In vitro rumen fermentation: Effect of head space pressure on the gas production kinetics of corn meal and meadow hay

An automated batch system, consisting in 20 bottles equipped with gas pressure sensors and venting valves, was used to test the effects of headspace pressure on the kinetics of gas production (GP). Two venting procedures were compared: with FT (fixed times) the gas accumulated in the headspace of bottles was released after 2, 4, 6, 8, 12, 24, 48, 72 and 144 h of incubation, whereas with FP (fixed pressure) the valves were opened at a threshold of 3.4 kPa. For each procedure, samples of corn meal and meadow hay (0.50 g) were incubated in 4 replications in 310 ml bottles with 25 ml of rumen fluid and 50 ml of medium for 144 h at 39 degrees C. Both with FT and FP, gas pressures at the times of venting, converted in terms of volumes, were adjusted or not for the amount of dissolved gas according to the Henry's law. Data were cumulated and they were best fitted by a first order model the which parameters are the asymptotic GP (A), the time at which half of A is produced (T1/2) and the sharpness (c) of the curve. The effects of the 2 procedures were evaluated using a Wilcoxon two-sample test. The headspace pressure obtained with FT peaked 18.0 +/- 2.84 kPa at 12 h on corn, while peaked 7.5 +/- 0.81 kPa at 48 h on hay. For corn, the un-adjusted GP achieved between 12 and 48 h of incubation were 21 and 8% lower with FT compared to FP (P=0.01), and FT also had greater standard deviations. A similar trend, less accentuated, was observed for hay. The T1/2 values were greater with FT compared to FP (+1.3 and +2.3 h, for corn and hay, respectively; P<0.05), suggesting that FT delayed the release of gas dissolved in the medium. After adjustment, the GP values provided by the 2 procedures continued to be different for corn: compared to FP, FT reduced GP at 12,24 and 48 h (P=0.01). Adjustments removed all the differences for hay due to the venting procedure. Using the FT procedure, headspace volume, venting frequency and amount of fermentable matter must be carefully balanced to avoid high headspace pressures, lowered gas release and, hence, altered GP kinetics. (C) 2010 Elsevier By. All rights reserved.