Summary Forage stored in conventional tower silos undergoes fermentation. This process generates toxic gases at concentrations likely to cause serious poisoning, which can result in death. Due to insufficient oxygen and the presence of these gases, it is difficult to determine the moment when the internal atmosphere is healthful. Since ventilation is the best means of ensuring the healthfulness of these enclosed spaces, the authors documented the critical period of forage fermentation and the production of toxic gases in a specific situation. They then formulated recommendations on the appropriate ventilation technique and on the parameters for using a practical ventilation system that is adapted to silos.