58 Evaluation of substrate:buffer ratios for in vitro cultures inoculated with equine cecal contents

Abstract

In vitro fermentation systems allow researchers to mimic the equine hindgut while providing quick, affordable, and publishable data. However, the substrate:buffer ratio within the system has a profound impact on subsequent fermentation parameters. The objective of this study was to evaluate effects of varying substrate levels on culture pH, gas production, dry matter disappearance (DMD), neutral detergent fiber disappearance (NDFD), and volatile fatty acid (VFA) concentrations utilizing the ANKOM RF Gas Production System (ANKOM Technology; Macedon, NY). Cecal fluid was collected from 4 cecally cannulated Quarter horses and used to inoculate fermentation bottles containing buffer and substrate. Substrate consisted of brome or corn included at 0.5, 1, 1.5, 2, 3, or 4 g/flask (dry matter basis) while buffer and inoculant remained constant at 125 and 25 mL/flask, respectively. Flasks were equipped with continuous gas pressure monitors and placed into a shaking incubator for 48 h at 39°C. Data were analyzed using mixed models with fixed effects of treatment, time, and treatment by time interaction, repeated effect of time, and random effects of run by horse within run. Changes in gas production, VFA production, and terminal pH were proportion to substrate amount for cultures containing 0.5 to 2 g of substrate, but were less for cultures with 3 or 4 g of substrate (linear and quadratic effects, P < 0.0001). These data suggest that cultures containing 2 g or less of substrate were substrate limited, whereas activity of cultures with more than 2 g of substrate may have been inhibited by buffer exhaustion or end product accumulation. Results of this experiment indicate that the composition of fermentative end products is impacted by substrate concentrations, and that cultures containing 125 mL of buffer and 25 mL of cecal inoculum should be limited to no more than 2 g of substrate.