Your nose and your eyes can tell you a lot about silage quality or problems.

"Sensory evaluation may suggest the need for further chemical or physical characterization of the feed should a problem be identified," say Robert J. Van Saun, Penn State University Extension veterinarian, and Jud Heinrichs, Penn State dairy scientist.

First, look at the contour of the bunker face, say Van Saun and Heinrichs. Ideally it should be very smooth and straight. This minimizes oxygen exposure to the silage. Bunker silos with irregular and uneven faces have greater surface area exposed to oxygen and therefore a greater chance at increased microbial activity.

As silage is reintroduced to air, mold and bacterial spores already in the silage can begin metabolism again. This metabolic activity will result in silage heating as well as alterations in acids and sugars available in the feed. This metabolic activity suggests unstable silage and can contribute to depressed feed intake and feed refusals.

This secondary heating is usually not sufficient to cause significant heat damage to the silage. The true value of bunk face management is not well known and is related to the density of the silage, the season of the year, and the amount of silage face that is removed each day.

Second, the color of the silage can indicate potential fermentation problems (See Table 3). Silages with excessive acetic acid will have a yellowish hue. Those with high butyrate will have a slimy, greenish color. Brown to black silage usually indicates heating from fermentation and moisture damage. These silages have the highest potential for molding and are unacceptable feeds. White coloration of silage is usually indicative of secondary mold growth.

Table 3 -- Odor and color evaluation of silages.






Acetic acid production (Bacillus)



Ethanol production (Yeast)

Sharp sweet


Propionic acid production

Rancid butter


Butyric acid production (Clostridium)


Dark brown to black

High temperature, Heat damaged


Third, silage odors can also help you evaluate fermentation. Normal silage has minimal odor because of the lactic acid. If acetic acid production is high, then silage may have a vinegar smell. High ethanol content from yeast fermentation may impart an alcohol odor to silage. Clostridial fermentation results in a rancid-butter smell. Propionic acid fermentation results in a sharp, sweet smell and taste. Heat-damaged silages will have a caramelized or tobacco smell. No silage should have a musty, mildew or rotten smell due to molding. Remember if the smell of the silage is really unpleasant to you, most likely it will be refused by cattle or at least reduce intake.