Then, at feedout, heating occurs and off odors are generated in the total mixed ration (TMR). Ultimately, feed refusals increase. This result is typical of secondary fermentation caused by yeast growth when silage and TMR are exposed to air.
Yeast consumes lactic acid and sugars, which increases silage pH. The rising pH leads to increased molds and spoilage bacteria. The growth of wild yeast, mold and bacteria generates heat, carbon dioxide and water. Cows tend to eat less, increasing the risk of rumen upsets. Lower feed intake also leads to a drop in milk volume and fat yield. Losses in dry matter consumption associated with aerobic instability amount to 5% to 7% under good management conditions and up to 10% to 20% in poorly managed silos.
How could this happen?
With the advent of bunker silos, large-capacity storage structures and the widespread use of TMR, producers saw a rise in feed instability in the feedbunk. Research concluded that silage deterioration was due to a cascade of events, led by yeasts developing under aerobic conditions and resulting in feed spoilage, refusals and a drop in milk yield.
Wild yeasts are found everywhere in nature: soil, dust, plant material, manure, grain, on your skin and inside your gut. Today, industrial processes mass-produce domesticated yeasts (mostly Saccharomyces spp. strains) for our benefit. These beneficial yeasts are used in the production of ethanol – for alcoholic beverages (beer, wine) – and for our daily loaf of bread. Other non-domesticated strains are commonly described by the term “wild yeasts.” We find them on grapes, in vineyards and in sourdough starter, but dairy producers get acquainted with them in a less enjoyable manner. Wild yeasts find their way into harvested forages, causing instability of fermented feeds, dry matter losses and further production problems, as described previously.
Yeasts are inhibited at a pH below 2.0; in well-packed, airtight silage; and when free water is below 0.62. Wild yeasts thrive when they are exposed to air (oxygen), and they come back to life at feedout when air enters the bunker face and when mixing the TMR. Once the feed is in front of the cows, producers further expose the feed to oxygen when they push up feed. With the help of fans for barn ventilation, the feed is further exposed to oxygen.
Yeasts grow on plant material, standing crops and post-harvest forages. Yeast numbers increase as plants mature with the help of air, moisture and warm ambient temperatures. While this is beneficial on wine grapes, it is the source of problems associated with yeasts present in ensiled forages. Yeasts proliferate on stressed or diseased crops, as well as on crops affected by drought, insects, parasites, fungal or viral diseases, with abrasion and pest wounds. These are all opportunities for yeasts to grow on the standing crop. Even small yeast counts on the plant will explode to a sizable population capable of significant feed problems.
Rock River Lab tested forage samples from 2014-17, and the data showed a rise in yeasts to more than the rule-of-thumb maximum level of 100,000 cells per milliliter per gram of feed. The lab came up with an equation where the 100,000-per-gram level meant 10% spoiled feeds. It would be very easy to reach 1 million per gram in a rapidly degrading TMR, which would mean 100% spoilage. Yeast proliferates with a new generation every one to two hours. In well-preserved feed with a starting population of less than 10,000 per gram, it may take only 12 hours to reach 1 million per gram. Dr. Limin Kung, professor of animal science at the University of Delaware, cited work completed in conjunction with Cumberland Valley Analytical Services and reported the distribution of corn silage yeast counts where 35% of corn silage samples had more than 1 million per gram and the average corn silage sample contained 15 million per gram.
What can be done?
Numerous articles and scientific publications discuss the best practices and recommended moisture levels and maturity stage guidelines to achieve the best possible silage. Good silage management increases the odds of improving aerobic stability and reducing dry matter losses on the dairy farm. However, let’s look at less frequently explored possibilities to help reduce the problem of yeasts in silages.
1. Modern agronomic practices
Reduced tillage increases soil structure and profile biodiversity, all good for crop health and performance. However, the plant material left in the field post-harvest is the perfect host to wild yeasts and other fungal organisms (molds, in particular). If a dairy producer chooses to practice minimum tillage or no-till, he or she is faced with an increased wild yeast challenge year after year.
One management method recommended to reduce fungal growth is to shred residual crop material to facilitate degradation. A study completed in Italy demonstrated that minimum soil disturbance will likely favor a shift to fungal population (yeasts and molds) in the surface layer (zero to 15 centimeters or zero to 5.9 inches) of soil. Forage lab analysis has also reported that yeast levels in forage samples analyzed are directly related to ash levels. The Italian researchers therefore concluded harvesting techniques that include higher amount of soil and dust in the forage will lead to higher levels of yeasts and risk of further feed spoilage at feedout. Caution should be taken to harvest the least amount of soil by chopping higher and raising the cutting bar of the harvesting equipment.
2. Hybrid selection
Planting different-maturity hybrids of the same crop will allow time to meet the recommended moisture and maturity at harvest time and achieve silo filling and packing to match harvesting capacity and volume. Match the bunker silo capacity with acreage of same-maturity crop to enable quick and complete silo filling to improve aerobic stability.
Choosing hybrids or varieties that are disease-, pest- and drought-resistant when planning for next crop season helps reduce plant susceptibility to hosts like yeasts and molds.
3. Increase crop rotation
In a system using the same crop year after year, a selection is made in favor of fungal micro-organisms best adapted to that specific crop. Rotational systems may disrupt that selection and improve silage quality.
4. Apply fungicide
Fungal diseases can lead to increased plant susceptibility to host yeasts and molds. Application of foliar fungicide to corn silage seems to increase dry matter yield, decrease the amount of fiber, increase sugar content, increase the amount of rumen-degradable silage and increase predicted milk per ton and milk per acre production. Several studies report positive impact of fungicide applied to corn.
A recent study from the University of Illinois showed that when treated corn silage was fed to dairy cows, they tended to have higher fat-corrected milk (FCM) and energy-corrected milk (ECM) feed conversion compared to control, and the potential benefits associated with these increases in efficiency led to a positive income from milk over feed costs. Currently, there are several chemical fungicides approved for corn, and the future will bring some interesting botanical products with fungicidal properties for forage crops.
5. Separate and treat affected crops
Producers may choose not to harvest heavily affected crops or to ensile separately in silage bags, allowing feeding of these affected crops after treatment with an organic acid blend to increase aerobic stability. Figure 1 can help producers better understand the need for a blend of acids.
The figure shows the results of various dosages of organic acid and the efficacy of these specific acids (propionic, acetic and sorbic) against different molds and wild yeast species.
When using temporary silage structures (bags or piles), test forage samples to detect the level of yeast challenge and evaluate fermentation profiles. High levels of acetic acid, when present in silage, help fight off possible yeast development at feedout or when moving silage. From these forage testing results, you can decide if treatment with an organic acid blend is needed. If the plan is to move the silage in the bag, do so quickly in colder weather and monitor stability.
When putting up silage, use a proven forage inoculant including Lactobacillus buchneri at a minimum level of 400,000 per gram of forage to help produce high levels of acetic acid in the finished silage. Meta-analysis proved that superior aerobic stability was achieved with products providing 400,000 per gram of L. buchneri (600,000 per gram for ensiled high-moisture corn). These levels led the FDA to grant a specific claim for improved aerobic stability for silage and high-moisture corn.
When faced with weather challenges, it is easier to deal with forage harvested earlier and at higher moisture rather than waiting for ideal conditions and harvesting too late and too dry. Drier forage will not ferment adequately, packing density goals will not be met, and this issue can lead to major forage losses. Forage harvested at higher-than-recommended moisture can be dealt with using a good inoculant and still give satisfactory results.
If rain is the cause for delay, remember that rain splashes soil particles on plant stems and leaves, inoculating the material with undesirable micro-organisms, including yeasts. With wet or lower-sugar-containing crops, use a proven general-purpose inoculant, like Lactobacillus plantarum to drop pH below 5.0 within three days of ensiling, to improve fermentation outcome. Additives like organic acid blends are also helpful in non-ideal forages. By reducing competition from non-beneficial organisms, organic acids assist the fermentation process by allowing beneficial silage bacteria to grow and generate an initial pH drop and make a critical difference.
In conclusion, hybrid selection management practices like crop rotation and tillage and fungicides all have benefits and disadvantages. However, when used in combination along with other good silage management methods, like using an inoculant or an organic acid blend, they can help mitigate the presence of yeasts in silages and further reduce their negative impact on feed stability and milk yield. 
PHOTO: Photo by Mike Dixon.
References omitted but are available upon request. Click here to email an editor.
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Eugene Rodberg
- Sr. Product Manager
- Kemin Industries
- Email Eugene Rodberg
