Effect of lowered pH and increased passage rate on methane and volatile fatty acid production from continuous culture

Abstract Text:

The present study was conducted as a 2×2 factorial treatment arrangement in a Latin Square design using continuous culture fermenters (n=4). Treatments were control pH (CpH; ranging 6.3 to 6.9) or low pH (LpH; 5.8 to 6.4) factorialized with solids passage rates (k_p) of either low (Lk_p; 2.5%/hr) or high (Hk_p; 5.0%/hr); total dilution by buffer was constant at 7.0%/hr. Fermenters were fed once daily (40 g DM; a 50:50 concentrate:forage diet) and periods lasted 10 d with 3 d of sample collection. The main effect of LpH decreased (P < 0.001) aqueous hydrogen compared with CpH by 3.82 µM, but there was no effect of k_p (P > 0.10). The main effect of LpH decreased headspace hydrogen (H₂(g); escaped from culture) by 60.1 mmol/L×d, and an interaction over time (P < 0.001) was explained by H₂(g) being greater for CpH/Hk_p than for LpH (both k_p) from 5 to 24 h post-feeding, and CpH/Lk_p being greater than LpH (both k_p) from 6 to 24 h post-feeding. Further, H₂(g) was greater (P < 0.05) with CpH/Hk_p compared with CpH/Lk_p from 15 to 24 h post-feeding. There was no main effect (P > 0.10) of pH on methane production, but the main effect of Hk_p tended (P = 0.08) to decrease methane production compared with Lk_p by 880 mmol/ L×d.  A treatment × time interaction (P < 0.01) was explained in that CpH/Lk_p had the greatest (P < 0.05) methane production from 11 to 23 h post-feeding, whereas LpH/Lk_p was not different (P > 0.05) from CpH/Lk_p at 24 h post-feeding.  Acetate molar percentage was 61.8%, 59.6%, 58.2% and 54.7% for CpH/Lk_p, CpH/Hk_p, LpH/Lk_p and LpH/Hk_p, respectively. Both the main effects of LpH and Hk_p decreased acetate molar percentage (P = 0.01) compared with CpH and Lk_p, respectively. Propionate molar percentage was 22.5%, 24.4%, 23.9% and 26.2% for CpH/Lk_p, CpH/Hk_p, LpH/Lk_p and LpH/Hk_p, respectively. The main effect of LpH increased (P = 0.02) propionate molar percentage, decreasing (P = 0.002) A:P ratio from 2.61 to 2.34 compared with CpH. The main effect of Hk_p increased (P = 0.006) propionate molar percentage, decreasing (P = 0.002) A:P ratio from 2.62 to 2.34 compared with Lk_p. There were no effects on butyrate molar percentage or total VFA production (P > 0.10). The results indicate increasing k_pand decreasing pH decreased A:P ratio independent of the current diet.

Keywords: hydrogen, methane, volatile fatty acids