When silage is exposed to air at opening the silo, or after its removal from the silo, fermentation acids and other substrates are oxidized by aerobic bacteria, yeasts and moulds. The aerobic stability of silage is a key factor in ensuring that silage provides well preserved nutrients to the animal with minimal amounts of mould spores and toxins. The concentration of fermentation products in silage is determined both by the initial chemical composition of the crop itself and by the type(s) of microorganisms that develop during the storage period.