When corn silage or haylage doesn't ferment properly, your nose may help you identify the problem, says Limin Kung, a University of Delaware dairy scientist.

For example, producers often think sweet-smelling silage is a good thing, Kung says.

“Unfortunately, this is not necessarily so, since the sweet smell is probably coming from a high concentration of ethanol, which is produced by spoilage yeasts mixed with acetic acid.”

High ethanol levels, generally easy to detect in corn silage, indicate that a significant amount of dry matter may have been lost.

“Most likely, this silage will heat very rapidly in the bunk,” Kung says.

Little can be done to correct the problem, he adds.

A rank, fishy smell indicates butyric acid.

“Butyric acid smell is commonly associated with grass and legume silages that are too wet — less than 30% dry matter,” Kung says.

If you detect the odor, check the silage's dry matter and ammonia content. Then limit the amount fed to high-producing cows.

A vinegar smell (from acetic acid) may be good or bad, depending on what the silage was treated with, Kung says.

“Silages treated with the new inoculant containing Lactobacillus buchneri tend to have a more vinegary smell than untreated silages,” he explains. “The moderately higher concentrations of acetic acid help to improve bunk life. However, high concentrations of acetic acid from uninoculated silages may indicate a problem because the acetic acid has come from a wild fermentation, and palatability is often poor.”

Silages suspected of undergoing a wild acetic acid fermentation can be aired out for a short period before feeding. Adding molasses to mask the smell and taste may also help. In worse-case scenarios, limit the amount of forage offered to cows.

Dry alfalfa silage, usually greater than 45-50% dry matter, may smell like tobacco mixed with molasses.

“Excess heat has caused proteins to bind with fiber and sugar molecules, and if the reactions are excessive, significant amounts of amino acids become unavailable to the cow,” Kung says.

“A mildly sweet tobacco or 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,” he points out. “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 nitrogen (ADIN). Then the ration should be adjusted to ensure a proper balance of degradable and undegradable protein.

When silage has undergone aerobic spoilage, the result is a musty or moldy smell.

“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, and are either in the middle of spoilage or have already spoiled. The bottom line is extensive losses in nutrients, dry matter and palatability.”

Use a propionic acid-based preservative in the TMR as a temporary fix. Increasing the feed-out rate may also help by limiting the amount of time silage is exposed to air, Kung advises.