There are two main classes of fungi that can be observed during the harvest, storage and feedout of ensiled forages: single cell yeasts and multicellular filamentous colonies of molds. In the field, fungal diseases of corn are characterized by yield and quality loss, and mycotoxin contamination.

Bryan keith
Technical Services Manager – Ruminant DFM and Silage Inoculants / Chr. Hansen Animal Health & Nutrition

Contaminant occurrence and concentrations vary by year due to variation in growing conditions, plant stressors and storage differences. Although the ensiling of forage, due to rapid rates of lactic acid production and subsequent decrease in pH, often controls numerous naturally occurring microorganisms, this is not predominantly the case for yeast.

Many types of yeast are not inhibited by acids and are able to tolerate pH 2 or below. After harvest, temperature and water activity (aw) are the primary factors influencing mycotoxin contamination of feeds, and molds grow over wide ranges of temperature (50-100°F), pH (4-8) and water activity (greater than or equal to 0.7); however, the presence of molds does not guarantee the presence of mycotoxins, nor does the absence of molds guarantee the absence of mycotoxins.

Oxygen is the enemy and leads to proliferation of yeasts and molds that convert sugars into energy yielding heat, CO2 and water. First, high epiphytic yeast populations are ensiled; during slow fermentations, growth of yeast occurs until oxygen is depleted in the silage. At feedout, yeasts are reexposed to oxygen and grow exponentially; lactic acid and residual sugars are consumed. As sugars are consumed, heating occurs and pH rises; thus, molds proliferate, resulting in aerobic instability.

Various molds capable of producing a variety of deleterious mycotoxins, which develop during storage of fermented feeds, represent potentially significant problems during feedout. Therefore, yeast and mold proliferation in fresh or poorly ensiled forages represents a significant loss of valuable nutrients, dry matter and value.


Throughout late summer through early winter of 2014, we collected numerous samples of freshly chopped corn prior to and after ensiling. These samples were analyzed for a variety of items, especially yeast and mold counts. Primarily centralized to the upper Midwest, but also reported elsewhere, freshly chopped corn silage repeatedly yielded yeast counts in the 1,000,000 to greater than 30,000,000 (log 6-7.5) cfu/g range, and mold counts from 100,000 to greater than 1,000,000 cfu/g (log 5-6; average = log 5.6), with the predominant species being Monacsus spp. and Fusarium spp. In shortly fermented corn silage (less than 75 days), mold counts were observed typically from 10,000 to 100,000 cfu/g (log 4-5; average = log 4.9), with the predominant species being Penicillium spp., Aspergillus spp. and Fusarium spp.

Yeasts in fresh or poorly ensiled forage are undesirable for three reasons:

  1. They compete with lactic acid bacteria for sugars, which are fermented into ethanol.
  2. They are responsible for “heating” the TMR and loss of vital nutrients.
  3. High levels of wild-type yeasts in rations for lactating dairy cows have been repeatedly associated with lower or inconsistent dry matter intake, milk fat depression, rumen upset and general lack of optimal performance.

Molds originating on the crop in the field or resulting from poor ensiling practices are undesirable for two primary reasons: They result in spoilage that alters nutrient composition, feeding values and feed inventory, and they produce a variety of mycotoxins that can cause acute toxicity or chronic, subacute mycotoxicosis in lactating dairy cows due to low-level consumption over time.

Mycotoxicosis is characterized by reduced dry matter intake or feed refusal, decreased nutrient absorption and impaired metabolism, suppressed immune function and altered microbial growth.

Testing forage and feed samples for mold spore counts and mold identification can be helpful to diagnose subacute mycotoxicoses. It could also be indicated by suboptimal lactational performance. As mentioned previously, oxygen is the enemy of stored fermented feeds, especially corn silage, due to the enhanced metabolic activity and replication of molds and yeasts in the presence of oxygen.

Best management practices for feeding silages with potentially high yeast and mold counts include:

  1. Periodic laboratory testing of corn silage and other feedstuffs for the presence of yeasts and molds (subsequent mycotoxin screening or analysis may be warranted).
  2. Feedout face should be smooth to minimize oxygen exposure.
  3. Remove at least 6 to 12 inches per day in cold weather months, 12 to 18 inches per day in warmer months, and minimize the time between silage removal and feeding.
  4. Daily initial removal from the silage face should be set aside and fed to heifers and far-off dry cows, leaving fresh, minimally oxygen-exposed feed for the highest-producing groups of cows.
  5. Discontinue feeding aerobically unstable feed, and discard any visibly moldy feed.
  6. Consider feeding a multi-component, direct-fed microbial that may help combat the deleterious effects of yeasts, molds and mycotoxins in lactating dairy cows while stabilizing potential inconsistencies in the rumen, leading to improved energetic efficiency and ROI. PD

Keith Bryan

Keith A. Bryan
Technical Services Manager
CHR Hansen Animal Health and Nutrition