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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

Murray, Jo-Anne M.D., Bice, Rachael K.T., Moore-Colyer, Meriel J.S.
Animal feed science and technology 2010 v.162 no.1-2 pp. 47-57
alfalfa, hay, drying, heat treatment, in vitro digestibility, fermentation, particle size, sugar beet pulp, horses, feces, intestinal microorganisms, gas production (biological), molasses, mathematical models, temporal variation, volatile fatty acids, propionic acid, acetic acid
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 1mm 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 (82.9mmol/l vs. 64.0mmol/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.