Effect of inulin on in vitro neutral and acid detergent fiber digestibility using equine fecal inoculum in the DaisyII incubator

Abstract

Cool-season grass fructans are a known health risk to grazing horses, but their effects on hindgut digestive processes are not well understood. Inulin is commonly used as the model fructan in many laboratory settings. The objective of this study was to evaluate the impact of increasing inulin levels on in vitro neutral detergent fiber digestibility (ivNDFD) and in vitro acid detergent fiber digestibility (ivADFD) of an orchardgrass (OG) hay incubated with equine feces in the Ankom Daisy^II Incubator. The hypothesis was that increasing inulin levels would inhibit ivNDFD and ivADFD. The OG hay used was 73% NDF and 46% ADF (% DM). NDF and ADF residues were prepared before incubations according to Ankom official procedures without sodium sulfite or α-amylase. Four inulin treatments were tested: 0 g (control), 4 g, 8 g, and 12 g per 2-L Daisy^II incubation jar. Preliminary experiments were used to determine appropriate inulin inclusion rates. Over a 2-wk period, 4 incubations were conducted with fresh feces collected from mature geldings that were managed similarly across the collection period. Each incubation jar was assigned to an inulin treatment and contained 11 filter bags per incubation (4 containing NDF residue, 4 containing ADF residue, and 3 blanks). Incubation jars were prepared with 2 buffers mixed in a 5:1 ratio, and inulin powder was dissolved in the buffer solution. The pH of each incubation jar was adjusted to 7.2 before fecal inoculum was added, and pH was measured again after inoculation. Daisy^II incubation jars contained a 10:1 buffer and feces solution, and filter bags were incubated at 39°C for 48 h. After incubation, final pH was measured, and filter bags were rinsed, dried, and reweighed to determine ivNDFD and ivADFD. PROC GLM (SAS 9.4) was used to evaluate the main effect of inulin treatment and differences in pH decline, ivNDFD, and ivADFD between treatments. Mean decline in pH during incubations was −0.4, −0.6, −0.9, and −1.1 pH units for the 0-, 4-, 8-, and 12-g inulin treatments, respectively; each inulin treatment resulted in a final pH that was significantly lower than the control (P < 0.05). The 8-g and 12-g inulin levels resulted in lower ivNDFD compared with the control (P < 0.05). Differences were not detected in ivADFD across treatments (P = 0.37). No difference was observed between the 4-g inulin and control treatments for ivNDFD (P = 0.38) or ivADFD (P = 0.99). In conclusion, 8-g and 12-g inulin levels inhibited ivNDFD but not ivADFD over 48-h incubations. Additional studies are warranted to investigate the impact of inulin on individual structural carbohydrates.