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0707 PD: Forage testing: Why, how and where

David J. Trotter and Keith D. Johnson Published on 06 July 2007

Forage testing assesses the nutrient composition of forages and permits a producer to develop feeding and marketing strategies. This [article] has been developed to inform livestock or cash-crop hay producers of the benefits of forage testing. Information on different analytical procedures, terminology, sampling procedures and the application of results are presented.

Forage quality defined

Since forages are predominantly used by livestock as a source of nutrition, forage quality is an expression of the characteristics that affect consumption, nutrition value and the resulting animal performance.

Factors influencing forage quality

Since many factors affect forage quality, no single factor can be used to make this prediction. Maturity stage at harvest, forage species and variety, leafiness, harvest and storage conditions, and the presence of pests are important factors that determine quality.


As forage plants mature, the concentration of structural carbohydrates and lignin increases. Structural carbohydrates, the plant’s fiber component, include cellulose and hemicellulose. While cellulose and hemicellulose can be partially digested by ruminant livestock and horses, another part of the fiber component, lignin, is not digestible. Research has shown an increase of one percentage unit of lignin will result in 3 to 4 percentage units decrease in digestible dry matter (DDM). As the amount of fiber increases and digestibility decreases, forage consumption is also reduced; therefore, the amount of digestible energy consumed decreases as the forage matures.

Species and variety

Forage quality differences between species and among varieties within a species are generally related to differences in structural carbohydrate consumption and digestibility.


Leaves contain more nonstructural carbohydrates and protein than stems. The nonstructural carbohydrates, which include plant sugars and starches, are highly digestible. When forage plants mature, the leaf-to-stem ratio decreases. Thus, forages with a greater amount of leaves are more likely to be of higher quality.

Harvest and storage conditions

Forage crops decrease in nonstructural carbohydrate concentration and digestible dry matter percentage during the curing process because of plant respiration. Decreases in digestible dry matter can also occur from the leaching of soluble nutrients during rainfall and the physical loss of leaves at harvest. Management techniques that minimize curing time (use of a mower-conditioner, drying agent or preservative) can minimize digestible dry matter loss.

Just as plant respiration reduces forage quality during curing, similar activity can cause quality changes during storage. If hay with excessive moisture is packaged and stored, mold may develop. Mold-producing organisms generate heat through respiration; the mold and heat reduce consumption as well as protein and dry matter digestibility.

Presence of pests

Diseases, insects, and some weeds have been responsible for reduced forage quality. Particularly noteworthy pests are leaf diseases and high populations of the alfalfa weevil and potato leafhopper.

Forage testing

Why conduct forage testing?

Forage testing is used to determine nutrient composition and potential animal performance. Once the factors that determine forage quality have been measured, a more accurate ration can be formulated for a specific class of livestock according to nutritional requirements for optimum performance. The significance of forage testing is illustrated in the following example.

In a dairy feeding trial, two groups of cows were fed different alfalfa rations during a 305-day lactation. The two alfalfa rations fed were two maturity stages, early-bloom and full-bloom. (The nutrient compositions of these forages and their associated milk yields are found in Table 1*.) The higher nutritive value of early-bloom alfalfa results in considerably more milk production from cows than does full-bloom alfalfa. These results clearly indicate the nutritional shortcomings of full-bloom alfalfa in the ration of lactating dairy cattle, especially those rations not properly supplemented.

Forage testing is used to indicate nutrient content of hay, silage or any other forage fed to livestock. Depending on the livestock class being fed, nutrient content of the forage may be adequate or inadequate for maintenance and production.

Methods of forage testing

Several methods of estimating or analytically determining nutrient content of forages have been used for many years. New forage testing procedures have also been developed recently. Some methods for determining forage quality include visual appraisal, chemical analysis, near infrared reflectance spectroscopy, plus in-vivo and in-vitro measurements. Each method has strong and weak points as well as specific uses in forage evaluation.

Visual appraisal

Visual appraisal is the oldest and most widely used forage evaluation method. The term visual appraisal is not limited to the sense of sight; it is also based on texture and odor. Forage appearance is evaluated by color, leafiness, maturity and the presence of foreign material (mold, dust, and weeds). “Texture” refers to stem texture and its relationship to maturity stage at harvest.

Since visual appraisal relies heavily on subjective evaluation, it is possible no two people will evaluate the same forage in the same way. Factors such as the intended use and the evaluator’s past experience can greatly influence the evaluation, and then it becomes very difficult to communicate descriptive terms. Even though visual appraisal is relatively fast, color, texture and odor do not necessarily relate well to animal performance.

Chemical analysis

A truly meaningful assessment of forage quality requires an evaluation of its chemical makeup. Determination of the chemical components requires laboratory procedures. The chemical analysis of a forage will identify nutritionally strong and weak points; proper supplementation can then be made to ensure optimum animal performance. The main limitation of this method of forage testing is slow turnaround time and expense. These common complaints indicate the need for a method of forage analysis that would allow for more rapid return of results to the producer at a lower cost.

Near infrared reflectance spectroscopy (NIRS)

The use of NIRS to determine the major chemical constituents in forages is relatively new. This computerized method of forage testing, which is based upon accurate chemical analyses, can solve the slow turnaround problem of chemical analysis. The forage can be analyzed in less than ten minutes using NIRS, compared to hours and even days for conventional chemical characteristics in the near-infrared region of the spectrum. Thus, their identification and measurement are possible through mathematical relationships.

In-vivo and in-vitro measurements

In-vivo and in-vitro measurements determine the digestibility of dry matter and fiber. Due to the cost and the length of time required to perform these procedures, they are not used in routine analysis of forages for on-farm use. They are, however, used extensively by scientists for evaluation of forage quality. The main difference between these procedures is that in-vitro procedures are performed in test tubes that simulate the rumen.

Components of forages

A well-equipped forage testing laboratory can analyze forages for many components, but more tests will be required if a more complete analysis is desired. However, additional tests can increase costs and delay return of the results to the producer. Usually forages need only be analyzed for the nutritional information that will be useful to the producer. Components that should be tested on a routine basis are dry matter, crude protein, acid detergent fiber, and neutral detergent fiber.

Definitions of common forage analysis terms

Dry matter (DM) is the percentage of the forage that is not water. Nutrient requirements of animals are expressed on a DM basis.

Crude protein (CP) is the total amount of nitrogen contained in a forage; percent nitrogen is multiplied by 6.25 to obtain crude protein. Crude protein is a mixture of true protein and nonprotein nitrogen; it indicates the ability of the forage to meet an animal’s protein needs. It is expressed as a percentage of dry matter and includes both nutritionally available and unavailable nitrogen. Generally, the higher the CP percentage the greater the forage quality.

Insoluble crude protein (ICP) is the amount of unavailable crude protein, including protein which was permanently linked to nonstructural carbohydrates during heating of forage stored at an improper moisture. During fiber analysis, ICP is incorrectly measured as lignin, which can result in errors in data interpretation and ration balancing. Therefore, when heating of the forage is suspected, ICP should be analyzed.

Acid detergent fiber (ADF) is the percentage of highly indigestible plant material in a forage; it is comprised of cellulose, lignin, cutin, silica, pectin and unavailable protein. It is negatively correlated to digestibility and therefore can be used to calculate dry matter digestibility, digestible energy and total digestible nutrients. A low ADF value indicates greater digestibility.

Digestible dry matter (DDM) is an estimate of the percentage of the forage that is digestible and used as an estimate of energy content. It is determined from the ADF concentration.

Total digestible nutrients (TDN) is the sum of all digestible organic nutrients (protein and energy) of a forage available to the animal, usually expressed as a percentage of DM.

Neutral detergent fiber (NDF) is the percentage of cell wall material in a forage. Partially available to animals, it is comprised of cellulose, hemicellulose, lignin, cutin and unavailable protein. It is considered an indication of plant maturity and therefore related to DMI. A low NDF indicates greater forage intake by livestock.

Dry matter intake (DMI) is an estimate of the amount of dry forage an animal will eat with only forage being fed. It is expressed in terms of forage dry weight and is calculated by using the neutral detergent fiber and crude protein concentrations.

Relative feed value (RFV) is a way of assigning to a forage a single numeric value which reflects the sum total of several forage quality attributes. The RFV of a forage is calculated as follows:

•DDM multiplied by DMI gives the forage’s digestible dry matter intake (DDMI).

•DDMI is then divided by a constant of 1.29.

•The result is the RFV.

The reference forage has a RFV of 100. A high RFV (above 100) indicates a higher-quality forage. Relative feed value can also be used to assign quality standards used in forage marketing.

The analysis of forages for mineral content is not routinely performed. However, mineral analysis may need to be considered for livestock on a high plane of nutrition (e.g. a lactating dairy cow). In addition, the mineral data also serves as a guide to the effectiveness of your soil fertility program. Although vitamins are required by livestock in very small amounts, their availability in forages is quite variable. Forage analysis for vitamins is rarely performed unless livestock health problems are suspected. Routine additions of vitamins and minerals to rations are recommended to meet nutritional requirements.

Sampling – obtaining a representative sample

As with soil testing, a representative sample is needed for a meaningful analysis. One forage sample should be sent to the laboratory for each lot of forage. The term “lot” refers to a specific quantity of similar forage. Lot size may vary from a bale to a barn full of hay, or a wagonload to a silo full of silage. A lot for forage is a crop harvested from a particular field, at a specific cutting, within a two-day period of time.

The important consideration in determining the lot is uniformity. Factors that affect forage quality (plant maturity, species and variety, leafiness, harvest and storage conditions, and presence of pests) must be considered when dividing the crop into lots. The following example illustrates the importance of dividing forages into lots for testing and feeding purposes.

To insure a balanced ration for his herd, a dairyman wants to analyze the alfalfa-grass hay he has on hand. His first cutting yielded 600 bales. It was not harvested until the end of May, two weeks late because of competition with corn planting. While the bales were still in the field it rained. The second cutting was made 35 days later and yielded 400 bales which reached the barn without getting wet.

The summary of the two harvests indicates obvious quality differences. Therefore, the dairyman labels the 600-bale first cutting “Lot 1” and the 400-bale second cutting “Lot 2” and has them analyzed separately. Based on the results, he will formulate and feed a balanced ration using Lot 1, then rebalance the ration when ready to feed Lot 2.

Sampling procedures

If forage testing is to be of value, the samples must be taken at the right time and proper methods used to ensure accurate representation of the lot. The proper time to take forage samples is as near as possible to the planned time sale or feeding. An allowance of one week for NIRS and three for traditional chemical analyses should be considered. Proper sampling methods for different types of forages are discussed in the following paragraphs.

Conventional square bales

A bale probe should be used to collect samples. The probe should be approximately 12 to 18 inches long, hollow and at least 3/8-inch in diameter. Most probes are designed to attach to an electric drill or brace.

A minimum of 20 average-looking bales that represent the lot should be selected for sampling. Steps in obtaining samples from these representative bales are as follows:

1. Take one core drilling from an end of each bale. Insert the probe near the center of the bale’s end at a right angle to the surface.

2. Place drillings in a clean plastic bucket.

3. Thoroughly mix drillings.

4. Put about one quart of the mixed drillings in a plastic bag for laboratory analysis.

5. Repeat procedure for each lot, making sure not to mix drillings of different lots.

If a bale probe is not available, reach inside each sample bale and carefully remove a handful of forage. Cut each sample into 2-inch sections with a pair of shears. Mix all samples from a lot and again remove about one quart for analysis.

Large round bales and stacks

Ten sampling bales or compressed stacks from each lot should be selected. The bale probe is used to make drillings from two different locations from each selected bale or stack. Drillings should be made at the top of each stack, while bales should be probed form the sides (not the ends). Mix the drillings from each lot and remove about one quart for analysis. If a bale probe is not used, sample these large packages by hand as described for conventional square bales, with the exception that two or three handfuls should be removed rather than a single handful.

Loose hay

Samples should be taken from at least twelve random locations in the mow or on the stack. To make bale probe drillings, stand on the stack and insert the probe vertically where the hay is compressed between your feet. If a bale probe is not used, obtain samples by hand using the procedure suggested for large package hay. In either situation, about one quart of the total sampling will be needed for laboratory analysis.


Sampling silage presents different problems as compared to sampling hay. Differences in storage structures, feeding sequence and the presence of silo gases at various stages during the fermentation process must be considered.

A vertical silo will usually contain more than one lot of forage; therefore, sampling a lot before feeding begins is very difficult. While bunker, trench and plastic-covered piles of silage are somewhat easier to sample, they are often more variable in nutrient composition. Sampling a lot when filling the silo would be a way of getting representative samples for analysis. This sampling method, however, gives an indication of feeding value when the forage entered the silo, but may not provide an accurate assessment of its nutritional and dry matter composition after termination. The following is the suggested sampling procedure for silage.

Collect about two gallons of silage in a clean plastic container by taking handfuls at random from 20 different locations on the exposed face or surface (avoid rotted silage on top) or by passing the container beneath the chute several times (once a minute), collecting one or two quarts at each pass while the silage is unloading. Mix subsamples thoroughly and place about one quart in a plastic bag for analysis.

To identify different forage lots, several bales of straw, shaving or shredded paper can be fed through the blower when the last of each lot is ensiled. The presence of silo gases should be of utmost concern when obtaining silage samples during the fermentation phase.

Preparation of samples

The plastic bag containing the forage sample to be analyzed should be tied or taped shut and labeled with your name, address and the lot number. In the case of silage, double-wrap the sample for further protection. Most testing laboratories will provide a description sheet for sample identification. On this sheet include your name, address, forage description and lot identification, and tests desired.

If mailing the sample, be sure the plastic bag is airtight. Place this bag inside a rigid cardboard box, along with the description sheet. The rigid outer container will prevent the bag from being punctured during shipment. With silage samples, freeze the sample prior to mailing and mail early in the week to avoid weekend delays.

Application of forage testing results

Forage testing helps producers, nutritionists, veterinarians, and researchers make knowledge-based decisions. The following sections describe four important uses of forage testing results.

On-farm improvement of forage production and quality

One of the most common factors influencing forage quality is plant maturity. Forage testing allows producers to see how nutrient composition changes as the crop matures. This is accomplished by comparing the test results of a forage cut at a very mature stage to a similar forage harvested on a more timely basis. Forage testing could also help the producer better assess the need to renovate existing fields or establish new forage stands.

The application of drying agents and preservatives to enhance forage quality by reducing curing time can be evaluated by comparing test results of forage that was or was not treated. This evaluation can then be applied to management decisions in future years. Testing forages prior to storage is helpful in determining specific precautions needed to maintain quality. Nutrient losses during storage can be determined by testing forages prior to storage and again before being used. This procedure allows the producer to evaluate storage facilities and techniques.

Forage marketing

Forage testing makes marketing on the basis of feeding value possible. This practice aids the seller in pricing hay and provides valuable information to the buyer about how to use a forage most effectively in a livestock feeding program.

The National Forage Testing Association is working to bring the various hay testing and grading systems currently used into one common system. It has developed a national certification program for forage testing laboratories which utilizes a standard set of important forage analyses (DM, CP and ADF) and includes visual standards. The adoption of a national system will make interstate transfer of forages more common since descriptive terms will be uniform.

Forage testing in the marketplace has been limited because of the lack of a rapid analytical procedure. The recent development of NIRS has been shown to be a solution to this limitation. The NIRS procedure has been tested at hay markets and has provided accurate and rapid nutritive analysis without disrupting sales.

Forage testing is not intended to be a mandatory requirement for forage marketing. Instead, it should be considered a tool for negotiating the price of hay or silage. The costs of producing high-quality forage is greater than those associated with forage of low quality; therefore, high- quality forages should be of greater value to livestock producers if forages represent a large part of the ration. If improved forage quality results in increased animal performance and increased animal performance provides greater profits, the costs associated with forage analysis are easily offset by the benefits gained.

Ration formulation

In the past, producers have formulated rations from “book values.” While book values do predict average nutrient composition, they do not take into account many factors that affect nutrient availability and intake. Only forage testing can precisely evaluate the composition.

Using forage testing to formulate livestock rations takes into account two basic goals:

• development of nutritionally balanced rations

• development of least-cost rations

Forage testing provides information that helps determine the best use of a forage and what supplements, in any, are needed to achieve desired levels of animal performance. It is important to keep in mind different livestock classes have different nutritional requirements which must be met by the ration. Livestock classes are generally separated by intended purposes of production, production levels, bodyweight and condition score, and frame size.

In order for forage testing to be successful in ration formulation, samples must be representative of separate lots and analyzed by a reputable firm. Nutrient requirements and livestock data must be known, accurate and complete.


Traditionally, increased yields and improved disease resistance have been stressed in forage breeding programs. Forage quality has been a concern in breeding programs recently, but progress in improving quality through plant breeding has been slow. Forage testing can help identify germplasms and varieties superior in quality to current varieties marketed.

Forage testing is also used to monitor compositional changes of plants as a result of management practices. Nutrient composition is altered by fertilizer applications, pest control practices, harvest schedules, and storage systems. New technology and management practices can first be evaluated in research studies by using forage testing. This allows scientists the opportunity to investigate the cause-and-effect relationships that new management advances have had on forage quality, prior to recommendation for on-farm use.  PD

—Excerpts from Oregon State University Forage Information System website

*Table ommited but is available upon request to

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