Non-additivity occurs when the nutritive value of a mixture of feedstuffs differs from that of the sum of its components. It is most commonly observed when one dietary constituent influences, either positively or negatively, the apparent digestibility of another under conditions where components such as nitrogen and sulphur are non-limiting. In general negative effects occur due to the depression of rumen pH or substrate competition, while positive effects have been identified when readily fermentable fibre sources such as sugar beet pulp have been included in rations containing poorly fermented forages such as cereal straw. With the increasing use of in vitro systems, not just to examine feed degradation characteristics but to derive parameters such as microbial protein yield, the following study was conducted to determine whether such interactions could be identified in vitro.