Rumen micro-organisms ferment feeds to short chain fatty acids (SCFA) with the production of ATP. Measuring SCFA concentration in vitro could therefore be used to predict the yield of ATP in vitro. This estimate could then be used to predict the yield of synthesised microbial biomass. However, feeds may interact when they are incubated together, and so it is quite possible that ATP yield is not additive. The objectives of this experiment were therefore to determine whether the yield of total SCFA produced by feed mixtures was additive, and also whether the ATP yield of feed mixtures could be predicted from the calculated ATP yield of the individual feeds.