Forage Analysis?

So, as producers, how do we evaluate or determine forage quality as it affects the cattle operation?

Because the forage plant characteristics are primarily sensitive to changes over time (in some cases quite rapidly), timely and regular analyses of forage are required to know if the forage meets the daily nutritional requirements of the animals. The analytical data also provides the information needed to “build” an effective supplementation program. Commercial laboratory analyses (wet chemistry or near infrared - NIR) include measurement of moisture, protein, and fiber (see Table 2 - Soil, Water, and Forage Testing Laboratory, Texas A&M University. http: // Dry matter intake and energy or total digestible nutrients (TDN) cannot be measured directly because this requires testing with the actual animal which may not be practical for all commercial laboratories. There are some labs, however, that do employ actual animal models for these measurements or more extensive digestibility analysis. These analyses are more expensive but produce very eye-opening results.

Commonly used values such as total digestible nutrients (TDN), net energy (NE, maintenance, gain, lactation), metabolizable energy (ME) and intake are estimated from equations based on repeated animal research results and are thus based on calculations. Two other indices commonly used to represent forage quality are relative feed value (RFV) and relative forage quality (RFQ). Unfortunately, these are often misused when it comes to warm-season forages.

The common nutrients in forages found on a forage analysis report include:

1. Moisture. Moisture content is reported usually as wet and dry matter (DM) basis. Wet basis is valuable since it provides a reference to how much ‘fresh' forage would be required to meet the dry matter (DM) requirement of the animals. Dry Matter, is calculated with all the moisture taken out of the sample and is used to make valid comparisons among different forages. Forage moisture will vary depending on how the forage is fed as shown in the table below.

2. Energy. For ruminants energy is one of the most misunderstood nutrients. The main sources of energy for ruminants are the products of carbohydrate fermentation in the rumen, i.e.:

Forages      Fiber      cellulose, hemicellulose       glucose, xylose, etc.       VFA's

The end products of microbial fermentation of these carbohydrates in the rumen are volatile fatty acids (VFAs). These once in this form, they are absorbed through the rumen wall and used as building blocks for energy in the body. These energy sources are used for growth, milk production, basic bodily functions, etc.

Forages have two basic types of carbohydrates: 1) those inside the cell (soluble carbohydrates or sugars which are highly digestible by the rumen microbes), and 2) those more resistant to degradation, usually part of the cell wall (which consist of fiber components, subject to limited or partial degradation by rumen microbes). Total digestible nutrients is an indicator of concentration of available energy. It is calculated by adding digestible protein, digestible crude fiber, digestible nitrogen free extract, and 2.25 times the digestible fat content. Although TDN has been in use for many years and in many cases has been replaced by other calculated values, this value is still easily understood and widely accepted as a measure of nutritive value by the cattle industry. Total digestible nutrients vary with maturity; the more mature the forage the lower TDN value. Values of TDN also vary with forage type:

• Alfalfa (60-70 percent)

• Cool-Season Grasses/Clovers (55-68 percent)

• Warm-Season Grasses (45–65 percent)

Examples of TDN values for specific forages include:

• Common bermudagrass, 55-65 percent (for 28-30 days old)

• Common bermudagrass, 40-45 percent (for mature, low quality forage)

• Prairiegrass hay, 45-60 percent (depending on maturity)

• Pearl millet, 70 percent

• Kleingrass, 70 percent

3. Crude protein. Proteins together with energy are the most important nutrients for livestock as they support rumen microbes that in turn degrade forage. True proteins make up 60-80 percent of the total plant nitrogen (N), with soluble protein and a small portion of fiber-bound N making up the remainder. Forage protein concentrations vary considerably depending on species, soil fertility, and plant maturity.

Crude protein (CP) is measured indirectly by determining the amount of N in the forage plant and multiplying that value by 6.25. The assumption is that N constitutes about 16 percent of tissue protein in the forage (100/16= 6.25). Ruminant CP requirements are influenced by the physiological state of the animal (production stage, age, etc.)

4. Fiber. As discussed, fiber refers to the cell wall constituents of hemicelluloses, cellulose, and lignin. Fiber extraction in forages is accomplished with the detergent analyses system, and is presently the most widely used system for analyzing forages. A point to note, however, is that this system does not measure digestibility.

a. Neutral Detergent Fiber (NDF): The NDF values represent the total fiber fraction (cellulose, hemicellulose, and lignin) that make up cell walls within the forage tissue. NDF values can vary by plant species from 10 percent in corn grain to 80 percent in warm-season grass straw. Values of NDF for grasses will be higher (60-65) than for legumes (45-45). A high NDF content indicates high overall fiber in forage. Generally, the lower the NDF value the better.

b. Acid Detergent Fiber (ADF): The ADF values represent cellulose, lignin and silica (if present). The ADF fraction of forages is moderately indigestible. Forages range from 3 percent in corn grain to 50 percent ADF in warm-season grass straw. Animals and laboratory testing have shown that high ADF values are associated with decreased digestibility; therefore, a low ADF is desired.


Defining and understanding forage quality and nutritive value is important to the producer and the nutritionist alike. With so many variables, it is critical to understand when, where and how forage quality changes and how to compensate for these changes at any point in the production year. In the final pat of this series we will continue this discussion of forage quality and evaluate relative feed values and other indicators that can be used to predict performance and determine what steps may need to be taken to compensate for changes.

Dr. Steve Blezinger is a management and nutritional consultant with an office in Sulphur Springs, TX. He can be reached at or at (903) 352-3475. For more information please visit us on at www.facebook/reveillelivestockconcepts.

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