Ronald L. Boman
USU Extension Dairy Nutritionist

I continue to receive reports of dairy cows not milking well because of the reduced quality of alfalfa hay in the ration. One dairy producer (typical of many) mentioned that his cows were producing about 10 lbs/day less than they had in previous years. He went on to say that his alfalfa hay was more mature than usual, but "it didn't get rained on." This rather mature alfalfa hay (that didn't get rained on) averaged only 17% crude protein with 34% ADF and 41% NDF, and a relative feed value of only 142.

Good dairy hay should be 21 to 22% crude protein and less than 28% ADF and 35% NDF with a relative feed value of 170 to 180 or even higher.

What is Relative Feed Value?

Relative feed value is an index that ranks cool season legumes (alfalfa) and grasses by potential digestible dry matter intake. It is calculated from digestible dry matter and dry matter intake.

Digestible dry matter is an estimate of the total digestibility of the feed and is calculated from percent acid detergent fiber (ADF).

Dry matter intake is an estimate of the amount of feed an animal will consume in percent of body weight and is calculated from percent neutral detergent fiber (NDF).

Calculation of Relative Feed Value (RFV)

       DDM = Digestible Dry Matter = 88.9 - (0.779 x % ADF)
       DMI = Dry Matter Intake = 120 / ( % NDF )
       RFV = (DDM x DMI) / 1.29

Example: (Plug in values for ADF and NDF on a dry matter basis)

       Alfalfa Hay or Haylage with 28% ADF and 35% NDF
       DDM = 88.9 - (0.779 x 28) = 67.1
       DMI = 120 / 35 = 3.43
       RFV = (67.1 x 3.43) / 1.29 = 178

Notice that Relative Feed Value DOES NOT include the protein percent of the alfalfa!!

Relative Feed Value is an excellent measure of alfalfa quality because it reflects the digestibility (% ADF) and the intake potential (% NDF) of alfalfa. Use it when buying alfalfa and on the alfalfa you grow to determine quality. Obviously the more mature the alfalfa at harvest the higher the percent ADF and NDF (lower Relative Feed Value). However, rain damaged hay will also be higher in ADF and NDF because the soluble sugars and carbohydrates can be leached out. Rain damaged hay is also less palatable. Astute dairy producers who purchase alfalfa not only utilize the chemical analysis and relative feed value, they also buy small quantities and feed some of it free choice to see if the cows readily consume it before purchasing larger quantities.