AbstractFormaldehyde, Bronopol, tylosin and pimaricin were tested as silage additives in simulated silage based on laboratory media and microbial inocula. Fermentation was restricted by 33 mM formaldehyde, 1.2 mM Bronopol and 0.2 mM tylosin. Pimaricin had little effect on fermentation but a level of 0.3 mM prevented mould growth when the silages were exposed to air. The antimicrobial effects of formaldehyde and Bronopol, the antibacterial effect of tylosin and the antimycotic effect of pimaricin support the results obtained in earlier work with cultures of silage microorganisms. The concentrations required to prevent growth of lactic acid bacteria were, however, about four times higher than in pure culture tests. The levels of formaldehyde and Bronopol required to prevent fermentation in the simulated silages are, however, less than in needed in the test‐tube silos containing forage.