PS5-19. Gas Densitometry for Determination of Methane Concentration in Mixed Samples of in-vitro Technique.

Abstract In vitro methods are commonly used to simulate enteric fermentation of ruminants. Under these experimental conditions, methane (CH4) and carbon dioxide (CO2) are the main gases produced during the fermentation. Determining the concentration of these gases typically requires special equipment (e.g., gas chromatograph, infrared or electrochemical gas analyzer), which are expensive, require frequent maintenance, and involve costly calibration procedures. However, because of the difference of density between CH4 (0.65063 mg/mL) and CO2 (1.7906 mg/mL), it has been proposed that their concentration in fermentation gas mixtures could be estimated using this property. The objective of this experiment was to determine CH4 concentration under in vitro conditions based on gas densitometry. To this aim, goat fecal liquor (GFL) was prepared by mixing goat feces with McDougall's buffer in a 1:4 relation, and 60 mL of GFL was introduced to 32 glass flasks (125 mL) randomly assigned to seven CH4 mitigation treatments (n = 4 flask/treatment) including calcium oxide (CaO), magnesium oxide (MgO), ground (1 mm) turmeric, garlic, cinnamon, onion, oregano or no additive included (only GFL). In each flask, 1.0 g of corn meal (substrate) and 50 mg of each additive were incubated in a 25 µm nylon bag. All flasks were flushed with CO2, sealed with rubber stoppers, weighed, and placed in a water bath at 39.5 ± 0.1 °C for 24 h. After incubation, each flask was thoughtfully dried and weighed on a precision scale. Total gas volume was determined by registering the displaced volume in a buretrol connected to each flask. Subsequently, each flask without gas was weighed to calculate gas mass and density. Gas density values were then introduced in the general equation for ideal gases, correcting for laboratory temperature and pressure to estimate the proportions of CO2 and CH4 in the gas mixture. The treatment with lower gas production was MgO (59.0 ± 7.35 mL/g DM) (P < 0.039), while Cinnamon promoted the lower CH4 concentration (28.7 ± 5.9 %) (P < 0.033), and both Cinnamon and MgO generated the lowest CH4 yield (21.7 ± 7.6 mL CH4/g DM and 23. ± 2.9 mL CH4/g DM, respectively; P < 0.002). Dry matter digestibility and pH were not affected by any of the evaluated treatments (P > 0.05). These results demonstrate that MgO and cinnamon, at the evaluated dose, can reduce CH4 emissions as determined through gas densitometry. This method is a plausible alternative approach to measure CH4 and CO2 concentration under in vitro fermentation systems.