Agronomy Guide
Purdue University Cooperative Extension Service  West Lafayette, Indiana
(Forages) AY-260
Forage Testing—Why, How, and Where
David J. Trotter, Keith D. Johnson, and Jerry H. Cherney Dearborn County Agricultural Agent, Forage Specialist, and Forage Researcher
Purdue University
Forage testing assesses the nutrient composition of forages and permits a producer to develop feeding and marketing strategies. This publication has been developed to inform livestock and/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
Forage Quality Defined
Since forages are predominately used by livestock as a source of nutrition, forage quality is an expression of the characteristics that affect consumption, nutritive value, and the resulting animal performance.
Factors Influencing Forage Quality
Even though 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.
Maturity. 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 that an increase of one percentage unit lignin will result in three to four 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 concentration, leaf content,
or the presence of unsuitable components that affect consumption and digestibility.
Leafiness. 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. So it stands to reason that 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. But management techniques that minimize curing time (use of a mower-conditioner, drying agent and/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 and 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