The preservation of forage crops into a fermented feedstuff known as silage has been an important practice in agriculture, especially in the dairy and beef cattle industries where there has been a documented increase in silage production since the early 1950s.

There are three primary reasons why silage-making is important. First, it allows for the conservation of forage during times of harsh weather conditions like extreme cold or drought when forage crops are unable to be cultivated.

Secondly, it serves as a means to save surpluses harvested during the growing season. Lastly, it allows animals access to forage crops that are difficult to graze.

Silage can be stored in a number of ways, including tower silos, bunker silos, bags and piles. Of these methods, bunker silos have become one of the most attractive methods of preservation because they have high storage capacities, low initial start-up costs and require relatively little maintenance over time.

This [article] review will discuss several issues related to covering silos with particular references to the effects of air on spoilage and types of plastic and edible covers that can be used to minimize this problem.

Aerobic instability of silage
The most important factor in producing good silage is preventing air infiltration into the silo. Once air has entered, the system aerobic microorganisms that were previously kept at bay, due to the anaerobic conditions, become activated and begin the spoilage process.

This is a major problem for silage producers because nothing can be done to stop aerobic deterioration once it has begun. Aerobic spoilage results in large losses of nutrients, primarily residual sugars, which mineralize to water and carbon dioxide.

There is also an accompanying generation of large amounts of heat along with increases in ammonia and pH. The increase in pH can be attributed to the degradation of volatile fatty acids (VFAs), like lactate and acetate, produced during ensiling.

The principle initiators of aerobic spoilage are considered to be lactate-assimilating yeasts. Populations of yeasts are capable of log-phase growth in a relatively short period of time.

Woolford noted that yeast populations of only 102 colony-forming units (cfu) per gram (g) dry matter (DM) can increase to 1012 cfu per g DM in three days. Silages with more then 105 cfu of yeasts per g DM are especially prone to deterioration; however, smaller populations have also been known to cause rapid spoilage.

Bacteria and molds also play important roles in aerobic spoilage; however, they are primarily restricted to the later stages of deterioration.

During the first few days of ensiling, after respiration of the plant material is complete, lactic acid bacteria cause a rapid decline in pH to around 4.0.

Enterobacteria are normally decreased by this reduction in pH; however, with introduction of air to the silo and subsequent rise in pH, numbers can grow as high as 108 cfu per g fresh matter. Studies have shown that E. coli 0157 is able to survive for at least three weeks at a pH of 4.0 to 4.6 in grass and whole-plant corn silages.

Another spore-forming genus commonly found in aerobically spoiled silage is Clostridium. Clostridial species are able to break down protein and amino acids.

For example, C. tyrobutyricum can degrade lactate to butyric acid, hydrogen and carbon dioxide. Silage that has undergone clostridial fermentation is characterized by a high pH, allowing for additional growth of aerobic microorganisms.

Molds commonly target areas of the silo where there is poor sealing. They are also usually segregated to the uppermost layers of the silo. Seeing a dense layer of mold is characteristic of long-term aerobic spoilage.

Impact of feeding spoiled silage
A variety of microorganisms found in aerobically spoiled silage can be considered dangerous to human and animal health.

Enterobacteria make nitrous oxide (N20) that can be reduced to a variety of gaseous nitrogen oxides. Nitrogen oxides are sometimes visible during silage fermentation as yellow-brown gases escaping the silo surface.

These gases are dangerous due to their ability to damage lung tissue and cause respiratory distress. In humans, this is commonly known as silo-filler’s disease. The symptoms commonly mimic the early stages of pneumonia.

Enterobacteria also form biogenic amines and ammonia. It has been seen that large amounts of these compounds affect silage palatability, decreasing overall intake in cattle.

Animal health problems associated with molds are not uncommon, due to their ability to produce mycotoxins.

The degree of seriousness often varies depending on the type and amount of mycotoxin present at feeding. Ailments can range from minor digestive upsets and infertility to serious kidney and liver damage.

Molds of the genus Paecilomyces produce patulin, a mycotoxin linked to hemorrhagic disorders in cattle. Another serious disorder in cattle, mycotic abortion, has been linked to consumption of the mold Aspergillus fumigatus.

Covering bunker silos
It has long been known in the agricultural community that the practice of sealing horizontal silos with a covering improves silage quality.

The most common method of covering bunker silos has been polyethylene sheeting weighted down with tires to create a boundary between the anaerobic environment of the silo and the aerobic conditions of the atmosphere surrounding it.

When no covering is used, the spoiled portion that forms on the uppermost layer effectively acts as the seal for the healthy layers beneath it.

As much as a 0.3 meters of spoilage can be seen on the top of uncovered horizontal silos. This material has little to no nutritional value to the animal; therefore, farmers will often discard it as compost. This material is not only a loss of nutrients, it is a considerable economic loss to the farmer.

Effect of plastic thickness and color on aerobic spoilage of bunker silos
Plastics that are thicker are more resistant to physical damage from the weather and animals. They also act as a better barrier to oxygen infiltration into the silo.

Forristal et al. looked at the effect of thickness and color of plastic when wrapped around grass bales. Bales were wrapped in two, four or six layers of plastic, of five different colors (black, clear, green, light green and white).

There was no effect observed with color; however, there was an effect with the number of layers used to wrap bales. Bales with two layers of plastic had 21.5 percent area of visible mold growth, while more layers of plastic yielded significantly less growth. Four layers of plastic had 1.7 percent and six layers had 0.6 percent.

There were no significant differences observed in any of the chemical characteristics between types of plastic.

Temperatures directly underneath plastic surfaces were significant between types; however, researchers concluded increases were too little to cause a change in the silage micro-environment. Researchers concluded that optimal silage fermentation is not dependent on thickness or color of plastic, but these results were based on using experimental mini silos.

Oxygen barrier plastics
Polyethylene sheeting used to cover bunker silos is not completely impervious to air, so a completely enclosed anaerobic system can never be achieved.

Holmes and Muck noted that thicker plastics of at least 6 millimeters (mm) should be more resistant to oxygen; however, there has been very little research on plastics of varying thickness to prove that point.

Creating thicker plastics will not serve to decrease the total amount of plastic used in agriculture.

Ideally, a plastic would be able to minimize waste as well as reduce the infiltration of oxygen into the silo. A plastic has been manufactured that attempts to address both of these issues.

This triple co-extruded film (TCF) is composed of two layers of polyethylene and a middle layer of polyamide totaling 0.45 microns in thickness. This film is also an oxygen barrier film, which could lead to decreases in surface spoilage on top of bunker silos.

In a recent experiment we filled three identical bunker silos (43 m x 7 m) with approximately 600 tons of whole-plant corn silage on August 25 through September 3, 2006.

Corn was harvested at about 30 percent DM using a pull-behind chopper. Roller clearance on the processor was set at about 1.35 mm. Silos were packed using a tractor weighing 14 tons and a tractor weighing 8 tons.

Two covering systems were compared. The first system was composed of extruded film (TCF, composed of two layers of polyethylene and a middle layer of polyamide, totaling only 0.45 microns in thickness) placed along the length of the sidewall before filling, with approximately 0.91 m of excess draped over the wall.

After filling the excess film was pulled over the wall, and another sheet of TCF was placed on top, extending 3.69 m width-wise.

A protective tarpaulin was then placed on top of the TCF. The TCF was weighted down with gravel bags where TCF met the concrete wall, down the middle of the silo at the point where treatments met and also every 3.66 to 4.57 m perpendicular to the silo wall.

The second treatment involved using a traditional 6 mm black/white polyethylene (BWP). The BWP was also 3.69 m wide, which allowed for 0.3 m of overlap between treatments.

No sidewall plastic was used with the BWP treatment. The BWP was weighted down with sidewall tires in a normal fashion.

Corn silage was sampled after five months of ensiling. Blocks of silage were taken at three heights extending in 15.24 centimeter (cm) increments downward and three sampling widths extending in 20.32 cm increments outward from the wall.

This sampling scheme yielded nine “blocks” in total that were 20.32 cm x 20.32 cm x 15.24 cm. The experiment was replicated twice per time-point, per treatment.

Preliminary data from this study indicates a lower dry matter, most likely an indication of infiltration of water along the sidewall.

The DM content of silage was fairly consistent among the three widths for the TCF covering system, suggesting the plastic on the sidewall protected this material from water.

Contamination with water in the control silos was probably responsible for marked differences in pH and fermentation end products when compared to TCF silages.

Weighing down plastic tarps with tires is a common method to secure the plastic and prevent air from penetrating into the silage mass.

Full-casing tires are often difficult to move and are able to hold water. Lanyon et al. noted a whole tire can weigh as much as 60 pounds; however, with water collection, the weight can increase between 40 to 70 percent.

Water collection in tires also serves as a breeding ground for mosquitoes and causes an increased risk of West Nile Virus in many states.

There has been some success with split tires, or tires cut in half, which reduces the weight, the number of tires needed and limits breeding grounds for mosquitoes. However, many split tires do not have enough weight to sufficiently hold the plastic flush to the silage mass.

Although not scientifically evaluated, all workers in the study of McDonell and Kung who had considerable experience covering bunker silos agreed that using gravel bags for weights was significantly easier, faster and cleaner than using tires.

Edible coverings for bunker silos
Even though good silage can be made using plastic and tires, there are several disadvantages to this system. First, there is a labor requirement needed to move plastic and tires onto the silo for sealing and their removal during feedout.

Disposal of polyethylene sheeting has become an environmental problem because it is non-biodegradable and difficult to recycle.

Non-biodegradable plastics currently make up as much as 30 percent of municipal solid waste across the country. Plastics used for covering bunker silos and piles fall into this category.

Because of this problem, along with cost of labor and risk of West Nile Virus associated with the plastic and tires method, researchers have looked at ways to develop edible coverings.

Berger and Bolsen noted that in order for the plastic and tires method to be replaced by an edible covering, there are several criteria that should be met.

The covering should be easy to apply, cheap and serve as an effective barrier against air and water. In addition, the material should be edible, nutritious and palatable to animals consuming ensiled materials.

To date, there have been numerous edible alternatives proposed using a wide array of materials. Apple pulp, candy, chopped straw, molasses, peanut butter, sawdust, sod, starch-salt matrices and wax are a few of the many coatings experimented with in conjunction with bunker silos.

Brusewitz et al. experienced cracking in the cover, allowing for air and water infiltration into the silos. Denoncourt et al. offered some possible reasons for the failure of this commercial covering.

The viscosity of the solution prevented it from staying completely on the surface of the silage. The solution that was left following seepage into deeper layers was not thick enough to form a stable barrier against air and the environment.

In other studies, the materials used were unsuccessful in keeping air out of their respective systems. Many of the studies had increased DM loss with edible coverings when compared to conventional plastic sealing systems.

Pritchard and Conrad had relative success with molasses as a covering for piles. The percent DM and average daily consumption were better for the molasses-treated silage (26.3 percent, 56 pounds) compared to the silage covered with plastic (25.0 percent, 54 pounds).

Even though the molasses covering did rate higher than plastic in the final outcome, researchers encountered some problems during application and storage.

During the first application, rain diluted the molasses, causing it to run off the pile. Also, during storage the molasses-treated surface was ridden with flies and maggots. The top 4 to 5 inches had to be discarded, but all of the silage underneath was fed to dairy cows with little refusal.

Recently, Berger et al. attempted to use a starch-salt matrix to cover whole-plant corn silage in bunker silos.

The starch had to be mixed with boiling water prior to application for gelatinization to be achieved. Two silos each were used between three treatments: uncovered, covered with 6 mm plastic or covered with 1.27 to 1.90 cm of starch-salt matrix and a thin layer of paraffin wax.

The salt-starch matrix/wax was fed to heifers at a rate of 2.0 pounds per day, with refusal averaging 9 percent of the total covering offered.

Ash content was significantly higher for the starch-salt matrix treatment compared to the uncovered and 6 mm covered treatments.

Because of the cost necessary to apply the salt-starch matrix/wax treatment, researchers sought an alternative application method in another trial. Instead of using a mortar mixer and cement trowel to apply the matrix, the consistency was altered so it could be sprayed on.

Conclusions
Infiltration of air and water into the silage mass during storage results in decreased forage quality. Besides a loss in total DM and nutrients, feeding spoiled silage has adverse effects on rumen function and animal performance.

Research continues on practical methods to exclude oxygen from the stored silage mass in bunker silos. Successful practices will be user-friendly and economical.

There have been too few studies investigating edible materials over multiple trials to conclude edible coverings are better than plastic. Several studies are currently underway investigating the efficacy of low oxygen, permeable plastic.  PD

References omitted but are available upon request at editor@progressivedairy.com

—Excerpts from 2006 Vita Plus Dairy Summit Proceedings