A major criticism of in vitro gas systems is that measurement of nutritionally wasteful products (i.e. CH4 and CO2, either directly from fermentation or indirectly from neutralization of VFA by the bicarbonate buffer) provides little useful information. However, as VFA and gas production are inversely related to microbial biomass yield for any given unit of substrate degraded, it has been suggested (e.g. Getachew et al., 1998) that an estimate of fermentation efficiency or partitioning factor (PF) can be obtained by relating gas production (ml) to the extent of degradation (mg). This study examined the relationship between in vitro DOMD, gas release and PF values using data obtained from an assessment of four maize silages.