Laboratory prediction of forage digestibility by the pepsin-cellulase method. The renewed equations

Abstract

Pepsin-cellulase digestibility determination is widely used to predict forage organic matter digestibility (OMd). Prediction equations wereestablished in 1989 on 85 fresh forage or hay samples of pure species of grasses and legumes and on permanent grasslands rich in grasses. Some species like red clover, perennial ryegrass/white clover, as well as fermented forages (silages, wrapped big bales) were not includedin this model. To extend and strengthen the prediction equations, we analysed the relation between the OMd and the pepsin-cellulasedigestibility from a database containing 384 measurements of OMd performed on sheep at INRA Theix in Auvergne or at INRA Pin-au-Haras in Normandy. The determinations of pepsin-cellulase digestibility were all performed at INRA Theix. The database contains269 measurements on fresh forages (121 grasses, 60 ryegrass - white clover mixtures, 32 legumes and 56 permanent grasslands), 64 measurementson fermented forages (39 grasses and 25 legumes) and 51 measurements on hays (37 grasses and 14 legumes)., OMd variedfrom 48.5 to 84.6 and pepsin-cellulase digestibility from 38.6 to 86.5, determined from the whole dataset. The analysis of the databaseallowed us to propose renewed prediction equations for fresh forages and hay with residual standard deviations between 2.2 and 2.9 pointsof digestibility, and to specify the differences in slope between grasses and legumes. A specific equation was proposed for rye-grass / whiteclover mixtures. New equations were also proposed to predict OMd of fermented forages with a residual standard deviation lower than2 points of digestibility. A validation study was conducted on 122 external samples for which pepsin-cellulase was determined in a commerciallaboratory. The standard error of prediction varied between 2.7 and 3.2 points of digestibility for the different forage types. It waslower than the standard error of prediction obtained from the chemical composition (crude fibre and crude protein) that varied between3.5 and 5.1 points of digestibility.