A note on the stoichiometrical relationship of short chain fatty acid production and gas formation in vitro in feedstuffs of widely differing quality

In vitro gas production tests are increasingly used in feedstuff research, mainly because rates of fermentation can conveniently be estimated via the accumulating gas volumes (for review see Getachew et al. 1998). In Germany, the Hohenheim in vitro gas test is widely employed in routine feed evaluation to predict in vivo digestibilities and metabolizable energy contents of feeds (Menke and Steingaβ 1988). Although they are waste products, fermentative gases (mainly CO2 and CH4) undoubtfully represent parts of the feeds which have been degraded. Conceptually, the use of in vitro gas tests is then only justified by a close linkage between the waste products and useful fermentation products, i.e. short chain fatty acids (SCFA) and/or microbial biomass. It was shown for straw (Blummel and O˜rskov 1993), cereal grain (Opatpatanakit et al. (1994) and hay (Makkar et al. 1995) that gas volumes in the bicarbonate buffered Hohenheim in vitro gas production test reflect SCFA production very closely. Gas volumes were produced quantitatively and qualitatively (Blummel and O˜rskov 1993) according to the stoichiometry of Wolin (1960), i.e. the amount of fermentative CO2 and CH4 could be accurately calculated from the amount and proportion of acetate, propionate and butyrate present in the incubation medium. As previously suggested by Menke and Steingaβ (1988) additional CO2 was produced from buffering the SCFA and about 54% of the total gas volume was attributable to this buffering reaction (Blummel and O˜rskov 1993), assuming that 1 mmol of SCFA released 1 mmol of CO2 from the bicarbonate buffer into the gas phase. However, the stoichiometry of Wolin (1960) considers only the fermentation of carbohydrates with glucose as a model and the presence of protein (or fat) in feeds might distort the stoichiometrical relationship between SCFA and gas production. It was for example argued by Cone (1998), that the stoichiometry does not hold good for protein rich feeds. In other words, the relationship between SCFA and gas production would be getting weaker and more variable. These conclusions were based on dilution experiments of starch and glucose with casein in an in vitro gas production test (Cone 1998). Obviously, uncoupling of SCFA and gas production would make the interpretation of gas profiles difficult, since the relationship between the useful products (SCFA) and the waste product (gases) would become obscured. It was therefore the objective of this technical note to re-examine the relationship between SCFA and gas production for 55 feeds varying largely in their protein and also fat content.