The effect of particle size on the in vitro fermentation of different ratios of high-temperature dried lucerne and sugar beet pulp incubated with equine faecal inocula

An in vitro gas production technique where equine faeces was the source of microbial inoculum was used to determine the effect of particle size (ground vs unground) on the in vitro fermentation of high-temperature dried lucerne (L) and molassed sugar beet pulp (SB) Two experiments were conducted in experiment 1 unprocessed (U) L and SB or ground L and SB (G to pass through a 1 mm dry mesh screen) were mixed in the following ratios 100 0 90 10 80 20 and 70 30 L and SB respectively In experiment 2 unprocessed L or ground L and ground SB were mixed in the following ratios 100 0 80 20 60 40 40 60 20 80 and 0 100 L and SB respectively Substrate combinations were fermented in vitro with equine faecal inocula using in vitro gas production (GP) In both experiments total gas pool was unaffected by particle size Conversely mathematical analysis of the cumulative gas production curves showed significantly different rates of fermentation in bottles containing ground substrates compared to unprocessed feedstuffs (P<0 001) Rate parameter values fractional rate of degradation (FRGP) and time taken to produce 50 or 95% of the total gas production (T-50 and T-95 respectively) were all increased (P<0 001) by grinding the substrates in both experiments In experiment 2 total volatile fatty acid (TVFA) concentration in the culture fluid post-fermentation was also higher (P<0 001) in bottles containing U material compared to G (82 9 mmol/l vs 640 mmol/l respectively) with a marked change in the molar proportions of volatile fatty acids (VFA) present with bottles containing G material containing more propionate and less acetate compared to the U substrates In conclusion particle size has a marked effect on the rate of substrate fermentation and TVFA concentration and VFA composition of the culture fluid which has important implications for predicting in vivo digestibility from in vitro digestibility measurements therefore further work is required to determine optimal particle size of substrates evaluated in vitro using equine faecal inocula (C) 2010 Elsevier B V All rights reserved