Potential and limitations of in vitro gas methods for studying the effectsof plant defensive compounds on rumen fermentation

The plant defensive compounds (PDCs) most widely studied in ruminant feeds are tannins, saponins and alkaloids. Various approaches have been used to q uantify and study the effects of these components. However, most of these a ssays are performed under conditions different from those in the rumen, and therefore the results obtained have limited applicability for predicting t he nutritive value of feeds which contain these components. An in vitro rum en fermentation system could be complimentary to these chemical and protein precipitation assays and the effects of the PDCs could be monitored in the system in terms of quantifiable parameters. The measurement of gas release d on incubation of a feed in a medium containing rumen microbes is easy. As a result in vitro gas methods have been widely used lately, and to a large extent thoughtlessly, to characterize feeds and to study the effects of PD Cs. Several types of gas measuring equipment, from manual, semiautomatic to automatic have been developed for this purpose. Gas measurement is a reflection of short-chain fatty acid (SCFA) production and the production of fermentation gases. However, besides these products, the substrate degraded during fermentation forms microbial mass. The relat ionship between microbial mass production and SCFA or fermentation gas prod uction is not constant and can vary widely, depending upon the extent of co upling and the spillage of energy released during fermentation. If the only measurement taken is the gas released during fermentation it could be misl eading, both for the purpose of characterization of a feed or in the study of the effects of PDCs on rumen fermentation. The results of our studies on tannins, saponins and alkaloids which demonstrate the need to combine the measurement of microbial mass production with gas measurements are presente d.