Silage smells and what they may mean

As a kid, silage always seemed to stink and it is a smell which has been imprinted on my brain.

Now I work with the stuff. I even have a personalised number plate with the word SILAGE on it! Whenever I  introduce myself to people from town and they ask me what I do and I mention the word silage, they instantly screw up their noses and say something like “How can you work with that stuff, it stinks?” or they ask “Are you still married?”

Prior to the maize silage making season, I thought it would be a good opportunity to run through some of those silage smells and outline what they may be telling us.

First, a word of precaution.  If you want to smell your silage, don’t stick your nose into a handful and inhale deeply.  Some silages may contain spores harmful to human health.  Put your silage sample on a flat surface and use your hand to waft the smell up to your nose.

That said, let’s get back to the odours.

Desirable smells:

Little or no smell.  The most desirable end product of the fermentation process is lactic acid. Lactic acid is nearly odourless.  So if the silage doesn’t smell or has a slightly earthy smell that is a good sign.

Slightly vinegary.  If you have used an inoculant like Pioneer brand 11C33 or 11CFT which contains L. buchneri, then a vinegar odour shows the bacteria have done their job. L. buchneri bacteria take lactic acid and convert it to acetic acid, making the silage less likely to heat when exposed to air. Acetic acid is what is in vinegar.

Undesirable smells:

Vomit/rotten meat.  This is the smell I remember as a child, mainly in grass silage though.  If the silage stinks, it has probably gone down a butyric acid fermentation pathway. This usually happens when the silage contains a lot of soil or was harvested too wet.  Not only do humans hate the butyric smell, cows do too and they usually need to be very hungry before they will eat the silage. It is important to note that this smell nearly always occurs in grass silage rather than maize silage.

Vinegar. If your maize hasn’t been inoculated with a proven L. buchneri inoculant (as outlined above) and it smells of vinegar (acetic acid), a less efficient fermentation has occurred, increasing feed losses and possibly decreasing palatability.

Alcohol, sweet, fruity, buttery or even butterscotch. The silage may smell really good to us but these smells all indicate the presence of yeast prior to the maize being fully fermented.  Once again this means an inefficient fermentation pathway and greater energy and drymatter losses. Some farmers wrongly associate these smells with good silage.

Acetone (nail polish remover). This smell usually occurs once the stack has been opened and exposed to air.  This is a precursor to rapid heating, and therefore losses.

Ammonia, caramel or tobacco.  All these indicate heating when the silage was being ensiled. Ammonia indicates the breakdown of feed protein and caramel and tobacco indicate the loss of sugar. Cows often love tobacco smelling silage but usually turn their noses up at ammonia.

What to do about smells.

As always, the best way to ensure your maize doesn’t end up stinking is by doing everything right in the first place.  I have covered how to make great silage in many previous articles but you may also want to read the following article: http://www.pioneer.co.nz/maize-silage/product-information/silage-technical-insights/storing-a-maize-silage-crop.html

If your silage does smell and your cows don’t want to eat it, the only solution is dilution.  Remove all obvious rot and mould, and then try to incorporate any bad smelling silage with a large amount of good smelling silage.  Some farmers have found the addition of powdered molasses can help.

Finally, the best way to get an assessment of silage quality is to send a sample to a commercial feed testing laboratory. 

• Ian Williams is a Pioneer forage specialist. Contact This email address is being protected from spambots. You need JavaScript enabled to view it.