Mould is caused by fungi and can affect both forage feeds and concentrates. It is of concern particularly because some types of fungi produce mycotoxins which can poison horses. Mould generally grows best in warm damp conditions which is why the moisture content of hay and concentrates is monitored commercially and raw materials with excessive moisture may be rejected by feed mills and processors. Generally, a moisture content below 11% is considered acceptable for most concentrate feeds. Commercial concentrates very often contain mould inhibitors, feed additives which prevent or reduce mould growth. The most commonly used inhibitor is propionic acid. Other additives are mycotoxin binders designed to render mould toxins less harmful if ingested. These include clays such as bentonite and zeolite, aluminosilicates, various yeast cell derivatives and enzyme deactivators. The use of these compounds helps improve feed safety and reduce risk of harm even if the feed is subject to exposure to moisture after leaving the mill.
Any feed suspected of harboring mould either by appearance or smell should not be fed to horses.