Dacron bag measurements (in-situ) to determine protein degradability of feedstuffs are often used as the “reference method” when comparing laboratory procedures. The model commonly adopted for interpreting data from “in-situ” dacron bag studies is that proposed by Ørskov & McDonald (1979) which is described by the asymptotic equation p = a + b (1-eα) where p is the proportion of material degraded after time t and a, b and c are constants. This model was later extended to deal with the presence of lag times and describes the instantly degradable soluble fraction (‘a’ in the above equation), and a fraction, ‘b’ which is degraded more slowly at an exponential rate V. A knowledge of the rumen fractional rate of outflow (k) for the feedstuff is also necessary to estimate effective degradability (ED). The value of ‘a’ (also referred to as the wash value) forms a major proportion of ED calculations. ED = a + (bc/(c+k)) as it is assumed to be wholly degradable.