There is considerable scope for modifying the extent of fermentation during silage production by selecting an appropriate silage additive, with chemicals such as formic acid restricting fermentation and/or fermentable carbohydrates stimulating the natural fermentation in the silo. Both approaches can achieve satisfactory preservation but can produce silages of quite different chemical compositions. The objective of this experiment was to compare the pattern of rumen fermentation and digestibility of a silage, which had 50 kg of molassed sugar beet pulp (MSBP) applied at harvesting, with those of an untreated silage and a silage treated with formic acid.