There has been increasing misuse of terms and units for measurements of digestibility and degradability of dietary components and a number of papers have had to be referred back to authors for further clarification. Accordingly, it is felt that an explanatory note would be of value as guidance for future submissions. Please note the comments/calculations below.

1. 1 Apparent digestibility of a dietary component (e.g. neutral detergent fibre) is the amount of a dietary component ingested that is not excreted in faeces. It is calculated using the following formula:

   $$\displaystyle{{{\rm Quantity\;\ ingested}-{\rm quantity\;\ voided\;\ in\;\ faeces\;\ or\;\ excreta}} \over {{\rm Quantity\;\ ingested}}}$$

For example:

$$\displaystyle{{200\; \,{\rm g}\,{\rm ingested}-50\,{\rm g}\,{\rm voided}} \over {200\,{\rm g}\,{\rm ingested}}}\; = {\rm digestibility\;} \;{\rm coefficient}\;{\rm of\;} \;0{\cdot}75$$

Please note that there are no units as digestibility is a coefficient describing the proportion digested.

1. 2 Concentration of digestible component in a feedstuff or diet is sometimes preferred, which is calculated using the following formula:

   $${\rm Coefficient}\;{\rm of}\;{\rm digestibility}\, \times \,{\rm content}\;{\rm of}\;{\rm dietary}\;{\rm component}.$$

For example, from the digestibility described above:

If the total dietary concentration of the component was 100 g/kg, then

$$0{\cdot}75 \times 100\,{\rm g/kg} = {\rm Content}\,{\rm of}\,{\rm digestible}\,{\rm component}\,{\rm of}\,{\rm 75}\,{\rm g/kg}$$

Please note that the unit is g/kg and needs to be qualified further to confirm if this on a fresh weight or dry matter basis.

Please also note that the above points (1) and (2) apply also to any measurement of digestibility (e.g. ileal digestibility) and in vitro studies.

1. 3 A similar situation applies for degradability and content of degradable component in ruminant studies.

Here the key papers are Orskov and McDonald (Reference Orskov and McDonald1979) and a subsequent paper by Sinclair et al. (Reference Sinclair, Garnsworthy, Newbold and Buttery1993). The latter used the following equation (derived from Orskov and MacDonald, Reference Orskov and McDonald1979):

$$P = a + b(1-{\rm e}^{-ct})$$

where P is the cumulative amount degraded at time t; a is the readily soluble fraction; b is the potentially degradable fraction in the rumen; c is the rate constant for degradation of b and t is the time (h).

If a and b are in g/kg, then the amount degraded has to be in g/kg; however, the latter needs to be divided by 1000 to obtain degradability as a coefficient representing the proportion degraded.

Thus, the bottom line is that degradability of a dietary component needs to be expressed as a coefficient, whereas content of a degradable component needs to be expressed as g/kg; as before, this latter needs further qualification to confirm if data are on a fresh weight or dry matter basis.

Finally all the above data, either as coefficients or g/kg, should replace % that is commonly employed.