Relative Feed Value (RFV) and Relative Forage Quality (RFQ)

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Charles C. Stallings
Extension Dairy Scientist
Nutrition & Forage Quality
(540) 231-3066; email: cstallin@vt.edu

Relative Feed Value (RFV) and Relative Forage Quality (RFQ) are methods used to evaluate hays. To calculate RFV it is necessary to have a forage analysis for acid detergent fiber (ADF) and neutral detergent fiber (NDF). Protein is not considered but higher RFV values are usually 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 (the expected digestible dry matter intake as % of body weight for full-bloom alfalfa). The RFV for full-bloom alfalfa would be expected to be 100. For an alfalfa hay containing 29% ADF and 36% NDF the RFV = (66.3 * 3.3)/1.29 = 170. Grasses typically have higher ADF and NDF concentrations and consequently have lower RFV. For instance a grass or mixed grass/legume hay having 32% ADF and 50% NDF would have an RFV = (64 * 2.4)/1.29 = 119. Note that grasses and corn silage have a greater NDF:ADF ratio than legumes.

	ADF %	NDF %	NDF:ADF	RFV
Alfalfa A (mature)	40	51	1.28	105
Alfalfa B (immature)	29	36	1.24	170
Grass	32	50	1.56	119

What the RFV calculation does not account for is fiber digestibility.

Relative Forage Quality (RFQ) Index is similar to RFV except NDF digestibility is used. NDF digestibility allows for a more precise estimate of the energy in the feed and many laboratories are offering an in vitro NDF digestibility to account for fiber digestion. 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 (non-NDF material) that are highly digestible thus elevating energy concentrations to higher levels than in grasses. When using RFV or RFQ it is best to compare hays that are within a similar classification such as alfalfa, grass, or mixed. RFQ gives more credit for digestible fiber in grasses and grasses will typically have higher RFQ than RFV but will still be less than many legumes. Type, quality and price should be taken into consideration when purchasing hays.