By Bill Mahanna, PhD, Dipl. ACAN, Pioneer Global Nutritional Sciences Manager
Yeasts can exert a profound impact on silage at the time of feeding in terms of quality and aerobic stability (heating). Yeasts are a natural component of the microbial epiphytic populations (naturally found on the crop) of corn silage, cereal silage and high-moisture grains at the time of harvest.
Yeasts are also found in grass or legume silages, especially those ensiled at less than 55% moisture. This may explain why producers are increasingly having aerobic stability issues in these nitrogenous feeds (that typically do not have aerobic stability issues) given the popularity of ensiling at lower moistures to avoid butyric acid (clostridia) problems (Muck, 2007).
Yeast populations and the metabolites they generate shift dramatically in aerobic (with oxygen) versus anaerobic (without oxygen) environments. In silage, aerobic conditions typically exist at the beginning of the ensiling process and during feedout, while anaerobic conditions predominate during the storage phase.
Yeasts can be categorized as fresh-crop, storage or feedout strains and are classified as fermenters or nonfermenters. They are further subdivided by their ability to utilize different substrates such as soluble sugar or lactic acid.
The sugar-utilizers dominate during the aerobic phase at the beginning of the ensiling process and during the anaerobic conditions of storage.
The acid-utilizers comprise the majority population in the presence of oxygen at feedout. At harvest, more than 90% of yeasts are sugar-utilizers, but the ensiling process provides selection pressure, ensuring that more than 90% lactate-utilizers are dominating at feedout (Dennis, 2007).
High counts of lactate-consuming yeasts are of greater aerobic stability concern because their metabolism of lactic acid elevates silage pH, creating an environment conducive to spoilage bacteria and mold growth.
Fresh-crop yeasts are usually nonfermenters and include ptococcus, Rhadotorala, Sporabolomyces and sometimes Torulopsis organisms. Heat, carbon dioxide and acetic acid are the main products produced by yeasts during aerobic conditions. Heat and its secondary effects can affect palatability, and carbon dioxide contributes to silage dry matter loss.
Fermentative bacteria, especially the lactic acid bacteria, utilize plant sugars to produce acids that drive silage to a stable, terminal pH. Residual sugars can be utilized during storage by anaerobic, low-pH resistant, storage-type fermenter yeasts like Saccharomyces and sometimes Torulopsis (Woolford, 1984).
Yeasts do not reproduce during anaerobic conditions. This explains why brewers add very high levels of yeasts (called pitching a big starter) to initiate the brewing process and also help prevent contamination from other microbes. While yeasts are not reproducing, they remain metabolically active, producing heat, carbon dioxide and ethanol and also byproducts, including acetic acid, aldehydes and esters (Dennis, 2007). It is also known that for every alcohol that is produced, a carbon dioxide molecule is generated, which further contributes to dry matter loss.
Ethanol production in silage is not entirely bad. Ethanol can help solubilize zein protein in corn kernels, allowing for increased starch digestibility over time in storage (Owens and Soderlund, 2007; Hoffman, 2007).
The fermenter yeasts that are active during feedout include lactic acid-utilizing Candida and Hansula species (Woolford, 1984). Yeast will reproduce during aerobic conditions (but not as fast as bacteria), explaining why overly dry, poorly compacted and slow-feedout silages with high air porosity often display such high yeast (and aerobic bacillus) counts.
Besides acetic acid and limited amounts of ethanol, aerobic conditions cause yeast to produce a large number of aromatic compounds depending upon the specific yeast strain and environmental conditions. As the temperature rises, more aromatic compounds are produced. The effect of warmer conditions producing more aromatic compounds is well documented in the brewing industry. This is why ales, fermented at 55-75°F, are considered fruitier and more aromatic compared to lagers, which possess a "cleaner" taste because they are cold fermented at 46-56°F and then stored for several weeks at 33°F (Dennis, 2007).
In silages, feedout yeasts are also capable of producing esters (fruity smell), ethyl acetate (fingernail polish smell), fusel alcohols (from amino acid degradation causing a harsh, solvent-type smell), aldehydes diacetyl — butter smell, or acetylaldehyde — green apple smell) and other compounds with solvent-like odors (Dennis, 2007).
Substrate levels also influence the level of byproducts produced by anaerobic, storage-type sugar-utilizers. As the level of sugars and temperature increase, more aromatic esters and fusel alcohols can be produced. A high level of sugars can also shift the production of alcohol to other metabolites. The production of these aromatic compounds in silages not only increases dry matter loss but can also significantly contribute to palatability problems (Dennis, 2007).
From a diagnostic perspective, aerobically challenged silages usually have yeast populations that exceed 100,000 (105) colony-forming units per gram (cfu/g) of ensiled feed. The identification of Hansula and Candida organisms usually is associated with high pH from the consumption of lactic acid, while the presence of Torulopsis usually does not have elevated pH since the organism primarily utilizes soluble sugars.
Volatile fatty acid profiles will typically show a reduction in lactic acid and an increase in acetic acid levels. However, samples taken deeper in the silage mass will typically show a more desirable pH and lactic acid level because growth of these yeasts is limited by lack of oxygen penetration (Mahanna and Chase, 2003).
Higher levels of acetate should not always be considered deleterious or evidence of high yeast contamination. Elevated acetic acid levels caused by yeasts, gram-negative acetic acid producers (e.g., Enterobacter sp.) or heterofermentative lactic-acid bacteria (e.g., euconostoc sp.) may contribute to poor bunklife or intake issues. However, silages treated with bacterial additives containing strains of Lactobacillus buchneri also exhibit lower lactic:acetic ratios yet have been shown to reduce yeast counts, improved bunklife and exert no negative effects on dry matter intake (Driehuis, 1999; Kung, 2005; Andesogan, 2006).
The increased availability of yeast counts and identification has caused some nutritionists to question if there is a relationship between high yeast silages and chronically low butterfat testing herds.
While yeasts certainly contribute to the cascading events leading to unstable silage, it is unlikely that low butterfat test can be attributed to yeast or their metabolites, per se, unless the unpalatable silage is causing sorting and reducing effective fiber intake.
A more likely culprit of fat test depression is underestimating the fiber or starch digestibility of forages, which contributes to reduced rumen pH, thus promoting the synthesis of ruminal biohydrogenation intermediates (example: trans-10, cis-12 conjugated linoleic acid; Lock and Bauman, 2007).
Short-term management of yeast-challenged silages involves approaches to increase daily removal rates to deprive yeasts of time to grow in oxygenated environments. Proper removal techniques to preserve a densely packed and clean horizontal silo face will also help minimize yeast aerobic activity.
Longer-term crop planning to minimize aerobic activity from yeast in silages includes properly sizing storage structures to allow aggressive feedout rates, rapid harvesting at proper maturity and moisture levels, the use of silage additives containing L. buchneri and proper compaction and sealing.
The Bottom Line
The proliferation of yeasts in silage can have a negative effect in terms of dry matter loss, heating and poor palatability. In the presence of oxygen, certain yeast species have the ability to metabolize lactic acid, elevate silage pH and also produce aromatic compounds such as esters, ethyl acetate and aldehydes that can significantly influence palatability. These effects can be minimized by proper harvest moisture, silage compaction/feedout methods and the use of additives containing proven strains of L. buchneri.
This article was originally published in December 2007 Feedstuffs issue, and is reproduced with their permission.