AbstractThe microflora of experimental batches of grass silage treated with formaldehyde, with formaldehyde plus formic acid, or with sodium benzoate and held at a constant temperature of 20°C was examined during fermentation and after exposure to air. The formaldehyde treatments inhibited the development of lactobacilli during fermentation but their biocidal effect did not persist and on exposure to air microbial numbers increased and the pH of the silages rose at a similar rate to that in untreated silage. Treatment with sodium benzoate had little effect on fermentation but its biostatic, particularly fungistatic, activity persisted through 17 days of exposure to air during which the pH did not rise and fungal numbers remained low. The yeast flora of all treatments was predominantly of fermenting species similar to that found in stack silages but the mould flora, of Geotrichum candidum, Mucor spp. and Penicillium spp., was dissimilar and could be related to the lack of heating in the experimental silages.