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1209 PD: Know these six phases to limit aerobic instability in silage

Gerald E. Higginbotham Published on 05 August 2009

With the harvesting of corn silage rapidly approaching, a review of the processes involved in making quality corn silage is probably warranted.

The fermentation process involves plant sugars being fermented by anaerobic (without oxygen) bacteria to organic acids which lowers the pH (measure of acidity) of the plant material. The efficiency of fermentation and amount of fermentation loss is influenced by a number of factors:

• the ability to achieve and maintain anaerobic conditions in the pit or bag

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• the amount of fermentable sugars in the crop

• the quantity and type of bacteria present on the crop

• the quantity and type of fermentation acids produced

High-quality corn silage results when lactic acid is the predominant acid produced during fermentation. Lactic acid is the most efficient fermentation acid and will drop the pH of the silage the fastest. Silage fermentation can be basically broken into six phases as shown in Figure 1. The first three phases are completed during the first four to five days.

• Phase I
The chopped forage is placed in the pit or bag with plant cells taking in oxygen and giving off carbon dioxide. In addition, bacteria present on the chopped material also use oxygen to convert plant tissue to carbon dioxide and water.

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Plant cell respiration produces heat which creates an optimum temperature of between 80° to 100°F for bacterial activity. If excessive oxygen is present in the silage, temperatures of the silage may climb to above 100°F. This will in turn decrease the nutrient content of the silage due to heat damage.

• Phase II
Plant cell respiration ends, and the production of acetic acid begins. As the acid is produced, the pH is lowered enough to prevent the growth of more acetic acid organisms. In silage with more than approximately 30 percent dry matter, acetic acid production occurs, and at less than 30 percent dry matter, butyric acid is produced. Butyric acid reduces silage acceptability by dairy cattle and results in low milk yields.

• Phase III
In this phase, lactic acid bacteria begin to grow and produce lactic acid, continuing to lower the pH. Lactic acid production is dependant on a lower pH, adequate content of lactic acid bacteria, sufficient soluble carbohydrate, moisture content and no oxygen conditions.

• Phase IV
This phase begins about the fifth day and continues for two to three weeks. Temperature of the silage gradually declines to around 80° to 85°F. More lactic acid is produced until the pH is low enough to reduce and stop any further bacterial action.

• Phase V
If enough acetic and lactic acid were formed to prevent further bacterial action, then the silage will remain fairly constant . Otherwise, the silage will be exposed to further decomposition by undesirable bacteria.

• Phase VI
This phase of silage fermentation begins once the silage is exposed to air due to feedout. Once the silage is exposed to oxygen, yeasts and molds begin to proliferate. They convert leftover sugars, fermentation acids and other nutrients into carbon dioxide, water and heat. Feedout losses can account for up to 30 percent of the total dry matter loss in the ensiling process.

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Minimizing oxygen exposure includes proper face management in bunker or trench silos. It is best to remove 4 to 6 inches from the bunker face daily. Slow feedout rates allow more time for losses due to the growth yeasts, molds and aerobic bacteria. This in turn decreases dry matter intake. For example, when corn silage that had been exposed to air for four days was fed to dairy cows, their dry matter intake dropped 38 percent, from 60 pounds to 37 pounds per day.

Figure 2 depicts the dramatic differences in dry matter losses associated with different levels of pit face management. Maintaining a firm face and cleaning up loose silage that has fallen to the floor of the silo on feedout will help minimize aerobic losses. Keeping an even, clean face on bunker or pit silos is an important management factor. It has been recommended that to remove silage from a bunker, use the edge of a bucket on a front-end loader to pull the silage down the face of the silo. Then scoop and load.

This method will minimize infiltration of oxygen into the silo face and eliminate loose and unpacked silage at the bunker floor. Silage should never be scooped from the face as this allows more air to enter, resulting in unnecessary spoilage.

To improve silage stability once exposed to air, various silage additives have been examined. Propionic-acid-producing bacteria have been included in some silage additives. These bacteria convert lactic acid and sugar to acidic acid and propionic acid. Higher levels of propionic acid would theoretically inhibit molds. Research conducted using these types of inoculants to improve silage stability has been inconsistent.

Another additive that has received considerable attention is Lactobacillus buchneri. Buchneri is a bacterial inoculant approved for use in grass silages, corn silage, legume silage and high-moisture grains. Buchneri has been demonstrated to improve aerobic stability of silages by reducing the growth of yeasts. The net result is that silages inoculated with L. buchneri are more resistant to heating at feedout (exposed to air) as compared to untreated silages.

When applied at the time of ensiling at the rate of up to 5 x 105 CFU per gram of fresh material, L. buchneri has been demonstrated to improve aerobic stability of high-moisture corn, corn silage, alfalfa silage and small grains relative to untreated controls.

The beneficial impact of L. buchneri appears to be related to the production of acetic acid. Although the precise mechanism has not yet been determined, it is likely that aerobic stability is improved because acetic acid inhibits growth of specific species of yeast that are responsible for heating upon exposure to oxygen.

In research trials yeast and mold growth in silage treated with L. buchneri has been lower at feedout than for untreated control silages. Yeast and mold levels in silage inoculated with L. buchneri also do not increase as rapidly as in untreated controls when exposed to air. As a result, the temperature of silage inoculated with L. buchneri does not readily rise upon exposure to air and tends to remain similar to ambient temperature for several days, even in warm weather.

Treating silage with L. buchneri most likely would be beneficial under circumstances where problems with aerobic instability are expected. Corn silage, small-grain silage and high-moisture corn are more susceptible to spoilage once exposed to air than legume or grass silage, and therefore L. buchneri inoculants may be a benefit. L. buchneri can also be applied to legume silage if aerobic stability is a problem.

It should be remembered that high ambient temperatures, slow filling, improper packing, low surface removal rate and poor feedbunk management are all factors that can decrease aerobic stability of silage. It is likely L. buchneri would improve aerobic stability in circumstances where untreated silage or silage treated with lactic-acid-producing bacteria have a history of being aerobically stable at feed out.

In fact, under such circumstances, the potential reduction in silage dry matter recovery due to this organism’s heterofermentative fermentation may actually make L. buchneri a less desirable silage inoculant than heterofermentative bacterial inoculants.

Research conducted to date has not shown that animal performance is improved when cattle are fed silages inoculated with L. buchneri. Intake and milk production have been similar in trials where cattle have been fed diets containing either L. buchneri treated or untreated high-moisture corn, alfalfa silage or barley silage.

It appears that inoculation of silage with L. buchneri has the potential to dramatically improve aerobic stability of ensiled feeds and may significantly reduce feed waste in circumstance where heating and molding of feeds are an ongoing problem. The economic benefit of using this product will depend on how much feed can be saved by reducing losses associated with aerobic instability. PD

References omitted but are available upon request at

—Excerpts from University of California 21st Century Dairying Newsletter, August 2008

Gerald E. Higginbotham, Dairy Farm Advisor, University of California – Davis

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