Silage is one of the most important feeds in dairy farming. Farmers rely on it to maintain milk production when grazing is limited, particularly through winter or dry seasons. Dairy cattle need consistent energy and protein to sustain milk yield, and well-made silage delivers exactly that when pasture cannot. This guide covers how silage is produced for dairy herds, and how to judge whether the result is good enough to feed.
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What Is Silage?
Silage is forage preserved through fermentation, sometimes described as pickled pasture. It is typically made from whole crops such as maize, oats, sorghum, or other cereals, using the entire green plant rather than just the grain. Thicker-stemmed plants are generally best suited to ensiling, while thinner-stemmed forages are more often conserved as hay.
How Silage Fermentation Actually Works
There is a common misconception worth clearing up immediately, because it affects how farmers interpret what comes out of their silo.
Silage fermentation is not a choice between two methods. There is one fermentation you want, and one you are trying to prevent.
Lactic acid fermentation is the goal. Anaerobic lactic acid bacteria consume plant sugars and produce lactic acid, which drives the pH down rapidly and preserves the forage. The result is silage with a clean, sharp, sour smell that cattle eat readily.
Butyric acid fermentation is a failure, not an alternative. It occurs when clostridia bacteria dominate instead of lactic acid bacteria, which happens when the crop is ensiled too wet, when the pH drops too slowly, or when soil contaminates the forage. Clostridia degrade protein and produce butyric acid, giving silage a rancid, unpleasant odor. Dairy cattle reject it, intake falls, and milk yield suffers. Butyric acid in your silage is not a grade of quality. It is evidence that something went wrong.
“We still come across guidance suggesting butyric fermentation is simply what happens with protein-rich crops, as if it were a normal outcome. It is not. Butyric acid means clostridia beat the lactic acid bacteria to the sugars, and on a dairy farm that shows up directly in the bulk tank. If your silage smells rancid, the answer is not to accept it as a lower grade. The answer is to find out whether the crop went in too wet or whether soil got into it.”
— Sue Su, Marketing Director of Silopak
Producing Silage for Dairy Cattle
Harvest at the Right Stage
For cereal crops, the optimal window runs from the flowering stage through to the milk stage, when the balance of sugar, starch, and digestible fiber is at its best.
Get the Dry Matter Right
Target 30 to 35% dry matter at ensiling, which corresponds to roughly 65 to 70% moisture. This range matters more than almost any other decision.
| Dry Matter at Ensiling | Outcome |
|---|---|
| Below 28% (too wet) | Clostridia thrive, butyric acid forms, protein degrades, effluent runs off |
| 30 to 35% (target) | Good compaction, rapid acidification, clean lactic fermentation |
| Above 40% (too dry) | Poor packing, trapped air, mold, heating in storage |
Chop, Fill, and Compact
Chop the crop to allow proper compaction and to make the feed easy for cattle to consume. Distribute it evenly through the silo in layers, and compact each layer hard. Density is a direct proxy for how much air you have driven out. Well-compacted silage typically reaches around 600 to 800 kg per cubic metre on a fresh weight basis. If your density is far below this, air is still trapped in the mass and fermentation will suffer regardless of what else you do right.
Keep Soil Out of the Crop
This point deserves its own heading because it is so often overlooked. Clostridia live in soil. Every clod of earth that enters the silo brings the bacteria responsible for butyric fermentation with it.
This means raising the cutting height rather than scalping the ground, avoiding harvest in very muddy conditions, and never using soil, mud, or dirt as a covering material. Traditional pit methods that seal the silo with earth introduce contamination directly into the feed and are a common cause of the butyric problem they are meant to avoid.
Seal Properly
The silo must exclude air completely, and it must stay sealed for the entire storage period. This is what a purpose-made silage film does, providing an airtight, UV-resistant barrier that holds through months of outdoor storage without tearing or degrading. Fermentation begins as silo temperature reaches roughly 27 to 38 degrees Celsius, and the main fermentation completes in about three to four weeks, though quality continues to improve over the following months as the material stabilizes.
Silage Color: What It Tells You
Color changes during fermentation because organic acids react with chlorophyll, converting it into brown pigments as magnesium is stripped from the molecule and pheophytin forms.
At moderate silo temperatures, the result is a yellowish-green or golden silage, which is what you want to see. If the silo runs hot, the silage turns dark brown or even black. Dark discoloration indicates excessive heating, which damages protein availability and signals that too much air was trapped in the mass.
Judging Silage Quality for Dairy Cattle
The table below sets out realistic quality grades. Note that butyric acid appears only as a marker of declining quality, never as a feature of good silage.
| Grade | Smell and Appearance | pH | Ammonia N (% of total N) |
|---|---|---|---|
| Excellent | Clean sharp sour smell, no butyric odor, no mold, not slimy, golden to greenish | 3.8 to 4.2 | Below 10% |
| Acceptable | Sour but slightly sharper, minor discoloration, no visible mold | 4.2 to 4.5 | 10 to 15% |
| Poor / failed | Rancid or putrid butyric odor, slimy texture, visible mold, dark discoloration | Above 4.8 | Above 15 to 20% |
For lactating dairy cows in particular, the difference between the top and bottom rows of this table shows up directly in intake and milk output. Poor silage is not merely lower-value feed; cows eat less of it, so the ration you formulated is not the ration they consume.
Note that pH targets shift with dry matter. The figures above suit maize and cereal silage in the 30 to 40% dry matter range. For the full picture across different forages and moisture levels, see our guide on determining pH in silage.
Frequently Asked Questions
Q: Is butyric acid in silage normal?
No. Butyric acid is a sign of failed fermentation, not a normal outcome or a lower grade of acceptable silage. It forms when clostridia bacteria dominate the fermentation instead of lactic acid bacteria, which happens when the crop is ensiled too wet, when acidification is too slow, or when soil contaminates the forage. Butyric silage smells rancid, has degraded protein, and is rejected by dairy cattle, causing intake and milk yield to fall. Good silage should be free of butyric acid entirely.
Q: What pH should dairy silage be?
For maize and cereal silage at 30 to 40% dry matter, the target is a pH of 3.8 to 4.2, with ammonia nitrogen below 10% of total nitrogen. A pH between 4.2 and 4.5 with ammonia of 10 to 15% is still workable but indicates room for improvement. Above pH 4.8 with ammonia over 15%, fermentation has failed. Bear in mind that the correct pH target depends on dry matter, since wetter silage must reach a lower pH to be stable.
Q: Should I use soil or mud to seal a silage pit?
No. Soil contains clostridia, the bacteria responsible for butyric fermentation and protein breakdown. Sealing a pit with mud or earth introduces those bacteria directly into the feed and is a common cause of the fermentation failure it is intended to prevent. Use a proper airtight silage film instead, which excludes air without contaminating the crop and resists UV degradation and tearing through months of storage.
Q: Why is my dairy silage dark brown or black?
Dark discoloration means the silo ran too hot during fermentation. This happens when excess air is trapped in the mass, allowing aerobic bacteria to respire and generate heat. High temperatures damage protein availability, so the feed value drops even if the silage looks otherwise intact. The usual causes are inadequate compaction, material ensiled too dry to pack properly, or a slow fill that left the crop exposed to air for too long.
Q: How much silage does a silo hold?
Capacity depends on packing density, which is itself a quality indicator. Well-compacted silage typically reaches around 600 to 800 kg per cubic metre on a fresh weight basis. If your silo is holding substantially less than that per cubic metre, it points to under-compaction, meaning air is still trapped in the mass and fermentation quality will suffer. Sizing the silo to your herd and feeding period matters, but achieving good density within it matters more.
Reviewed and updated by the Silopak Editorial Team on 14 July 2026. The fermentation and quality grading guidance in this article has been revised to reflect current silage science, particularly regarding butyric acid as an indicator of fermentation failure.
