Organic acids, most commonly fumarate and malate, when offered to ruminants have been shown to produce a decrease in methane emissions along with an increase in in vitro DM digestibility in rumen fluid taken from sheep (López et al., 1999). They are classed as ‘preservatives’ in the list of feed additives authorised by EU legislation, and as such are permitted for use in livestock (Castillo et al., 2004). However, the addition of fumaric acid to the diet has been associated with inappetance and problems of reduced rumen pH. Fumaric acid encapsulated in a soya oil capsule enables a slow release of fumaric acid in the rumen, which should overcome these problems. Encapsulated fumaric acid (EFA), when offered to lambs as 10% of the diet, reduced methane emissions by 75% and increased feed conversion efficiency by 20% (Wallace et al., 2006). To date, no in vivo studies have been carried out on dairy cattle. Therefore, the objective of this study was to determine the effect of addition of EFA on animal performance and methane production from grazing dairy cows.