Silage Making Process: Ensuring Quality from Harvest to Feed

Silage is one of the most reliable methods for preserving forage crops over long periods, especially when fresh feed is scarce. But the quality of silage at the feeding stage is almost entirely determined by decisions made at harvest. Get those decisions wrong, and you end up with spoiled, nutrient-depleted material that does more harm than good. Get them right, and you have a stable, fermented feed that can sustain livestock for months. Here is a clear, step-by-step breakdown of how the silage making process works – from the moment a crop is cut to the day it is fed out.

Overview of the silage making steps 🔗

The silage making process moves through four core stages: harvesting, chaffing (chopping), filling and compacting the silo, and sealing. Each stage feeds directly into the next, meaning a mistake at any point can compromise everything that follows.

Harvesting at the right time and moisture 🔗

Timing is everything at harvest. The crop must be cut when it contains the right balance of moisture and fermentable sugars. According to the University of Florida IFAS Extension, forages harvested with excess moisture (above 70%) risk seepage losses and clostridial fermentation, which produces butyric acid, degrades protein, and creates offensive odors. On the other hand, forages that are too dry create packing problems, as stiff stems trap air and promote yeast and mold growth.

For corn silage, the recommended dry matter (DM) at harvest is 30-35%, which corresponds to a whole-plant moisture of roughly 65-70%. Grasses and legumes typically need to be wilted in the field after cutting to bring their moisture down to suitable levels – legumes in particular, due to their high buffering capacity, require wilting to 35-45% DM before ensiling. NC State Extension notes that crops like corn are especially suited for silage because of their naturally high water-soluble carbohydrate (WSC) content, which fuels the bacterial fermentation needed to acidify and preserve the material.

Chaffing: chopping to the right particle size 🔗

Once harvested, the crop is chopped into uniform pieces using a forage harvester. This step, called chaffing, does more than just reduce size. Chopping breaks open plant cell walls and releases the plant juices that contain fermentable sugars – sugars that lactic acid bacteria (LAB) will later consume during fermentation.

Penn State Extension recommends a theoretical length of cut (TLC) of 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 traps air in the forage mass, making proper compaction difficult and leading to spoilage. Chopping too fine, however, reduces effective fiber content, which affects rumen function in animals consuming the silage. The ideal cut achieves a balance: tight enough to pack well, but not so fine that it loses its nutritional structure.

Precision forage harvesters with sharp, well-maintained knives are essential for achieving consistent particle size. Dull blades tear rather than cut cleanly, leading to uneven material that packs poorly and ferments inconsistently.

Rapid filling of the silo 🔗

After chopping, the material must be moved into the silo as quickly as possible. Speed matters here because, from the moment a crop is cut, plant enzymes and aerobic bacteria begin consuming sugars – the very sugars needed for fermentation. IFAS Extension explains that this initial aerobic phase can last anywhere from a few hours to up to 48 hours in poorly managed silos, and every carbohydrate consumed by aerobic bacteria during this window is permanently lost from the final silage product.

Ideally, the silo should be filled within 1-3 days to minimize this aerobic exposure. For bunker silos, this means coordinating harvesting, transport, and compaction equipment to keep the entire operation moving continuously.

Compaction and air exclusion 🔗

Compaction is arguably the most physically demanding – and most critical – step in the entire process. Its sole purpose is to squeeze out oxygen from the forage mass and create the anaerobic (oxygen-free) environment that lactic acid bacteria need to begin fermentation.

Why oxygen is the enemy 🔗

As long as oxygen is present, aerobic microorganisms continue to break down plant sugars, generating carbon dioxide, water, and heat. Temperatures rising above 100°F are particularly damaging – IFAS Extension warns that excessive heat degrades protein by bonding it with carbohydrates, making it far less digestible for livestock. Heat-damaged silage is typically brown in color with a tobacco-like smell – a clear sign of poor compaction during filling.

How to compact effectively 🔗

For bunker silos and silage piles, mechanical compaction is essential. The standard method involves spreading the chopped material in thin layers – no more than 15-20 cm (6-8 inches) per pass – and running heavy machinery, typically tractors, back and forth over each layer before adding the next. ABS Global UK describes this as driving over thin layers continuously to pack the grass as tightly as possible, with the target being a silage density of around 200-250 kg of dry matter per cubic meter.

Edges and corners are the most common weak points. These areas tend to hold air pockets because machinery cannot reach them as effectively as the central mass. Extra manual compaction at the edges – using a tracked vehicle or a compaction roller – is necessary to ensure uniform density across the entire silo face.

Bayer Crop Science notes that the respiration phase typically lasts three to five hours when compaction is done correctly. The faster and more completely oxygen is excluded, the less nutrient loss occurs before fermentation can begin.

Sealing and fermentation 🔗

Once the silo is filled and compacted, it must be sealed immediately – ideally within 12 hours of harvesting. Sealing is what transforms the storage structure into a controlled fermentation environment.

Airtight sealing methods 🔗

For bunker silos, the standard approach is to cover the entire surface with a plastic oxygen-barrier film, then weigh it down with old tires, sandbags, or gravel bags placed edge-to-edge. The plastic must be laid carefully to avoid punctures or gaps. Even a small hole can allow oxygen infiltration that causes aerobic spoilage to spread through the top layers of the silage. For round bales, stretch wrap film applied with a minimum of four overlapping layers achieves a comparable anaerobic seal.

Upright (tower) silos rely on gravity for compaction and generally do not require mechanical packing except at the very top layer, which should be manually leveled and sealed.

The fermentation phases 🔗

Once the silo is sealed, fermentation proceeds through distinct phases. IFAS Extension identifies five fermentation phases in the ensiling process. After the initial aerobic phase ends (Phase 1), anaerobic bacteria begin producing organic acids that drop the pH from above 6 toward 5 (Phase 2). At this point, lactic acid bacteria (LAB) take over as the dominant microorganisms (Phase 3). LAB convert plant sugars into lactic acid, which is more effective than other organic acids at lowering pH rapidly. In well-made silage, the pH stabilizes between 4.0 and 4.5.

This acidic environment effectively “pickles” the forage, inhibiting the growth of clostridia, listeria, and other spoilage-causing organisms. The fermentation is typically complete within 21 days, though some crops like corn silage continue to improve in digestibility for several months after ensiling as cell wall structures break down further.

The role of silage additives 🔗

While natural LAB populations on the crop surface are sufficient for fermentation in many cases, bacterial inoculants can accelerate the pH drop and improve fermentation consistency, especially for crops with low WSC content. Inoculants containing homolactic bacteria produce only lactic acid, making the acidification faster and more efficient. For crops prone to aerobic spoilage at feed-out – such as high-starch corn silage – inoculants containing heterofermentative bacteria like Lactobacillus buchneri are particularly effective, as they produce acetic acid that suppresses yeast growth when the silo is opened. NC State Extension also notes that molasses and enzymes can serve as fermentation stimulants, promoting a rapid pH drop and reducing dry matter losses.

Best practices for feed-out 🔗

Opening the silo reintroduces oxygen to the silage face, triggering the very aerobic spoilage process that the entire ensiling procedure was designed to prevent. Managing feed-out carefully is therefore just as important as the earlier stages.

When to open the silo 🔗

Silage should not be opened before fermentation is complete. Most silages need at least 21 days of sealed fermentation before feeding can begin. Opening prematurely interrupts the fermentation process, increases dry matter losses, and can result in unstable silage in the feed bunk. Corn silage, in particular, tends to improve in quality and digestibility the longer it is stored before feeding, with significant improvements observed at 60-90 days post-ensiling.

Managing the exposed face 🔗

At feed-out, the key principle is to minimize the surface area of silage exposed to air at any given time. The Organic Farmer recommends removing only what will be consumed within a short period rather than opening large sections of the silo face. The exposed face should be kept smooth and vertical. Rough, uneven cuts create a larger surface area that accelerates aerobic deterioration and heating.

Using sharp cutting equipment – silage defacers, block cutters, or front-end loaders with cutting blades – produces clean, uniform slices without loosening or disturbing the surrounding compacted material. Standard bucket loaders that tear at the face are problematic because they disturb the packed silage and expose more material to air than necessary.

Rate of face advance 🔗

The rate at which the silo face is worked through matters too. A face that advances too slowly means exposed silage sits in contact with air for too long before being consumed. As a general guide, the face should advance by at least 15-30 cm (6-12 inches) per day during warm weather, when aerobic spoilage accelerates. In cooler conditions, this rate can be somewhat slower without significant quality loss.

Introducing silage to animals 🔗

When introducing silage to animals that have not been fed it before, a gradual transition is recommended. The Organic Farmer advises against feeding silage to young animals under six months of age, as their digestive systems are not yet developed enough to handle fermented feeds efficiently. For adult livestock, increasing the silage proportion in the ration over 7-10 days allows the rumen microbiome to adjust and prevents digestive upset.

Putting it all together 🔗

Every step in the silage making process is connected. Harvest timing sets the moisture and sugar content available for fermentation. Chop length determines how well the material can be compacted. Compaction dictates how quickly oxygen is excluded. Sealing controls whether LAB can dominate the fermentation. And feed-out management determines how much of that carefully preserved nutrition actually reaches the animal. Cutting corners at any one stage can undo the work done at all the others. When each step is executed correctly, the result is a stable, nutrient-rich feed that can sustain livestock through months when fresh forage is unavailable – a straightforward application of microbial science in service of sustainable agriculture.

What do you think? If a farmer consistently finds that the top layer of their bunker silo is spoiled after opening, which step in the silage making process is most likely responsible – and what specific changes would you recommend? Beyond livestock feed, could the fermentation principles used in silage production be applied to manage other types of organic waste more sustainably?

References

1. https://ask.ifas.ufl.edu/publication/AG180
2. https://content.ces.ncsu.edu/forage-conservation-techniques-silage-and-haylage-production
3. https://extension.psu.edu/from-harvest-to-feed-understanding-silage-management
4. https://www.absglobal.com/uk/how-is-grass-silage-made/
5. https://www.cropscience.bayer.us/articles/bayer/silage-harvest-moisture-and-proper-fermentation
6. https://theorganicfarmer.org/silage-making/