A comparison of filter bag methods with conventional tube methods of determining the in vitro digestibility of forages

In vitro digestibility of forages is commonly estimated by two-stage methods in which the various samples are kept completely separate from one another, using tubes. A possible alternative approach, which may save labour, is to use larger vessels, within which up to as many as 25 samples are incubated, each contained in its own filter bag. The two approaches were compared for estimating apparent dry matter (DM) digestibility, apparent digestible organic matter in DM, true DM digestibility, true digestible organic matter in DM and digestibility of neutral detergent fibre. The forage samples analysed comprised all 72 combinations of two forage species (Lolium multiflorum and Medicago sativa), three plant parts (whole crop, leaf and stem), three degrees of particle breakdown (0.5, 1.0 and 1.5 mm sieve size when milling) and four field replicates. Rumen fluid from sheep was used for two held replicates and rumen fluid from cattle for the other two. There was no discernible effect on digestibility of the sieve size used when milling, e.g. true digestible organic matter in dry matter using filter bags was 674, 677 and 663 g kg(-1), respectively, (SE 6.4) with the 0.5, 1.0 and 1.5 mm sieves. There were smaller differences between the two forage species (in respect of whole crop, stem and leaf) with the filter bag than with the tube method. The standard errors and coefficients of variation were higher with the filter bag than with the tube method; of 16 coefficients of variation calculated for each method, the mean with filter bags was 4.0% and the mean with tubes was 2.7%. Linear regression indicated that true digestibility using tubes could be predicted more precisely than apparent digestibility using tubes from the results using filter bags. The difference between apparent and true digestibility, when estimated using filter bags, appeared unrealistically low. The estimates of forage digestibility when using rumen fluid from sheep were very similar to those when using rumen fluid from cattle. It is concluded that the traditional methods, using tubes, are likely to give more precise results than using filter bags, although at the cost of requiring more labour. (C) 2000 Elsevier Science B.V. All rights reserved.