For over a half-century the U.S. dairy industry has embraced corn silage as the most economical and energy-dense forage fed to cows and heifers. Many millions of acres are planted in silage corn every year and many billions of pounds of corn are harvested and put up as silage in horizontal and vertical storage systems. The purpose of ensiling corn is to recover and preserve as much organic matter as possible after harvesting, retaining its nutritive value in a fermented and stabilized state and having it available to feed to animals year-round.

Proper management of the corn crop is critical at the time of harvest and most of the quality of corn silage is dependent upon how well it fermented after harvesting. But eventually the pit or silo must be opened, at which time many cubic feet of feed are exposed to air and rain and heat and cold on a daily basis. Once this high-moisture organic matter is exposed to the elements it begins a secondary fermentation and begins to spoil – aerobic spoilage.

The single, most effective management tool in reducing aerobic spoilage is minimizing the amount of air penetrating the surface of a bunk or silo as well as the hours of exposure. Even though exposure of the silage to air during the feeding process is inevitable and unavoidable, the key to reducing aerobic instability is to manage the face or silo in such a way as to minimize the exposed volume or square footage to as little as possible on a daily basis.

Recommended management tips for limiting aerobic spoilage include the following:

  • Keep the face of a pile smooth with the use of a mechanical facer or other equipment.
  • Avoid gouging and leaving uneven surfaces with front-end loaders.
  • Knock down only enough silage to be used for the day.
  • Avoid creating shelves and ledges.
  • Avoid transporting and re-piling silage at a different location.
  • Remove more silage per day from the face during hot weather.

The silage face of a pit should be maintained as a smooth surface perpendicular to the floor and sides in bunker, trench and drive-over storage systems. This will minimize the surface area exposed to air. An average removal rate of 6 to 12 inches from the face per day is a common recommendation. However, during periods of warm, humid weather, a removal rate of 18 inches or more might be required to prevent aerobic spoilage.

Air can penetrate into the face of even a well-packed silo as much as 3 feet. Removing only 6 inches of silage per day from a face means that the silage on the face today has been exposed to air for six days. Six days is more than adequate time for yeasts to establish and begin degrading organic matter. Removing 12 inches per day results in silage at the face being exposed to air for only three days. The key is to remove enough silage daily to reduce the hours silage is exposed to air, minimizing the time for yeast growth. Silage piles and silos should be sized and managed in such a way that at least 12 inches of feed can be removed per day, thus reducing the amount of “air time” penetrating the face.

Large silage piles sectioned off with multiple faces left untouched for many weeks or months should be avoided as should poorly faced piles with extensive shelving. Exposing large faces for many months at a time will result in the loss of many thousands of pounds of silage to spoilage. During hot weather more silage must be removed since higher temperatures accelerate the spoilage process.

Although proactive management of silage is the most effective way of avoiding aerobic spoilage, additives such as bacterial inoculants and organic acids, when properly applied, can help mitigate spoilage. Organic acids such as propionate, benzoate and sorbate each have anti-fungal properties that can reduce the proliferation of yeast in silages. However, the silage pH range in which these acids are effective is extremely narrow. When the pH level in the silage rises over 4.5, the efficacy of organic acids drops to 60 percent or less. Silage that is dry usually has a higher pH (more alkaline), also making organic acids less effective. Organic acids require more water for application, making them more difficult and expensive to apply.

It’s important to understand that a low pH and high levels of lactic acid alone in well-fermented silage will not prevent aerobic instability. Although the addition of organic acids or bacterial inoculants may provide a minimal reduction of aerobic instability, limiting air exposure through well-managed feedout and aggressive face management are the primary means of achieving long-term and consistent aerobic stability in silages.

Extensive research has been done on a strain of bacteria called Lactobacillus buchneri (L. buchneri). Buchneri is a naturally occurring bacterium that can convert small amounts of lactic acid to acetic acid. Acetic acid is another organic acid that also has anti-fungal properties and is capable of reducing yeast numbers in silage. The addition of L. buchneri to silage at the time of harvest may be most beneficial in situations where harvesting is taking place in hot weather or delayed packing. Because L. buchneri only produces acetic acid, it should never be the sole inoculant applied at the time of harvesting. It’s much more critical that lactic acid-producing inoculants are applied at higher levels to encourage the production of lactic acid and a rapid fermentation.

Poorly managed silage piles disrupt the quality and consistency of the feed. Maintaining aerobic stability in silages is critical to limiting dry matter losses in the silage as well as maintaining the nutritional components and palatability of those forages. Extended periods of exposure will also impact moisture content, which will have a significant effect on weigh-outs in the feed trucks and mixer wagons, which will, in turn, impact levels of nutrition being fed to the herd.