A forage test determines the nutritional qualities of hay. The best way to determine hay quality is through a forage test from a single lot of hay. A lot is from one field which has been cut, baled, handled and stored under uniform conditions. The sample should be a consolidation from the center of 15 to 20 bales.
Understanding Forage Analysis
Forage tests usually contain information on moisture (%), dry matter (DM%), crude protein (CP%), acid detergent fiber (ADF%), neutral detergent fiber (NDF%), total digestible nutrients (TDN%), and net energy calculations for lactation (NEl,mcal/lb), maintenance (NEm, mcal/lb), gain (NEg, mcal/lb) and relative feed value (RFV).
In most analysis reports, test results are give on an “As Fed Basis” and “Dry Matter Basis.” The “As Fed” numbers reflect nutrient concentrations in the forage as it was received in the forage lab, including all water present. Water dilutes the concentrations of all other nutrients, so the “As Fed” numbers are less than “Dry Matter” numbers.
The “As Fed” values are necessary when figuring out the weight of actual hay for a given nutrient. For example, if a hay sample contains 16.7% crude protein on an “As Fed” basis, then to produce enough hay to provide 1,000lb of protein for a winter feed mix, it would be necessary to make 5,988lb of hay (1,000/0.167=5,988).
“Dry Matter” reports the concentration of a given nutrient with all water extracted. “Dry Matter” values are most commonly used in forage analysis information because the moisture contents of forages vary.
Understanding Forage Analysis Terms
Moisture, expressed in percentages, if the water present in the forage analyzed. Dry matter (DM) is the percentage of the forage that is not water.
Crude Protein (CP) is the sum of true protein and non-protein nitrogen. It is a measure of a forage’s ability to meet the protein needs of livestock. Most protein in forages is true protein, with exceptions for nitrate-accumulating summer annual grasses such as sudangrass and pearl millet. Although high-protein forages are often high in energy, CP content is of little value in determining energy content. Protein is one of the most expensive nutrients to supplement, so high protein forages are desirable for most livestock.
Acid detergent fiber (ADF) is the percentage of highly indigestible plant material present in a forage. ADF is made of cellulose, lignin and silica. ADF is a useful predictor of energy and digestibility. Low ADF means higher energy value and digestibility since lignin and silica are not digestible by ruminants. Because of this, low ADFs are desirable. All energy estimates presently used in forage testing are calculated from ADF alone.
Neutral detergent fiber (NDF) represents all of the structural or cell wall material in the forage. NDF is inversely related to the amount that a cow or calf is able to consume.
Energy Terms
Total digestible nutrients (TDN) reports the percentage of digestible material in a forage. TDN is calculated from ADF and expresses the difference in digestible material between forages.
Net energy of maintenance and lactation) are expressions of energy value of forage in megacalories (Mcal/lb); therefore they refer to the forage’s ability to meet the energy requirements of dairy and beef cattle.
Net energy for gain is the amount of energy in a forage available for growth (weight gain) after the maintenance needs have been met.
Relative feed value (RFV) is used to compare one forage to another on an energy basis. It is calculated by taking into account the digestibility calculated from ADF and the potential intake, calculated from NDF, for a given forage. To help compare, the RFV of a mature, full bloom alfalfa was set at 100. All grass hays will be lower than 100. A very high energy alfalfa might have an RFV of 124, meaning that is contains 24% more energy than mature alfalfa. RFV is becoming the most price determinant in quality-tested hay auctions.
From "Interpreting Forage Quality Reports." Jimmy C. Henning, Garry D. Lacefield and Donna Amaral-Phillips. Cooperative Extension Service, University of Kentucky.
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