A comparison of in vitro rumen fluid and enzymatic methods to predict digestibility and energy value of grass and alfalfa hay

Relationships between in vivo organic matter digestibility (DOM, %) or metabolizable energy (ME MJ/kg OM and different laboratory measurements have been calculated for 22 samples of grass hay and 20 samples of alfalfa hay. Laboratory measurements included Weende constituents (crude protein, CP; crude fiber, CF; ether extract, EE; and ash), pepsin–cellulase digestible organic matter (CDOM, %), gas production (GP, ml/24 h) and a combination of in vitro rumen fluid fermentation in 100 ml glass syringes and determination of the undigested neutral detergent residue. Digestible dry matter (DDM, %) was calculated from dry matter quantity of the feed minus total residue after incubation. On the basis of the quantity of organic matter (OM) in the original samples and the NDF residue true digestible OM (TDOM, %) was calculated. All in vitro procedures are significantly correlated to in vivo DOM and ME. Prediction of energy value of feeds using gas production alone and DDM is less accurate compared to other methods. Combination of GP with CP, NDF, ADF and ash improves prediction accuracy. Rumen fluid–neutral detergent method (TDOM) and cellulase method (CDOM) predict OM digestibility and energy value of hay with similar accuracy. The best regression equations for prediction of ME (MJ) obtained are the following:ME (MJ/kg OM)=7.4+0.07×TDOM−0.004×NDF+0.02×EE(R=0.92; SEE=0.25; n=42)ME (MJ/kg OM)=12.3−200×1/CDOM−0.003×ADF+0.01×EE+0.002×NDF(R=0.91; SEE=0.27; n=42)Using glass syringes instead of the centrifuge tubes used in the method of Tilley and Terry [Tilley, J.M.A., Terry, R.A., 1963. A two-stage technique for the in vitro digestion of forage crops. J. Br. Grassl. Soc. 18, pp. 104–111] simplifies the procedure from an operative point of view. The combination of the two methods (gas production and two-stage method with rumen fluid) could give information either of digestion kinetics or undigested products.