Validation of Equations Used To Estimate Relative Feed Value of Alfalfa Hay

Twenty alfalfa hay sources, grown under irrigation in Wyoming, were fed to 24 fine-wool wether lambs (average weight=44.9kg) to determine dry matter intake (DMI), expressed as a percentage of body weight, and digestible dry matter (DDM). These data were used to evaluate equations utilized in predicting relative feed value (RFV) of forages. High and low values of the hays for NDF and ADF were 56.0 to 39.5 and 43.6 to 32.7, respectively. They were ground through a grinder-mixer equipped with a 5-cm screen prior to feeding. Lambs were housed in metabolism crates in a temperature-controlled room. Six lambs were fed each forage with four forages evaluated in each of five periods. Because intake was ad libitum, the percentage of NDF and ADF in the diet of each sheep was calculated by dividing the amount of NDF or ADF consumed by the amount of DM consumed. Mean values for each hay were used in data analysis. Equations evaluated were: DMI=120/%NDF; DDM=88.9-(0.779×% ADF); and RFV=(% DDM×% DMI)/1.29. An estimated value was calculated for DMI, DDM, and RFV for each of the hays using these equations. Actual RFV of each hay was calculated using actual DMI and DDM. Regression analysis was used to determine the relationship between actual and the estimated values using actual values as the dependent variable. The model for DMI [actual DMI=2.81 - (0.07×estimated DMI)] accounted for less than 1% of the variation of intake (R 2 =0.01), whereas the bias was 20.7%. The equation for DDM [actual DDM=43.8+(0.25×estimated DDM)] accounted for 20% of the variation in DM digestion and the bias was 2.4%. Because of the large amount of variation unaccounted for in DMI and DDM in the above equations, the model for actual RFV accounted for less than 1% of the variation in estimated RFV, and the bias was 21.9%. These data suggest that NDF or ADF may not be related to forage quality to the extent to allow ranking of similar forages.