AbstractThe purpose of this study was to evaluate the efficiency of two inoculants of lactic acid bacteria (LAB) on grass silages. The evaluation was made in terms of silage composition, rumen fermentation, digestion and animal production. Over a 2‐year period two lots of silage were prepared in two 100‐tonne capacity clamps. In each case an untreated silage and an inoculant‐treated silage were prepared simultaneously from an identical herbage source using independent sets of silage‐making equipment. The inoculant used in the first year was Pioneer Brand 1188 (Pioneer Hi‐Bred UK), and in the second year a slightly different inoculant from the same source (Pioneer Hi‐Bred) was used. The two inoculants were essentially similar in composition, comprising a mixture ofLactobacillus plantarumandStreptococcus faecalis; the strains included in the second inoculant were selected for their capacity to tolerate higher temperatures. The fermentation of the inoculant‐treated silages appeared to be dominated by homofermentative LAB compared with the untreated silages. The use of either inoculant was associated with a change in the rumen volatile fatty acid (VFA) pattern, with a significantly greater molar proportion of propionate and a corresponding reduction in both acetate and butyrate. There was a trend indicating an enhanced efficiency of microbial protein synthesis within the rumen of the heifers for the inoculant‐treated silages, although this was only significant when concentrate was offered with the silage in one experiment. The inoculant‐treated silages had significantly greater intake characteristics than did the untreated silages, though this was not reflected in a statistically significant improvement in average daily liveweight ga