Interference of indirect gas produced by grass silage fermentation acids in an in vitro gas production technique

Volatile fatty acids (VFA) resulting from rumen fermentation are measured in in vitro gas production techniques (IVGPT) by including bicarbonate in the incubation medium and measuring indirect gas produced. During ensiling, anaerobic fermentation of plant organic matter also produces VFA, along with lactic acid. The aim of this study was to establish whether silage fermentation acids (SFA) affected gas production profiles and subsequent carbohydrate parameter estimations of fresh and dried grass silage. Results indicate that a substantial volume of SFA gas is produced by fermentation of both fresh and dried grass silage. SFA gas production reaches an asymptote at 30 min post-inoculation and is 81% higher in fresh versus dried grass silage (P<0.001). Cumulative gas productions were fitted to the model: gas = A(1 − exp(−cat)) + B(1 − exp(−cb(t − l))) × exp(5(t − l))/(1 + exp(5(t − l))), where ‘A’ is the asymptotic gas production (ml) from fermentation of quickly degraded carbohydrate (QCHO), ‘ca’ the fractional rate of gas production of A (fixed at >0.2 h−1), ‘B’ the asymptotic gas production (ml) from fermentation of slowly degraded carbohydrate (SCHO), ‘cb’ the fractional rate of gas production of B (h−1), ‘t’ time (h) of fermentation and ‘l’ is the discrete time lag (h) before fermentation of SCHO begins. Results indicate that failure to correct for SFA gas resulted in overestimation of A, l and total rumen degraded carbohydrate (P<0.001), as well as underestimation of B (P<0.001) and cb (P<0.05). Multiple linear regression equations were developed to predict SFA gas volume produced by fresh grass silage at 30 min of incubation (R2 = 0.91, SEE = 0.62) and to predict the proportion of gas produced in the first 30 min of IVGPT that was derived from SFA for fresh grass silage (R2 = 0.95, SEE = 0.046) and dried grass silage (R2 = 0.71, SEE = 0.125).