This handbook has been written as a guide for those who wish to (1) set up the newer detergent system of fiber fractionation (^, 5, P, 13)j^ (2) adapt their apparatus and equipment to the new analyses, and (3) estimate the di- gestibility directly by a modified {15) Tilley and Terry {3) two-stage in vitro rumen fermenta- tion procedure.
Laboratory evaluation of forage is essentially aimed at obtaining analytical data that predict the extent of biological degradation under speci- fied conditions, animals, organisms, and time. In general, an in vitro rumen fermentation re- flects the factors known and unknown limiting availability of forage to the digesting organism. The analyst can assay only known constituents. The assays are for things that are expedient to handle, are known, or are considered important, and its evaluation is valid only if the principals
concerned are dominant in the sample. For ex- ample, it makes little sense to expect lignin to be a good predictor of digestibility if silica or some other factor is a more important variable. In vitro fermentations will be influenced by all factors and inhibitors known and unknown. The in vitro, however, does not disclose anything re- garding the nature of the limiting factor. This latter task of identification remains the duty of chemical studies.
An attempt has been made to cover the prin- cipal problems in technique encountered in these procedures. These include sample preparation, filtration and washing, and efficiency in han- dling large numbers of samples. Ultimate lab-
oratory efficiency is tied to the precision of dupli- cates and the repeatability of values. Handling of samples in drying and weighing and in ashing and reweighing is also discussed.