What Is Silage Made From and How Is It Preserved?

Silage is made from green, moisture-rich crops that are chopped, packed tightly to exclude air, and sealed in an airtight environment where naturally occurring bacteria ferment the plant sugars into organic acids, primarily lactic acid. The resulting drop in pH, typically to around 4 or below, effectively pickles the forage and prevents it from rotting. Corn (maize) is the most widely ensiled crop worldwide, but grasses, alfalfa, sorghum, clover, and a range of other plants can all be turned into silage. The process is surprisingly simple in concept yet full of biological and practical nuances that determine whether you end up with high-quality livestock feed or a spoiled, potentially dangerous mess.

Which Crops Can Be Ensiled

Almost any green crop with enough fermentable sugar can be made into silage, but some plants lend themselves to the process better than others. Whole-plant corn is the gold standard in many regions because it delivers a combination of high energy (from starch in the grain), adequate moisture, and plenty of water-soluble carbohydrates for the bacteria to work with. As corn matures, its starch content rises significantly; harvesting at the right stage matters because starch levels can increase by a third or more between early and late maturity stages.¹

Grasses, including ryegrass, timothy, and fescue, are ensiled extensively in cooler, wetter climates where corn does not grow as well. Legumes like alfalfa and clover are also common, though they present more challenges. Legumes tend to have lower sugar content and higher buffering capacity, meaning their natural chemistry resists the pH drop that good fermentation requires. That is why alfalfa silage often benefits from additives to get the fermentation going quickly.²

Beyond the mainstream crops, farmers have successfully ensiled sorghum, proso millet, sugar beet tops, and even agricultural by-products like potato hash. A study on potato hash silage showed that inoculating it with lactic acid bacteria reduced pH, cut ammonia levels, and boosted lactic acid production compared to an untreated control.³ The common thread is moisture and sugar. If a crop is too dry, it will not ferment well. Too wet, and you risk a different set of problems, including excess effluent and the wrong kinds of bacteria taking over.

How Fermentation Preserves the Crop

The preservation mechanism is lactic acid fermentation, the same basic process that gives yogurt and sauerkraut their tang. Once the chopped forage is sealed off from oxygen, lactic acid bacteria already present on the plant surfaces begin consuming water-soluble carbohydrates. Through glycolysis, they convert glucose into pyruvate, which the enzyme lactate dehydrogenase then converts into lactic acid.⁴ This acid accumulation is what drives the pH down and creates an environment too acidic for the spoilage organisms, including clostridia, molds, and enterobacteria, that would otherwise decompose the forage.

A well-fermented silage reaches its stable, low pH within the first one to three weeks, though the process can continue at a slower pace for months. The speed and completeness of this initial pH drop are critical. If the fermentation is sluggish, undesirable bacteria like clostridia can proliferate first, producing butyric acid and breaking down proteins into ammonia. The result is foul-smelling, nutrient-poor silage that animals may refuse to eat. This is why everything about silage-making, from crop selection and chop length to packing density and sealing speed, is aimed at giving lactic acid bacteria the fastest possible head start.

Why Packing Density and Sealing Speed Matter So Much

Packing the crop as tightly as possible serves one overriding purpose: squeezing out trapped air. Oxygen is the enemy of good silage. As long as oxygen remains in the pile, plant cells continue to respire, consuming sugars that the lactic acid bacteria need, generating heat, and creating conditions where aerobic spoilage organisms thrive. The faster you eliminate oxygen, the sooner anaerobic fermentation can dominate.

Research on maize silage at two packing densities, roughly 350 versus 700 kilograms per cubic meter, found that the high-density group lost far less dry matter over 60 days of ensiling (about 3.4% versus 9.4%) and consistently produced more lactic acid throughout the fermentation period.¹ High packing density also reduced total gas production in whole-plant corn silage, a sign that less carbon was being wasted through unwanted metabolic pathways.⁵

Sealing speed is equally important. Delayed sealing allows yeast populations to explode during the first few days. In one study on maize silage, yeast counts were dramatically higher in silage that was sealed with a delay compared to silage sealed promptly, and those elevated yeast populations were strongly linked to later aerobic instability.⁶ Every hour of exposure to air before sealing is an hour the wrong microbes are getting established.

Storage Systems and Their Trade-Offs

The three most common storage systems are bunker silos (horizontal concrete-walled structures covered with plastic sheets), silage bags (long polyethylene tubes filled by a machine), and tower silos (vertical structures, either sealed or open-top). Each has different strengths depending on farm size, climate, and how quickly the silage will be fed out.

Bag silage tends to preserve nutrients well because the sealed polyethylene limits oxygen exposure quite effectively. A study comparing bag, trench, and bunker storage in smallholder dairy systems found that bag silage retained higher crude protein and starch while producing less ammonia and lower levels of the mycotoxin aflatoxin B₁ than trench or bunker silages.⁷ Bunker silos, on the other hand, handle large volumes efficiently and are easier to fill quickly during harvest, but their open faces during feed-out expose the silage to air daily. Tower silos keep a small surface area relative to volume, which limits spoilage, but they are expensive to build and maintain.

For smaller operations, even heavy-duty plastic wrap around individual round bales can create effective silage. The principle is always the same: create an airtight seal and maintain it for the duration of storage.

Biological and Chemical Additives

When the crop alone does not provide ideal fermentation conditions, farmers turn to additives. These fall into two broad categories: biological inoculants (live bacteria added to boost fermentation) and chemical preservatives (acids or salts that directly inhibit spoilage organisms).

Biological inoculants are the more commonly used option. They typically contain strains of lactic acid bacteria at high concentrations. A large meta-analysis found that inoculation significantly improved fermentation and dry matter recovery in grasses, alfalfa, and other legumes, while also reducing clostridial contamination, mold growth, and ammonia production.⁸ The same analysis found that inoculated silage fed to dairy cows increased milk yield. Interestingly, though, inoculation did not meaningfully improve the fermentation of corn, sorghum, or sugarcane silages, crops that already have enough natural sugar and bacteria to ferment well on their own.

There are two main types of bacterial inoculants, and they do different things. Homofermentative strains produce almost exclusively lactic acid, driving pH down quickly. Heterofermentative strains produce a mix of lactic and acetic acid, and that acetic acid is what improves aerobic stability, the ability of silage to resist spoilage once the seal is broken and air gets in. Dual-purpose inoculants containing both types pushed corn silage fermentation in a more heterofermentative direction, kept yeast counts low, and extended aerobic stability beyond 30 hours compared to untreated or molasses-treated silage.⁹

Chemical preservatives, most commonly formic acid and propionic acid, take a different approach. Rather than promoting fermentation, they partially suppress it while directly lowering pH and inhibiting mold and yeast growth. Formic acid applied to summer forage crops preserved more water-soluble carbohydrates than bacterial inoculants did, and it significantly reduced total microorganism and mold counts.¹⁰ Chemical preservatives are particularly useful for crops that are difficult to ensile, like wet, low-sugar forages, but they cost more per ton than biological inoculants.

What Goes Wrong: Aerobic Spoilage

The most common way silage fails is when air gets back in. This can happen through tears in plastic, poor face management on bunker silos, or simply leaving the silage open too long during feeding. Once oxygen is available, yeasts wake up and begin metabolizing the lactic acid that was keeping the silage stable. As yeasts consume the acid, pH rises, and that opens the door for molds and other spoilage organisms.

The speed of this deterioration is striking. In non-fermented feed exposed to air, yeast populations climbed from about ten million to ten billion colony-forming units per gram of dry matter. Even well-fermented silage with high moisture content eventually succumbed to spoilage when exposed to air for extended periods.¹¹ Lactate-assimilating yeasts, those specifically capable of consuming lactic acid, were the main culprits driving instability.⁶

Spoiled silage is not just a waste of feed. The heat generated during aerobic spoilage can reach temperatures high enough to damage proteins, reducing the feed’s nutritional value even before visible mold appears. And the molds that colonize spoiled silage can produce mycotoxins, which brings a whole different set of problems.

Mycotoxins, Listeria, and Silo Gas

Silage safety involves three distinct hazards: mycotoxins in the feed, pathogenic bacteria growing in poorly fermented material, and toxic gases produced during the early stages of ensiling.

Mycotoxin contamination can originate in the field before harvest or develop during storage if molds proliferate. Ruminants consuming silage are often exposed to mycotoxins including aflatoxins, trichothecenes, zearalenone, and fumonisins.¹² Forage crops are naturally in contact with yeasts and filamentous fungi from the moment they are growing in the field, and poor post-harvest management accelerates contamination.¹³ Even haylage, the drier wrapped-bale form sometimes fed to horses, can harbor fungi capable of producing mycotoxins, despite overall low mold counts.¹⁴

Listeria monocytogenes is another concern, especially in poorly fermented silage where pH stays above 5. Genomic analysis of an abortion outbreak in beef heifers traced strains of L. monocytogenes from the affected animals back to both water and silage sources on the farm, with nearly identical genetic sequences confirming the silage as a likely origin of infection.¹⁵ Good fermentation, meaning rapid pH decline below 4.5, largely prevents Listeria from surviving.

Silo gas, primarily nitrogen dioxide, poses a direct threat to both humans and animals housed near storage structures. These toxic gases can form during the first days of maize and grass ensiling as plant nitrates break down.¹⁶ Nitrogen dioxide is heavier than air and can pool in enclosed spaces, causing severe lung damage or death. Farmers are advised to ventilate silo areas thoroughly and avoid entering enclosed storage spaces in the days immediately after filling.

Silage Versus Hay as a Feed

Silage and hay are two solutions to the same problem: preserving forage grown in summer for feeding during months when pasture is unavailable. Hay relies on drying the crop to roughly 15% moisture, which halts microbial activity through water removal. Silage relies on fermentation in a wet environment. The choice between them depends on climate, farm infrastructure, and the nutritional goals for the herd.

A direct comparison of grass grown under identical conditions and conserved either as silage or barn-dried hay found that while fiber content was similar between the two, hay retained far more water-soluble carbohydrates (177 versus 23 grams per kilogram of dry matter) because fermentation in the silage consumed most of those sugars. Silage, however, preserved more crude protein. Cows fed the hay consumed more feed overall, which translated into greater nutrient and energy supply when concentrates were limited.¹⁷

Neither form is categorically better. Silage tends to win in wet climates where field-drying hay is unreliable, and it preserves more of the crop’s protein. Hay wins on palatability and sugar content, and it avoids the fermentation-related risks of spoilage and mycotoxin development. Many dairy farms use both, adjusting the ratio in the diet depending on the animals’ production stage and the other ingredients in the ration.

Silage Effluent and Environmental Concerns

When very wet crops are ensiled, the combination of packing pressure and fermentation produces a liquid runoff called silage effluent. This is not just water. It is a concentrated, acidic waste stream with extremely high biochemical oxygen demand, typically at a pH between 3.5 and 5, making it corrosive to steel and concrete and devastating to waterways if it escapes.¹⁸ A small amount of silage effluent reaching a stream can deplete dissolved oxygen rapidly enough to kill fish.

The simplest way to reduce effluent is to wilt the crop before ensiling, raising its dry matter content above roughly 30%. At that threshold, most crops produce little to no free liquid during storage. For crops that cannot be adequately wilted, proper effluent collection systems, directing the runoff into sealed tanks for later land-spreading, are essential. Regulations in many countries require farms to have effluent containment, but compliance varies widely, and accidental releases remain a real source of water pollution in agricultural regions.

Silage as a Biogas Feedstock

Silage is not only animal feed. Across Europe especially, maize silage has become one of the most common substrates for anaerobic digestion in agricultural biogas plants. The logic is straightforward: the same qualities that make silage a good preserved feed, high organic matter content, pre-acidified pH, and an active microbial community, also make it an excellent feedstock for methane-producing digesters.¹⁹

The methanogenic microbes that produce biogas in a digester are different from the lactic acid bacteria that ferment silage, but they can adapt to silage-based feedstocks over a series of passages. Research has shown that inocula from existing agricultural biogas plants, cattle slurry, or even raw sewage sludge can be adapted to digest maize silage effectively in two-stage reactor systems. This dual use of silage, as both a livestock feed and an energy crop, has reshaped land-use decisions in parts of Germany and Scandinavia, where energy-crop subsidies have incentivized growing corn specifically for biogas rather than for cattle.

Assessing Silage Quality on the Farm

Farmers have traditionally evaluated silage using their senses: color, smell, and texture. Well-preserved silage has a slightly sweet, vinegary smell and a greenish-yellow to olive color. Silage with a rancid or ammonia-like odor signals butyric acid fermentation or excessive protein breakdown. Visible mold, a dark brown color, or a slimy texture all indicate spoilage. These sensory cues remain useful, though they are blunt instruments.

Laboratory analysis of volatile fatty acid profiles gives a much more precise picture. The ratio of lactic acid to acetic acid, the presence or absence of butyric acid, ammonia levels relative to total nitrogen, and pH collectively describe how well the fermentation went. More recently, researchers have demonstrated that nanomechanical sensor arrays can measure volatile fatty acids in silage vapor, discriminating between samples based on the slow desorption patterns of different acids.²⁰ Technology like this could eventually give farmers rapid, on-site quality readings without waiting for lab results, though it remains in the research stage.

Total mixed rations, where silage is blended with grains, protein supplements, and minerals before feeding, add another layer of complexity to quality assessment. The fiber and protein balance of the total ration affects rumen fermentation dynamics, volatile fatty acid production, and even methane output. Rations with moderate fiber and balanced protein supported more efficient microbial fermentation and lower methane emissions in laboratory digestibility trials.²¹ Getting the silage right is only the first step; getting the whole diet right is what actually determines animal performance.