The Ensiling Process: Steps to Create High-Quality Silage

Preserving green fodder for months without refrigeration might sound like a challenge, but it’s precisely what ensiling accomplishes. Used worldwide to feed livestock year-round, ensiling is a controlled fermentation process that converts fresh, high-moisture crops into stable, nutrient-rich silage. It is estimated that 200 million tons of dry matter are ensiled globally every year, making it one of the most important forage conservation techniques in modern agriculture. Understanding how the process works – and what can go wrong – is key to producing silage that delivers real nutritional value to animals.

What is ensiling? 🔗

Ensiling is the process of storing green, high-moisture forage crops – such as maize, sorghum, or grass – under anaerobic (oxygen-free) conditions to trigger lactic acid fermentation. Silage is the end product of fermenting a high-moisture crop containing 40-80% water, and ensiling that fodder now contributes over 50% of the nutrients for beef and dairy cattle production worldwide.

The science behind it is straightforward: epiphytic lactic acid bacteria (LAB) naturally present on the crop ferment water-soluble carbohydrates into lactic acid and, to a lesser extent, acetic acid. This acid build-up lowers the pH of the stored mass, creating conditions hostile to spoilage microorganisms. The result is a preserved feed that can remain stable for months – or even years – if managed properly.

The four phases of the ensiling process 🔗

The ensiling process is not a single event – it unfolds in four distinct, sequential phases. The major chemical and microbiological changes that occur during fermentation can be divided into four distinct phases: aerobic, anaerobic fermentation, storage, and feedout. Each phase has specific microbial players and conditions that determine the final quality of the silage.

Phase 1: The aerobic phase 🔗

This phase begins immediately after the crop is chopped and placed in the silo. Even with the best packing, some oxygen remains trapped within the forage mass. During this initial phase, naturally occurring aerobic bacteria consume available carbohydrates, producing carbon dioxide, water, and heat until the oxygen is excluded by packing.

Plant cells also continue to respire, and proteolytic enzymes keep breaking down proteins. The silage temperature can rise noticeably during this window. The respiration phase typically lasts three to five hours, depending on the oxygen supply present. This phase directly reduces the carbohydrates available for beneficial fermentation later – which is why speed and compaction at filling are so critical. The longer this phase runs, the lower the quality of the finished silage.

Phase 2: The fermentation phase 🔗

Once oxygen is depleted, anaerobic conditions take hold and lactic acid bacteria become the dominant microbial population. This phase starts when the silage becomes anaerobic and continues for between several days and several weeks, depending on the properties of the ensiled forage and ensiling conditions. As fermentation proceeds successfully, LAB develop and become the predominant population, with pH decreasing to 3.8-5.0.

The quality of fermentation depends heavily on which type of LAB dominates. There are two types of lactic acid bacteria: homofermentative and heterofermentative. Homofermentative bacteria produce primarily lactic acid, while heterofermentative bacteria produce lactic acid, acetic acid, ethanol, and carbon dioxide. Homofermentative strains are preferred because they work faster, generate a sharper pH drop, and conserve more nutrients. In well-managed corn silage, at least 70% of the total acid should be lactic acid.

Phase 3: The stable phase 🔗

Once pH has dropped sufficiently, bacterial activity slows dramatically and the silage enters a stable, near-dormant state. For as long as air is prevented from entering the silo, relatively little occurs. Most micro-organisms from the fermentation phase slowly decrease in numbers, with some acid-tolerant organisms surviving in an almost inactive state.

This is the storage period, and it can last for months. The major factor affecting silage quality during the storage phase is entry of oxygen into the silo. Oxygen increases yeast and mold growth, resulting in dry matter loss and heating in the ensiled forage. A well-sealed, properly compacted silo will maintain quality through this phase with minimal losses.

Phase 4: The feed-out phase 🔗

The feed-out phase begins the moment the silo is opened. At this point, the silage face is re-exposed to oxygen, reactivating dormant spoilage organisms. Once silage is re-exposed to oxygen, yeasts and molds become active again, converting residual sugars, fermentation acids, and other soluble nutrients into carbon dioxide, water, and heat. Feedout losses can represent up to 30% of the total dry matter loss in the ensiling process.

The visible signs of aerobic deterioration – heating and off-odors followed by fungal surface growth – appear only after significant nutrient loss has already occurred. Some molds also produce mycotoxins, which can affect livestock health and reduce animal performance. Managing the feed-out phase carefully is therefore just as important as the earlier stages.

Factors that influence ensiling success 🔗

Even with correct procedure, poor outcomes are common when key variables are ignored. Three factors – moisture content, crop type, and silo sealing – have the greatest influence on fermentation quality.

Moisture content 🔗

Moisture is one of the most critical factors in silage production. Too little, and the crop cannot be adequately compacted; too much, and nutrients leach away as effluent. The ideal moisture concentration for ensiling corn is 65-70%, which closely coincides with the stage of development that ensures near maximum production of total digestible nutrients per acre.

The recommended moisture range also varies by silo type. Recommended moisture contents are 65-70% for horizontal silos, 63-68% for conventional tower silos, 55-60% for limited-oxygen silos, and 65% for silo bags. Harvesting outside these windows leads to either clostridial fermentation (too wet) or aerobic spoilage and mold (too dry).

Crop type and water-soluble carbohydrate content 🔗

Not all crops ferment with equal ease. The amount of water-soluble carbohydrates (WSC) in the plant directly determines how quickly LAB can produce enough acid to stabilize the silage. Maize and sorghum are naturally high in WSC, making them well-suited to ensiling with minimal intervention.

Legumes like alfalfa present more of a challenge. Legume crops contain relatively low levels of carbohydrates compared to corn silage and require field wilting to increase the concentration of carbohydrates and reduce moisture content in the forage mass. For crops with insufficient fermentable substrate, additives such as molasses or sugar-releasing enzymes can be applied to supplement WSC levels and support a successful fermentation.

Chop length and compaction 🔗

Particle size affects how tightly the forage mass can be packed, which in turn determines how quickly oxygen is excluded. The recommended theoretical length of cut is 3/8 to 3/4 of an inch for corn silage and 3/8 to 1/2 of an inch for alfalfa silage. Chopping too long makes compaction difficult, trapping air in the forage mass and resulting in silage that heats and spoils.

A general rule is to try to achieve a packing density of about 14 pounds per cubic foot. Good sealing with plastic sheets and concrete barriers will keep the carbon dioxide in and prevent oxygen from entering the silo. Any holes or gaps in covers should be sealed with UV-resistant tape, especially in low-moisture silages where porosity is higher.

Preventing spoilage: keeping air out at every stage 🔗

The single most consistent cause of silage degradation is oxygen ingress – whether during filling, storage, or feed-out. Controlling air exposure at each stage is therefore the most important practical step in silage management.

During filling and sealing 🔗

Filling speed matters. In general, the faster the silo is filled the better. Rapid filling minimizes the risk of feed losses due to inclement weather and advancing crop maturity, reduces overall ensiling costs, and improves fermentation by minimizing exposure of the chopped forage to oxygen. Silos should ideally be filled within one to three days.

After filling, the silo must be sealed promptly and completely. Covers prevent not only air entry but also rainfall, which leaches nutrients including water-soluble carbohydrates, protein, and vitamins from the silo. Weighted plastic sheets are standard for bunker and pile silos, and any punctures should be patched immediately.

Using inoculants to stabilize fermentation 🔗

Where natural LAB populations are low – as is common with grass and legume silages – bacterial inoculants can help. The most widely used inoculant additives are based on homofermentative lactic acid bacteria such as Lactobacillus plantarum, which convert sugar to lactic acid and give rise to a rapid fall in pH and an efficient fermentation, preserving both energy and protein levels in the crop.

For silages that are exposed to air at feed-out, heterofermentative strains such as Lactobacillus buchneri are also used. These produce acetic and propionic acids that inhibit yeast and mold growth, improving aerobic stability when the silo face is opened – at the cost of a slightly slower initial fermentation.

Managing the feed-out face 🔗

During feeding, the exposed silage surface should be managed to minimize the area in contact with air. Removing silage evenly across the face, rather than digging into the pile from multiple angles, reduces the surface area exposed at any one time. Silage removed from the silo should be fed promptly, since yeast and mold activity can spike once silage is exposed to oxygen, leading to a kickstart of the fermentation process, spoilage, and a major loss of nutritional value.

Signs of good vs. poor silage 🔗

Well-fermented silage has a pleasant, slightly acidic smell, a firm texture, and a pH between 3.5 and 4.5 for corn-based varieties. It should be free of visible mold and should not show excessive heating when the face is opened. If silage has undergone clostridial fermentation, it will have a pH above 5, high ammonia-nitrogen levels, more butyric acid than lactic acid, and a characteristically foul smell. Such silage can reduce feed intake and milk production in dairy cattle and should be discarded rather than fed.

Consistently high-quality silage results from disciplined management of every stage: harvesting at the correct moisture and maturity, rapid filling and thorough packing, immediate sealing, and careful feed-out practices. No single step compensates for failures elsewhere in the process.

What do you think? Given that feed-out losses can account for up to 30% of total dry matter loss, how much attention do farms in your region typically give to managing the silage face during feeding? And with legume crops being harder to ferment than maize, do you think smallholder farmers in forage-scarce areas have enough access to practical guidance on crop-specific ensiling strategies?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC5677030/
2. https://forages.oregonstate.edu/nfgc/eo/onlineforagecurriculum/instructormaterials/availabletopics/mechanicalharvest/silage
3. https://www.fao.org/4/x8486e/x8486e09.htm
4. https://corn.agronomy.wisc.edu/Silage/S005.aspx
5. https://www.cropscience.bayer.us/articles/bayer/silage-harvest-moisture-and-proper-fermentation
6. https://extension.psu.edu/from-harvest-to-feed-understanding-silage-management
7. https://extension.purdue.edu/extmedia/nch/nch-49.html
8. https://extension.psu.edu/corn-silage-production-and-management
9. https://content.ces.ncsu.edu/forage-conservation-techniques-silage-and-haylage-production
10. https://www.ansc.purdue.edu/beef/articles/SilageHarvestingStorage.pdf
11. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/silage-fermentation
12. https://bonsilageusa.com/blog/the-four-stages-of-ensiling/