Estimation of direct and indirect gas production in syringes: A tool to estimate short chain fatty acid production that requires minimal laboratory facilities

A technique is proposed that involves volumetric measurement of gas production in phosphate and bicarbonate buffered in vitro incubations in syringes. Gas production is measured before and after CO2 absorption by injection of NaOH (10 M) at the end of the incubation. After correction for the amount of CO32− in the rumen inoculum, measurements allow calculation of direct and indirect CO2 production, reflecting metabolism and acidification, respectively, as well as methane (CH4) production. This allows estimation of approximate short chain fatty acid (SCFA) production, energy lost as CH4 and it might provide information on the ratio of the glucogenic to lipogenic SCFA produced. Results from in vitro gas production tests in syringes with straw (0.325 g), and eight different tropical seeds (0.125 g) as protein supplements, were used to evaluate whether such calculations could indeed predict measured amounts of SCFA and CH4 produced, as determined by gas chromatography. The concordance correlation coefficient (CCC), representing both precision (ρ) and accuracy (Cb) has been used to assess effectiveness of this reproducibility. It was found that indirect CO2 production equimolarly reflects total SCFA production (CCC = 0.79; ρ = 0.86; Cb = 0.93), whereas absorption of gaseous CO2 provided an accurate prediction of CH4 produced (CCC = 0.96; ρ = 0.96; Cb = 1.00). The ratio of CH4/gasindirect allowed estimation of the ratio of lipogenic to glucogenic SCFA produced (R2 = 0.73; R.S.D. = 0.20). However, the current approach did not allow accurate quantification of proportions of individual SCFA produced (CCC = 0.19; ρ = 0.83; Cb = 0.23 for propionate proportions). The potential of a simple gas production technique to provide information on feed quality is illustrated, with the only apparatus required being gas tight syringes and incubation facilities.