Grass silage is a low-cost feed for dairy herds, typically fed through winter when grazing is unavailable. Cows need consistent nutrition to sustain milk yield, and rumen capacity is limited, so the quality of what you put in front of them matters enormously.
Here is the thing most guides skip: grass is considerably harder to ensile well than maize. Grass has lower sugar content and higher protein, which gives it a higher buffering capacity, meaning it resists the pH drop that preservation depends on. Getting good grass silage is not simply a matter of following the maize playbook. It requires a different approach.
contents
Why Grass Is Harder to Ensile Than Corn
Two factors work against you with grass.
The first is chemistry. Maize is rich in water-soluble carbohydrate and low in buffering capacity, so lactic acid bacteria have plenty of fuel and the pH falls readily. Grass has less sugar and more protein, and protein buffers against acidification. The bacteria have less to work with and more resistance to overcome.
The second is soil contamination, and this one is often misunderstood. Grass is cut low to the ground, typically leaving a stubble of only 5 to 7 cm, whereas maize is cut much higher, often leaving 20 to 30 cm of stubble standing. That difference in cutting height means grass harvesting picks up far more soil. Soil carries clostridia, the bacteria responsible for butyric fermentation and protein breakdown. This is why raising your cutting height is one of the single most effective things you can do for grass silage quality.
Grass Silage Quality Parameters
Nutritional Value and Energy
| Parameter | Target |
|---|---|
| Dry matter (DM) | 30 to 45% |
| Crude protein | Typically 14 to 17% |
| Crude fiber | 22 to 25% |
| Raw ash | Below 10% (higher indicates soil contamination) |
| Net energy for lactation | 6.0 to 6.4 MJ per kg DM |
Ash content deserves particular attention. Because plant material itself contributes only a modest amount of ash, a figure above 10% is a direct signal that soil has entered the crop, and with it, clostridia.
Fermentation Quality
| Parameter | Target | Why It Matters |
|---|---|---|
| Butyric acid | 0.0% in DM | Any butyric acid indicates clostridial fermentation, meaning failure |
| Ammonium N | Below 8% of total N | Higher figures mean protein has been degraded |
| Acetic acid | 1.5 to 3.0% in DM | Some acetic acid aids aerobic stability, but excess signals inefficient fermentation |
| Mold | Below 5,000 cfu per gram | Indicates air ingress and mycotoxin risk |
| Yeast | As low as possible | High yeast counts drive heating and spoilage at feed-out |
pH: It Depends on Dry Matter
There is no single correct pH for grass silage, because the target moves with dry matter content. Wetter material must reach a lower pH to be stable, since water dilutes the acid. Drier, well-wilted material is naturally more stable and does not need to acidify as far.
| Dry Matter | Target pH |
|---|---|
| Below 25% (unwilted, high risk) | Below 4.0 |
| 25 to 30% | 4.0 to 4.3 |
| 30 to 40% | 4.2 to 4.5 |
| Above 40% (well wilted) | 4.3 to 4.8 |
A pH of 4.6 is perfectly acceptable in well-wilted grass at 42% DM, and a serious warning sign in unwilted grass at 24% DM. Read the two figures together. For the measurement procedure and full context, see our guide on determining pH in silage.
Producing High-Quality Grass Silage
1. Harvest at the Right Dry Matter
Target 30 to 45% DM. At this maturity, the balance of sugar, buffering capacity, and fiber is at its most favorable.
Harvesting below this range carries a high risk of clostridial contamination and effluent losses. Harvesting above it makes compaction difficult, because high-fiber, dry material resists packing and traps air.
Particle size should generally be under 4 cm, and the rule to remember is that the drier the crop, the shorter you should chop it, because dry material is harder to compact and needs help.
2. Compact Hard
Compaction is arguably the most important step. Target a density above 650 kg per cubic metre.
A practical constraint governs this: do not fill faster than your compaction machine can pack. As a working rule, filling should not exceed roughly one tonne of DM per hour per tonne of machine weight. If your packing tractor weighs 10 tonnes and the crop is at 40% DM, the silo should not receive more than about 25 tonnes of fresh material per hour.
If you cannot slow the fill, add weight to the machine. Water-filling the wheels or mounting concrete blocks on the tractor both work. Every additional kilogram of packing weight translates into better density and better silage.
3. Keep Soil Out
Ash content below 10% is the target, and it is achieved in the field rather than the silo. Raise your cutting height. Avoid harvesting in very wet or muddy conditions. Set mower and rake heights carefully so they are not scraping earth into the swath.
Contamination risk rises when the crop is wet, below about 25% DM. If you must harvest in those conditions, cutting at a higher stubble height of 7 to 8 cm reduces how much soil comes in with the grass.
4. Use an Inoculant
Because acidification is the central challenge with grass, a homofermentative lactic acid bacteria inoculant is the most direct intervention. It supplies the bacteria that drive the pH down, compensating for grass’s lower sugar and higher buffering capacity.
Chemical treatments exist for material outside the workable DM range. Wet or dirty grass can be treated with agents that suppress clostridia, and very dry silage can be treated to inhibit fungal growth where compaction is inevitably poor. However, for most operations, inoculation is the safer and more economical route, and it addresses the root problem directly rather than compensating for it.
“Farmers coming to grass silage from a maize background often underestimate how differently it behaves. Maize wants to ensile. Grass resists it, because the sugar is lower and the protein buffers the acid. So with grass you have to earn the fermentation: wilt to the right dry matter, cut higher to keep soil out, pack harder than you think you need to, and support the bacteria with an inoculant. Miss any of those and grass will punish you in a way maize would forgive.”
— Sue Su, Marketing Director of Silopak
Sealing and Storage
Every gain made in the field can be lost at the silo if air gets in. Grass silage in particular has less margin for error, so the seal has to hold for the entire storage period.
Choosing a silage film can feel confusing for a newer farmer, but the requirements are straightforward: it must be genuinely airtight, resist tearing during handling, and withstand months of UV exposure without becoming brittle. Silopak has manufactured silage film and bale net wrap since 2011 in a range of sizes to suit different systems. Good production also depends on capable machinery, and our group company supplies the agricultural equipment side of that equation.
Grass silage remains one of the most economical ways to feed livestock through a feed deficit. It simply demands more care than maize to get right.
Frequently Asked Questions about Grass Silage
Q: Why is grass harder to ensile than maize?
Grass has lower water-soluble carbohydrate content and higher protein than maize. The lower sugar means lactic acid bacteria have less fuel to produce acid, and the higher protein buffers against the pH drop, resisting acidification. Maize, by contrast, is high in sugar and low in buffering capacity, so it acidifies readily. Grass also picks up more soil during harvest because it is cut closer to the ground, and soil carries the clostridia responsible for butyric fermentation.
Q: What pH should grass silage be?
It depends on dry matter. Unwilted grass below 25% DM should be below pH 4.0. Grass at 25 to 30% DM targets 4.0 to 4.3, and at 30 to 40% DM targets 4.2 to 4.5. Well-wilted grass above 40% DM can be stable at 4.3 to 4.8. A single pH figure quoted without reference to dry matter is not meaningful, because wetter material needs to reach a lower pH to be safe from clostridia.
Q: How does cutting height affect grass silage quality?
Cutting height directly controls soil contamination, and soil contamination directly controls clostridia. Grass is typically cut to leave only 5 to 7 cm of stubble, which is much lower than maize, so it picks up considerably more earth. Raising the cutting height, particularly to 7 to 8 cm when the crop is wet, reduces the soil entering the silo. Ash content above 10% in your analysis is the clearest confirmation that soil is getting in and that cutting height needs to rise.
Q: What density should grass silage be packed to?
Target above 650 kg per cubic metre. Achieving this depends on matching your filling rate to your packing capacity: as a rule of thumb, do not fill faster than one tonne of dry matter per hour per tonne of machine weight. If you cannot slow the fill, add weight to the packing tractor by water-filling the wheels or mounting concrete blocks. Better density means less trapped air, which means faster acidification and better silage.
Reviewed and updated by the Silopak Editorial Team on 14 July 2026. The pH guidance here is presented in relation to dry matter content, and the cutting height figures refer to stubble height at mowing.
