WHAT THE SMELLS FROM SILAGES CAN TELL YOU 

Limin Kung, Jr., Ph.D.
University of Delaware

            Can you tell the quality of silage by smell?  The answer to that question is sometimes yes, but sometimes no. When producers detect ”sweet” smelling silage, they often think that this is good.  However, this is not necessarily so, since the “sweet” smell is probably coming from high concentrations ethanol (produced by spoilage yeasts) mixed with acetic acid.  A high concentration of ethanol that is usually easily detected in high moisture corn and in corn silage, is an indicator that a significant amount of dry matter may have been lost.  This silage will also most likely heat very rapidly in the bunk.  In fact, the best type of silage fermentation, that is called homolactic acid fermentation, should have little or no distinct smell because lactic acid has almost no smell. 

            Butyric acid in silage smells rank, rancid, and sometimes fishy (due to protein degradation).  The butyric acid smell is commonly associated with silages that are too wet.  You probably will never smell this odor if silage that has a dry matter content greater than 35 to 40%.  Ironically, silage with a high butyric acid content is very stable when exposed to air and will not heat, but this silage is low in energy content.  

            If your silage smells like “vinegar”, this is a sign that heterolactic fermentation (a less desirable type of fermentation) occurred because vinegar is acetic acid.  Silage with high concentrations of acetic acid is usually stable when exposed to air but sometimes-high concentrations of acetic acid can depress dry matter intake in cattle.  

            Another common smell in silages is often associated with dry alfalfa silages (usually greater than 45-50% DM) and can be described as a “tobacco/molasses” type of smell.  This smell comes from the fact that excess heat has caused proteins to bind with fiber and sugar molecules via the “non-enzymatic browning reaction”.  A mildly, sweet tobacco/molasses type smell is okay in alfalfa silage but any hint of this smell in corn silage is a definite indicator of heat-damaged protein.  Normally, heat damaged protein in corn silage is only a problem if the DM is in excess of 45% DM.  In all silages, when the smell turns from tobacco-like to burnt, you can be sure that excessive heating has taken place.  Silages suspected of having excess heat damaged protein should be tested for bound nitrogen (also known as unavailable nitrogen or acid-detergent insoluble fiber, {ADIN}) and protein requirements adjusted accordingly.   

            The last smell that is commonly associated with silages is the “musty- moldy” smell that comes when silage has undergone aerobic spoilage.  The smell of moldy silage is very distinct.  However, this is not always an indication of mycotoxin formation.  Silages that smell moldy are usually hot and steamy (or have already gone through a heat).  In addition to extensive losses in nutrients and dry matter, such silages are very unpalatable. 

            Table 1 lists some things that could be done when silage has a particular smell.  Overall, silage should be wilted to the correct moisture, chopped to the correct length, and silos should be filled rapidly, packed tightly and sealed quickly.  Use of various silage aids can be helpful but should not be used in lieu of good management. 

Table 1.  Silage smells and what to do about them. 

Silage smell	Management Check
vinegar, acetic acid	- Use a microbial inoculant next season to obtain a more desirable fermentation.
butyric acid, rancid, fishy	- Check silage DM.  Wilt to > 30% DM next season.  - Use a microbial inoculant next season to obtain a more desirable fermentation.
ethanol	- Use microbial inoculant or propionic acid-based preservative on next season’s crop.
moldy, musty	- Increase feed out rate.  - Use microbial inoculant or propionic acid based preservative on next crop.  - Use propionic acid-based preservative in TMR (a temporary fix)
strong tobacco, burnt smell	- Check silage DM.  Lower moisture content of silage.  - Check chop length, pack tightly, and pack quickly.  - Use microbial inoculant or propionic acid-based preservative on next season’s crop.