by Tamara Scully

Silage, when properly managed, provides farmers with a means of preserving the harvest – retaining the dry matter and the nutrients – to feed their livestock. But silage spoils. And every step along the way is another chance for spoilage organisms to cause degradation of the feedstock and cause problems for the animals.

The role that wild yeasts play in silage spoilage was discussed by Dr. Limin Kung of the Dairy Nutrition & Silage Fermentation Laboratory at the University of Delaware during the recent virtual 2021 Catskills Regional Agricultural Conference.

“This is certainly a topic of a lot of research and we really don’t know a lot about what happens in the silo,” Kung said of the role of wild yeasts in silage spoilage.

What is known is that it is aerobic instability – and not fermentation – that is the primary cause of significant silage losses, accounting for more than 60% of total silage losses in ensiled systems, he said.

When air is allowed into a silage pile, respiration, fermentation, seepage and aerobic instability occur. Under good silage management practices, about 10% – 15% of the dry matter is lost during storage and feedout. But when management is poor, losses can rise to over 30%.

“Any percentage loss is a reduction in net farm income,” Kung stated.

When there is no air allowed into the silage pile, good lactic acid bacteria rapidly cause the pile to ferment. This fermentation occurs as the bacteria utilize the sugars in the forage to form lactic acid. The lactic acid causes a decrease in pH, and acetic acid is also produced. Temperature of the pile initially increases as a result, but rapidly decreases and then remains stable. Reaching this stabilization point as quickly as possible is the goal.

“If we are able to maintain the anaerobicity of the system…we create a really high-quality silage,” Kung said.

The goal when making silage is to pack it densely, cover it well and prevent air from accessing the pile. Properly defacing the pile and not allowing air access into the pile during filling, storage and feed out are important factors in maintaining quality.

Wild Yeasts

Silage contains many microbes, including yeasts and molds. In a 2011 study, silage from various New York, Idaho, Washington and Ohio farms was analyzed and a wide variety of wild yeasts were identified, with variations across samples.

“We don’t understand why there’s so many yeasts, and why they are so different across locations and farms,” Kung said.

What is known is that there are two types of yeast: anaerobic fermenting and aerobic lactic acid metabolizing yeasts. Those that cause fermentation are found in anaerobic systems, turning sugars into ethanol, carbon dioxide and water.

But the yeasts which oxidize lactic acid into carbon dioxide are the primary initiators of spoilage in silage and in total mixed rations.

Wild yeasts cause a loss in silage dry matter as carbon, which could have been used by livestock for energy. It’s instead lost to carbon dioxide. Primary spoilage initiated by yeast increases the aerobic instability of the silage.

Excess air present during the fermentation stage of ensiling interferes with the lactic acid fermenting organism’s ability to utilize sugars. The sugars in the ensiled forages will still decrease, but not as quickly as they should. As a result, the pH cannot decrease as quickly. This in turn exposes the silage to prolonged periods of heating, which decrease the energy available to livestock, and also decrease the feedstock’s nutritional value. The stable period, which should occur after a rapid fermentation stage, is destabilized, and the fermentation stage is prolonged.

Even after the silage is fermented, air entering the system can spoil the best silage. Air entry during storage and feedout allow the lactic acid-producing yeasts to activate, at which time they begin to destroy the highly degradable nutrients (the sugars) in the silage. Heat is produced, and the pH increases as the amount of lactic acid decreases. This occurs very rapidly, over a period of hours. The temperature of spoiling silage can easily reach 140º F or more, and can even spontaneously combust.

At this stage, a second array of damaging microbes, including bacteria and molds, become activated by the conditions resulting from the spoiling silage.

“Molds are not the cause of the aerobic instability,” Kung said. But the presence of molds indicates that spoilage has occurred.

Minimizing Yeast

A high level of yeast in the field does not mean that yeast levels will be high in the silage. Yeast can rapidly increase in numbers at warm temperatures, and getting forage samples from field to lab can cause exponential levels of yeast growth to occur.

A well-managed silage pile will actually decrease yeast numbers during storage. Well-preserved silage should have no more than 500,000 cfu yeast/gram. But the ideal target is 100,000 or less cfu/gram, Kung said.

In comparison, a probiotic feed additive might be given at a rate of 0.15 grams of yeast per cow per day. So the amount of bad yeast fed in spoiled silage is exponentially larger than the amount of good yeast given to promote health.

“You have to be worried about feeding a huge amount of spoilage yeast and the potential it has for negative effects on your cows,” Kung said.

Studies have shown that rumen digestibility of fiber is decreased as the amount of bad yeast per milliliter of rumen fluid is increased. And as mold levels increase in TMR, milk production of lactating cows drops significantly. Spoiled rations also cause reduction in dry matter intake.

Monitoring for silage spoilage after removal from the pile is important, as spoiled silage will contaminate TMRs upon mixing. TMRs should be mixed fresh just prior to feeding. In the summer, spoilage happens faster. Even fresh TMR will rapidly spoil when mixed with a small amount of spoiled silage. In studies, TMR inoculated with additives to prevent spoilage was mixed with as little as 10% spoiled silage, and it became spoiled within 24 hours. When the amount of spoiled silage increased to 40%, it took less than five hours for contamination to occur.

Once silage is “cured,” temperatures of 95º – 100º F indicate that spoilage is occurring. Taking the temperature in the feed bank two hours after feeding to determine if spoilage is occurring is recommended. Musty smells or visible mold are also signs of spoilage.

Filling the bunk quickly, but not too quickly; ensuring a dense pack; properly covering with plastic and using enough tires to weigh down and keep the air out; avoiding exposure of the pile to air as much as possible during feedout – all of these are essential to keep wild yeasts from spoiling your silage.

The effects of silage spoilage by wild yeast aren’t well understood. The presence of toxins is a possibility. The yeast may be causing digestive changes in the cow. And wild yeast may decrease DMI through changes in taste, smell or feel of the feed. Avoiding spoiled silage from wild yeast populations is prudent and possible.

“We do know how to minimize their numbers,” Kung said.