Temperature, concentration of organic acids, moisture and pH are factors that directly affect microbial activity and subsequently silage quality during storage and feedout. However, the presence of air (oxygen) is the most important factor that dictates the final quality of a silage as it is fed.
When silage is “open to the elements,” the potential for spoilage greatly increases. The feedout phase poses a major issue since the silage is inevitably exposed to air, which can deeply penetrate into the remaining silage mass.
Aerobic spoilage translates to losses of dry matter (DM) and nutrients, which can account for over 70 percent of the losses associated with storing crops by ensiling.
Additionally, there is the risk of the production of undesirable compounds by the spoilage micro-organisms, which can affect performance, health and fertility.
The aerobic spoilage of silages is highly correlated to the population of spoilage yeasts (e.g., species of yeasts that may use lactate as a food source, such as Candida, Hansenula and Pichia).
Those are the main cause of initiating virtually all heating events in silages exposed to air, and it has been reported that the population can multiply as high as 10 billionfold in only three days under ideal conditions.
These lactate-assimilating species are able to survive the ensiling process; therefore, the number of the total spoilage yeast population (lactate users and non-users) at the time of ensiling alone is not always a reliable indicator of future problems with aerobic instability.
These spoilage yeasts become active when the silo is open and air penetrates the silage mass, degrading lactic acid. In consequence, the pH of the mass will start to rise allowing opportunistic micro-organisms – bacteria and molds – to also become active and consume nutrients, generate heat and result in massive spoilage.
Effects on silage quality and animal productivity
Despite the importance of this topic, there are only a few studies reported in the literature.
Researchers at the University of Wisconsin reported declining milk yield in dairy cattle as the level of spoilage yeasts rose in high-moisture corn, which was kept in a loose, aerobically challenged pile as fed throughout the trial.
Over the course of the 14-day study, yeast levels increased, the energy content of the corn declined and milk production decreased. Surprisingly, feed intake was not affected.
In 2001, a group at KSU published a report of a study where steers were fed diets containing varying degrees of surface-spoiled corn silage.
Dry matter intake decreased as the level of spoiled corn silage increased, and there were substantial reductions in digestibility of DM, protein and NDF.
It is important to note that the largest single decrease in digestibility occurred with the first increment of spoiled silage (25 percent of the total silage, 5 percent based on badly spoiled silage).
It was observed that the physical integrity of the forage mat in the rumen had been partially or completely destroyed as a result of feeding the spoiled silage.
There is a common belief in the field that feeding diets with a high spoilage yeast population (more than 1,000,000 CFU per g) will lead to milkfat depression.
Data from Cumberland Valley labs showed that the samples of corn silage sent for total yeast count last year averaged more than 15,000,000 CFU per g. That field “evidence,” in addition to the scientific facts reported in the literature 15 years ago, spiked the interest of a group at the University of Delaware.
Initially, the UD group isolated several species of spoilage yeast from corn silage and HMC from different parts of the U.S. The most common species (C. valida) was then added at controlled levels in a TMR to study effects on the 12-hour NDFD.
The 1× level of inclusion (equal to 100,000 CFU C. valida per g TMR) did not affect NDFD statistically, but addition of 100× level reduced NDFD by 7 percent. The highest level studied (one billion CFU per g TMR) reduced NDFD by 23 percent when compared to negative control.
In another study, the UD group fed a silage-based spoiling TMR with more than 66,000,000 CFU of spoilage yeasts per g of TMR to heifers. The heifers ate 2.6 pounds less DM than heifers fed the same TMR that was fed fresh and had around 110,000 CFU of yeasts per g.
While there are many reasons for changes in production in high-producing lactating cows, the reductions in NDFD and intake reported compared to negative controls (i.e., cows given clean feed and silage) in these studies would certainly contribute to reduced production levels.
Clearly, feeding spoiled silage is a practice that should be avoided. There are fewer nutrients available to the cow, and the spoilage yeasts represent a competitor to the rumen microbes for nutrients; the spoilage yeasts may also produce metabolites that may be detrimental to rumen fermentation.
The growth of spoilage yeasts may also lead to the growth of other spoilage organisms, some of which could be known toxin producers.
The bottom line is that the exact causes of reduced intake or performance due to feeding spoiled silages are not fully understood, and the best option is prevention.
If there is a need to control spoilage yeasts and stop spoilage during feedout, some damage is already done: There has been probably initial heating with nutrients and DM losses. Furthermore, it is more efficient to control spoilage yeasts during ensiling rather than after the fact.
Controlling spoilage yeasts and improving aerobic stability
The population of spoilage yeasts in silages can be controlled by good management practices including the use of silage additives proven to prevent spoilage.
Getting the air out of the forage while filling the silo, covering it properly and keeping air out during storage are mandatory practices. During feedout, the face should be clean and well-managed, with enough material being removed between facings to control aerobic spoilage.
Several chemical additives are available to enhance aerobic stability of silages, most commonly based on buffered propionic acid.
However, the efficacy of this type of additive is pH-dependent (when the active form of the acid is formed), and also the efficacy of low application rates is debatable and may even be detrimental.
In regards to microbial inoculants, Lactobacillus buchneri has been commercially used in the U.S. for over 10 years as an enhancer of aerobic stability in a variety of forage crops.
Efficacy data for one strain (L. buchneri NCIMB 40788) has been reviewed by the FDA to allow a claim for improving aerobic stability in silages and high-moisture corn. In contrast to conventional inoculants, this bacterium is applied at higher rates to fully observe the expected benefits.
Furthermore, there is scientific data to show that, if stable silages are due to inoculation with L. buchneri, this benefit can be transferred to the resulting TMR. PD
Renato Schmidt has a Ph.D. in animal nutrition from University of Delaware and is employed by Lallemand Animal Nutrition as a forage products specialist. Bob Charley has a Ph.D. in applied microbiology from the University of Strathclyde in Glasgow, Scotland and is employed by Lallemand Animal Nutrition as a forage products manager.
References omitted due to space but are available upon request. Click here to email an editor.
Renato J. Schmidt
Forage Products Specialist
Lallemand Animal Nutrition North America
