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Determining Relative Feed Value

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. 

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, a common tool for determining hay quality (intake and energy value) and in pricing hay,
  • developed for alfalfa; however
    • fibre characteristics differ among forage species and cannot be used to compare grass or grass-alfalfa hay to alfalfa,
  • used to determine the quality of standing alfalfa hay; and
  • 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

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.

Table 1 - Use Table 1 to determine the Relative Feed Value of the standing alfalfa.

Growth Stage A B C D E
Stem Height (inches)
16 234 220 208 196 186
18 223 211 199 188 178
20 213 201 191 181 171
22 204 193 183 173 165
24 196 185 175 167 158
26 187 178 169 160 152
28 180 171 162 154 147
30 173 164 156 148 141
32 166 158 150 143 136
34 160 152 145 138 132
36 154 146 139 133 127
38 148 141 134 128 123
40 142 136 130 124 118
42 137 131 125 120 114
44 132 126 121 116 111
46 128 122 17 112 107

Using Relative Feed Value for Comparing Hay

RFV provides a convenient way of comparing alfalfa hay quality and predicting feed value in the field, but 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 RFV 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.

Formula for calculating RFV

  • 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).

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