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Dairy Pipeline: October 2004
Charles C. Stallings
Extension Dairy Scientist,
Nutrition and Forage Quality
(540) 231-3066
email: cstallin@vt.edu
Relative feed value (RFV) is a term that has been used in the marketing of hays. It can be calculated for pure grass and legume or mixed hays. To calculate this value it is necessary to have a forage analysis for acid detergent fiber (ADF) and neutral detergent fiber (NDF). It does not consider protein but higher RFV values would usually be associated with higher protein. The ADF analysis is used to predict the digestible dry matter = (88.9 - (.779 * % ADF)) and NDF predicts dry matter intake = (120/% NDF). RFV is calculated by multiplying digestible dry matter by dry matter intake and then dividing by 1.29. For an alfalfa hay containing 30% ADF and 40% NDF the RFV = (65.5 * 3)/1.29 = 152. Grasses typically have higher ADF and NDF concentrations and consequently have lower RFV. For instance a grass or mixed hay having 35% ADF and 50% NDF would have an RFV = (61.6 * 2.4)/1.29 = 115. What this calculation does not account for is ADF and NDF digestibility. Grasses typically have fiber digestibility's greater than legumes because legumes have more lignin associated with the fiber. Legumes make up for this by having more cell contents that are highly digestible thus elevating energy concentrations to higher levels than in grasses. When using RFV it is best to compare hays that are within a similar classification such as alfalfa, grass, or mixed. Commercial labs typically have the ability to provide RFV for hay crop forages. If you are interested, contact your local Extension Office for a list of commercial labs.