Timothy-meadow fescue herbage was ensiled with formic acid (FA) (expressed as 100% solution) at the rates of 0, 2, 4 or 6 L t )1 . The silages were fed along with concentrates to bulls fitted with cannulae in the rumen and duodenum. The ration comprised grass silage (700 g kg )1 ), barley (240 g kg )1 ) and rapeseed meal (60 g kg )1 ). The application rate of FA had no effect on the site or extent of the digestion of dietary organic matter (OM) and neutral-detergent fibre. The flow of total N at the duodenum increased linearly (P < 0AE05) with application rate of FA, reflecting mainly an increased (P < 0AE01) flow of microbial N. The apparent efficiency of net microbial protein synthesis in the rumen increased (P < 0AE05), the proportion of propionate in the volatile fatty acids (VFA) in the rumen was not affected (P > 0AE05) but that of butyrate increased (linear and quadratic effects, P < 0AE01) with increasing rate of FA. It is concluded that an increase in the rate of FA at ensiling leads to a higher utilization of energy and/or protein-yielding substrates for rumen microbes and to a modified rumen VFA pattern with an increased proportion of butyrate.