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Illinois Livestock Trail by UNIVERSITY OF ILLINOIS EXTENSION


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From the Fields
by Jim Morrison


From the Fields

"What do the terms and values on a forage analysis mean?" That question was recently asked. Once the important decision has been made to have hay (or other forage) analyzed, being able to understand the analysis is just as important. Since a forage testing laboratory uses only a few grams of material to represent tons of forage, it is critical to obtain a representative sample and then interpret the analysis correctly.

A typical forage analysis includes measurements of moisture or dry matter, crude protein, acid detergent fiber, and neutral detergent fiber. Ash and nitrate nitrogen are sometimes measured. Relative feed value is commonly calculated, along with net energy and total digestible nutrients. Recently, some laboratories have added the calculation for relative forage quality.

Analytical or measured values (crude protein, fiber, ash, etc.) should be reported on a 100 percent dry matter basis. The testing laboratory should be a clear distinction between results or values derived analytically and those values derived from calculations (relative feed value, total digestible energy, etc.).

Acid detergent fiber (ADF) is the residue remaining after boiling the forage sample in acid detergent solution. ADF contains cellulose, lignin, and silica and it is often used to calculate digestibility. The greater the number, the lower the digestibility.

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Page 2, From the Fields, December 2, 2007

Neutral detergent fiber (NDF) is the residue left after boiling a sample in neutral detergent solution. NDF represents the indigestible and slowly digestible components in plant cell walls. It is often used as a measure of forage intake. The greater the NDF number, the lower the intake.

The crude protein (CP) value is 6.25 times the nitrogen content of forage. The greater the number, the higher the CP content.

Dry matter (DM) is the percentage of the sample that is not water. As noted above, a forage sample should be evaluated on a 100 percent DM basis. Rations are commonly calculated in that manner.

Relative feed value (RFV) is an index for ranking forages based on combining digestibility and intake potential. It is calculated from ADF and NDF. RFV of 100 is equivalent to full bloom alfalfa hay. The higher the RFV, the better the quality.

Relative forage quality (RFQ) was developed to take advantage of the advances in forage analysis technology. Since RFQ includes digestible fiber, it is believed to be more representative of the way an animal would perform on a given forage. Therefore, whenever RFV and RFQ are different, RFQ is the better value to use. RFQ can be substituted for RFV without making economic and other management changes.

Additional information can be found in the publication "Understanding Forage Quality", at the National Forage Testing Association web site http://www.foragetesting.org/ , and from nutritionists.

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