Gas production from straw incubated in vitro with different levels of purified carbohydrates

The effect of the type (starch (S); cellulose (C) and pectin (P)) and level (0–4 mg/ml) of added carbohydrate on gas production from ammoniated straw (AS) at a fixed pH (6.7) was studied in vitro in three incubation runs. The S, C and P were also incubated without AS at levels of 1, 2.5 and 4 mg/ml. On a dry weight basis, gas produced from P as the only substrate was higher than S, and from S higher than C up to 12 h of fermentation (P<0.05). From 24 to 36 h, gas volume from S was higher than P, with no differences after 48 h. Gas produced from AS in mixed incubations was estimated by subtraction of the carbohydrate contribution, calculated by linear regression. Gas volume from AS was higher with P than with S or C at all times (P<0.05). Proportion of acetate was higher, and propionate lower, in P than in S or C media (P<0.05). Addition of C or S reduced gas volume from AS compared with level 0 from 24 or 30 h onwards, respectively (P<0.05). However, inclusion of P increased linearly gas production until 8 h (P<0.01). In vitro, carbohydrates affected straw fermentation at an optimum pH, being negative with starch and cellulose from 24 h onwards. A positive effect of pectin on straw fermentation was observed in the first 8 h of incubation, and no effect was seen thereafter.