The extent of rumen degradability of a feedstuff depends on the interaction between rate of degradation and residence time. In situ techniques require that substrates are ground to obtain a homogenous sample and to reduce result variability. However undegraded feed particle losses increase with bag pore size and fineness of grinding. If these particles are degraded at a similar or lower rate than the retained material, degradability, especially for short-term incubations, will be overestimated. In contrast if the feed particles lost are more readily degraded the degradability measurements obtained will be unaffected. Where improvements due to grinding have been recorded in situ these are assumed to result from variations in the proportion of fine particles that are immediately soluble or rapidly degradable. Gas-based in vitro feed evaluation systems offer the possibility of quantifying this effect directly and a study was therefore conducted to investigate the impact of particle size on the rate and extent of fermentation using the Reading Pressure Technique (Mauricio et al., 1999).