Forage lipids contain a high proportion of the polyunsaturated fatty acids (PUFA) C18:2n-6 and C18:3n-3. However, microbial biohydrogenation in the rumen results in extensive loss of these beneficial fatty acids. This experiment investigated the duodenal flow of fatty acids in beef steers offered grass or red clover silage, the latter of which has been shown to reduce biohydrogenation of C18:3n-3 (Lee et al. 2003a). In addition the effect of silage prepared from a ryegrass with high water soluble carbohydrate (WSC) content was investigated, as previous research (Lee et al. 2003b) has shown that increasing the availability of WSC lowered rumen pH and increased the proportion of propionate, relative to other volatile fatty acids, conditions which have been linked to reduced biohydrogenation (Jenkins, 1993).