The aim of the experiment was to assess the effect of applying formic acid at ensilage or formic acid/formaldehyde to the standing crop of herbage before ensiling upon the subsequent digestion of the silages by Jersey heifers. Unwilted grass silages were made from perennial rye; rass ensiled at a DM of 220㎏^(-1) with application of 2.22 liters ton of formic acid and wilted grass silages was made by spray to the standing crop of herbage with 90 liters ㏊^(-1) of a solution containing 50 liter formalin, 500㎖ of 10% Tween 20, and 2.5 liter water giving an application rate of 61g formaldehyde ㎏^(-1) herbage crude protein. The silages were fed to cattle equipped with rumen cannulae and duodenal re-entrant cannulae. Results from fermentation in the rumen and from the digestion of organic matter showed major differences between silages. The efficiency of rumen microbial nitrogen synthesis was also affected by additive treatment. Microbial N synthesized per ㎏ organic matter apparently digested in the rumen was 20g for the formic acid silage and 17.68 for the formic acid/formaldehyde silage. Degradability of feed N in the rumen was similar for two silages.