A semi-automated in vitro gas production technique for ruminant feedstuff evaluation

This technical note describes the Reading Pressure Technique (RPT) – an in vitro feed evaluation system based on a semi-automated gas production technique. Substrates are incubated together with buffered rumen fluid in sealed fermentation flasks. A pressure transducer, interfaced with a PC allows accumulated head-space gas pressure values to be directly entered into a spreadsheet. These pressure measurements are then used to generate gas volume estimates using a quadratic function derived from simultaneous pressure and volume measurements. Earlier attempts to derive volume from pressure resulted in predicted volume being under estimated. This discrepancy was most likely due to diffusion of head-space gases into the liquid phase, a factor not considered in Boyleʹs Law, the initial relationship used. In comparison with the syringe technique (Theodorou et al., 1994), the modifications improved both the accuracy (reduced operator error) and rate at which measurements could be taken (5–6 s per flask) greatly increasing the analytical capacity (to 336 flasks or 75 substrates per incubation series). The ability of this system to provide information not just on both rate and extent of degradation, but also fermentation efficiency and degradation kinetics offers a wide variety of applications, for example the systematic evaluation of tropical feedstuffs. In conclusion it is suggested that the simplicity, low cost and high capacity of the RPT system make it ideal for those situations where either budget constraints or the level of technical expertise required render the more complex systems inappropriate.