Corn silage is the most economical high-energy forage a farm can produce, which is why it anchors so many cattle rations. But its quality is not guaranteed by the crop alone. It is decided by how you harvest and, above all, how you pack it. This guide focuses on the part that most determines success and gets the least attention: packing density and oxygen exclusion.
The foundations, maturity, chop length, cutting height, kernel processing, and dry matter, all matter, and we cover the dry matter target below. But two identical crops can produce very different silage depending on how well the pile was packed. Packing is where good corn silage is won or lost.
Get the Dry Matter Right First
Corn silage should be harvested at 32 to 38% dry matter. This window gives the best balance of starch content, fermentation quality, and packing ability.
Kernel processing, cracking the kernels so the starch inside becomes digestible, should be done regardless of where you sit in that range, since whole kernels pass through the animal undigested. It is not something reserved for higher dry matters; it is standard practice for quality corn silage.
Harvesting below 32% dry matter causes real problems, and it is worth being clear about them:
- Energy loss. The plant has not reached full starch deposition, so you harvest less energy. What remains is proportionally more fiber and less of the starch that makes corn silage valuable.
- Effluent and nutrient loss. Very wet silage seeps effluent, carrying soluble nutrients out of the pile.
- Excessive protein degradation during a slow, wet fermentation.
- Clostridial fermentation. Wet conditions favour clostridia bacteria, which produce butyric acid instead of lactic acid. The result is unstable silage: secondary fermentation, rising pH, depleted sugars, and heating.
Above 40% dry matter brings the opposite problem: the crop is too dry to pack well, trapping air that leads to mould and heating. The 32 to 38% window is the target for good reason.
The Clostridia Problem and Why It Matters Beyond the Silo
The bacteria responsible for wet-silage failure are clostridia, and their effects reach further than many farmers realize.
Clostridia enter the silage mainly through soil contamination at harvest, picked up when the crop is cut too low or harvested in muddy conditions. In the silo, they produce butyric acid, driving up pH, destabilizing the fermentation, and degrading protein.
The consequences can follow the feed all the way to the finished product. Clostridium tyrobutyricum spores that survive in contaminated silage pass through the cow into the milk. In cheesemaking, those spores cause “late blowing,” a defect where gas production during ageing splits or bursts the cheese. This is a well-documented problem in hard cheese production and a genuine reason dairy processors care about silage quality. It all traces back to soil in the silo.
The lesson: raise your cutting height, avoid harvesting in muddy conditions, and keep soil out of the crop.
Recognizing Heated, Spoiled Silage
When silage heats, whether from wet clostridial fermentation or from trapped air in an under-packed pile, it spoils and loses feed value: energy, protein, and digestibility all fall.
The signs of overheated corn silage are recognizable: mould growth, visible decay, elevated temperature in the pile, a compressed or crusted surface, colour darkening from brown toward black, and a distinctive caramel or tobacco smell. That caramel smell in particular indicates heat damage that has bound up the protein and made it far less available to the animal. Silage in this state should not be fed.
“When we talk to farmers about corn silage quality, packing density is the factor that gets underestimated most. People focus on the crop and the additive, then under-pack the pile and wonder why it heated. Oxygen is the enemy of good silage, and the only way to exclude it is to pack hard enough. A good crop packed loosely makes mediocre silage. An average crop packed well often beats it.”
— Sue Su, Marketing Director of Silopak
Packing Corn Silage for Maximum Quality
Here is where the real work is, and where quality is decided.
Pack in thin layers
Fill the pile in thin layers, around 6 inches, rather than dumping deep loads. Thin layers can be compacted far more thoroughly, reducing porosity and driving out the air that fermentation cannot tolerate. Deep layers trap air pockets no amount of surface packing will remove.
Hit the target density
Aim for a packing density of at least 15 pounds of dry matter per cubic foot. This is the benchmark for well-excluded oxygen and stable fermentation. On a fresh weight basis the figure is higher and varies with dry matter, but the dry matter density is the number to manage against. Below this, air remains in the pile and spoilage follows.
Use enough packing weight
Density comes from weight and time. Do not fill faster than your packing equipment can compact. If needed, add weight to the packing tractor. Using two tractors packing in different directions increases both the weight applied and the coverage, which helps on larger piles.
Work safely
Do not build the pile too steep. A steep face is dangerous for packing tractors working side by side and risks rollover. A progressive wedge shape is safer and packs more evenly.
Seal top and bottom
Oxygen enters from every exposed surface. Use silage film as an oxygen barrier both below and above the pile. A well-sealed pile can be built up substantially without having to discard spoiled material from the top later, because the seal prevents the surface spoilage that otherwise wastes the top layer.
Use the right inoculant
An inoculant speeds and stabilizes fermentation. For piles that tend to heat, choose one containing Lactobacillus buchneri, which produces acetic acid and significantly improves aerobic stability, keeping the silage cool and stable through feed-out. This is particularly valuable for feeding through hot summer conditions when spoilage risk at the face is highest.
The Seal Is Everything
All the packing work is protected, or wasted, by the seal. Oxygen is the worst enemy of preserved forage, and film is what excludes it. A puncture or a poor seal lets air in and undoes the fermentation regardless of how well the pile was packed.
Silopak manufactures LLDPE silage film from premium virgin resin, engineered as a tight, durable oxygen barrier that holds through more than 18 months of storage. For the full detail on corn silage pH and harvest timing, see our dedicated guide on corn silage pH.
Frequently Asked Questions
Q: What dry matter should corn silage be harvested at?
Corn silage should be harvested at 32 to 38% dry matter. This range gives the best combination of starch content, fermentation quality, and packing ability. Below 32%, the crop is too wet: it loses energy, seeps effluent, degrades protein, and is prone to clostridial fermentation that produces butyric acid. Above 40%, it is too dry to pack well, trapping air that causes mould and heating. Kernel processing should be done throughout this range, not only at higher dry matters.
Q: What packing density should corn silage reach?
Aim for at least 15 pounds of dry matter per cubic foot. This density excludes enough oxygen for stable fermentation. Achieving it depends on packing in thin layers of around 6 inches, applying sufficient tractor weight, and not filling faster than the equipment can compact. Under-packed silage retains air pockets that lead to heating and spoilage, so packing density is one of the most important quality factors in corn silage, and one of the most commonly neglected.
Q: What causes the caramel or tobacco smell in corn silage?
That smell indicates heat damage. When silage overheats, whether from wet clostridial fermentation or from trapped air in an under-packed pile, chemical reactions bind up the protein and produce the characteristic caramel or tobacco odour, along with darkening from brown toward black. Heat-damaged silage has lost energy, protein availability, and digestibility. The caramel smell may seem pleasant, but it signals that the feed value has been degraded, and heavily heated silage should not be fed.
Q: How does corn silage affect cheese production?
Through clostridia contamination. If silage is contaminated with soil at harvest, clostridia including Clostridium tyrobutyricum establish in the silage and their spores pass through the cow into the milk. In hard cheese production, these spores cause a defect called late blowing, where gas produced during ageing cracks or bursts the cheese. This is why dairy processors supplying cheesemakers care about silage quality, and why keeping soil out of the silo, through higher cutting and clean harvesting, matters beyond the farm.
Reviewed by Sue Su, Marketing Director of Silopak, on 28 September 2026. This guide has been revised to correctly identify clostridia as the bacteria behind butyric fermentation and to focus on packing density as the key quality factor.
