Relative Feed Value - A Tool for Determining Alfalfa Quality
The Concept of Relative Feed Value
Feed value is an important factor when buying or selling hay. Buyers and sellers require an accurate and effective way of communicating the quality of hay using a method that best describes the feed value to livestock. Traditionally, protein content has been used as a measure of hay quality. However, other important factors, such as fibre content, which in turn influences digestibility and intake, are important factors when determining forage quality. Relative Feed Value (RFV) attempts to use a single value to describe forage quality, and has become a common tool for determining hay quality (intake and energy value) and in pricing hay. Relative Feed Values have been developed for alfalfa, but because fibre characteristics differ among forage species, RFV cannot be used to compare grass or grass-alfalfa hay to alfalfa. Relative Feed Value can also be used to determine the quality of standing alfalfa hay. Relative Feed Value is expressed as a percentage of alfalfa at 100 per cent bloom, whose RFV is 100.
Predicting Relative Feed Value of Standing Alfalfa
Relative Feed Value can be estimated on standing alfalfa. This allows producers to determine the appropriate harvest date of their alfalfa to suit their feed needs. Some losses occur after harvesting, so harvesting should occur at a higher RFV than the target quality of the baled hay to allow for these losses. For example, if the desired RFV of the baled feed is 150, harvest should occur at 170.
A method of predicting Relative Feed Value is outlined below:
Relative Feed Value is a useful technique to determine harvesting date for alfalfa.
1. Select a representative 0.09 m2 (one square foot) area in the field (repeating this procedure on five to 10 plots across the field will give a good measure of the field's RFV).
2. Determine the stage of development of the most mature stem in the plot, grouping it into one of the following categories:
Stage A - late vegetative, with stems over 30 cm (12 inches) tall with no visible buds or flowers.
Stage B - early bud, has one or two nodes with visible buds but no flowers or seed pods.
Stage C - late bud, has more than two buds with visible buds, but no flowers or seed pods.
Stage D - early flower, has one node with at least one open flower and no seed pods.
Stage E - late flower has two or more nodes with an open flower and no seed pods.
3. Use Table 1 to determine the Relative Feed Value of the standing alfalfa.
Using Relative Feed Value for Comparing Hay
Although R F V provides a convenient way of comparing alfalfa hay quality and predicting feed value in the field, the concept does have some limitations. It is only useful for comparing alfalfa forage. It cannot be used for comparing grass or alfalfa-grass forage due to the fact that grass does not have the same developmental stages of alfalfa and the nature of grass fibre is different than that of alfalfa. It is also important to balance the R F V of the forage with the fibre requirements of the type of animal being fed.
Deriving Relative Feed Value
Relative Feed Value is calculated from feed analysis values. The formula for calculating RFV is: RFV=(DDM * DMI)/1.29, where RFV = Relative Feed Value
DDM = Digestible Dry Matter is an estimate of digestible fibre in the forage. This is determined from the Acid Detergent Fiber value (the highly indigestible parts of the forage, including lignin, cellulose, silica, and insoluble forms of nitrogen): DDM (%) = 88.9 - 0.779ADF (% of DM), therefore the lower the ADF value, the higher the digestible dry matter (DDM) will be.
DMI = Dry Matter Intake, which is an estimate of how much forage an animal will consume, based on the Neutral Detergent Fiber (the portion of the forage that consists of cell walls, which are only partially digestible. Low NDF and ADF values are desirable in a forage) in the forage. DMI (% of body weight)=120/NDF(% of DM).
Prepared by: Michel Tremblay
Contributors: Dr. Ken Albrecht, University of Wisconsin; Mike Rankin, University of Wisconsin Extension; Terry Grajczyk, Saskatchewan Agriculture; Murry Feist, Saskatchewan Agriculture; Dr. Scott Wright, Alberta Agriculture.