Silage is forage preserved through fermentation. During that process, lactic acid bacteria convert plant sugars into acid, driving the pH down until the material becomes stable and spoilage organisms can no longer survive. Getting that pH drop right is the difference between feed that keeps for a year and feed that rots in the silo. So how do you determine pH in silage, and what should you actually be aiming for? This guide covers both: the target ranges, and how to measure pH on your own farm.
contents
What pH Should Silage Be?
Many farmers look for a single magic number, but the correct target pH depends on the dry matter (DM) content of the material. Wetter silage must reach a lower pH to be stable, because water dilutes the acid and gives spoilage bacteria more room to survive. Drier, well-wilted silage is naturally more stable and does not need to acidify as far.
| Dry Matter Content | Target pH | Typical Material |
|---|---|---|
| Below 25% (very wet) | Below 4.0 | Unwilted grass, high risk of clostridia |
| 25 to 30% | 4.0 to 4.3 | Lightly wilted grass silage |
| 30 to 40% | 3.8 to 4.2 | Whole-crop maize silage |
| Above 40% (well wilted) | 4.3 to 4.8 | Heavily wilted grass, haylage |
This is why pH figures quoted in isolation can be misleading. A pH of 4.5 is perfectly acceptable in well-wilted silage but a clear warning sign in very wet material. Always read pH alongside dry matter.
How to Measure pH in Silage
Knowing the target is only useful if you can actually measure what you have. There are two practical methods.
Method 1: Digital pH Meter (most accurate)
- Take a representative sample. Collect silage from several points across the face or bale, not just the surface. Surface material is always less representative because it has had the most air exposure.
- Prepare the sample. Take roughly 20 to 25 grams of chopped silage and mix it with about 100 ml of distilled water.
- Let it stand. Allow the mixture to sit for 15 to 30 minutes so the acids dissolve into the water, stirring occasionally.
- Calibrate the meter. Use buffer solutions (typically pH 4.0 and 7.0) before measuring. An uncalibrated meter is worse than no meter.
- Measure. Insert the probe into the liquid, wait for the reading to stabilize, and record it.
Method 2: pH Test Strips (quick field check)
Squeeze juice directly from a handful of fresh silage onto a pH test strip and compare the color against the chart. This is far less precise than a meter, and the color reading can be affected by the silage juice itself, but it is fast and useful for a rough check at the feed face when you simply want to know whether something has gone badly wrong.
When to Test
Test after fermentation has completed, typically 21 to 30 days after ensiling, and again at feed-out if you suspect a problem. Testing too early gives a misleading high reading, because fermentation is still in progress and the pH has not yet reached its final level.
“A pH reading only means something when you know the dry matter it belongs to. We regularly hear from farmers worried about a pH of 4.4, when their silage is well wilted and that figure is entirely normal. Equally, we hear from farmers happy with 4.4 on very wet material, where it actually signals that fermentation stalled. The number on its own tells you very little. The number in context tells you everything.”
— Sue Su, Marketing Director of Silopak
Read pH Together With Ammonia
pH alone gives an incomplete picture. Ammonia nitrogen, measured as a percentage of total nitrogen, tells you whether protein survived the fermentation.
| Ammonia N (% of total N) | What It Means |
|---|---|
| Below 10% | Good fermentation, protein well preserved |
| 10 to 15% | Moderate protein breakdown, room for improvement |
| Above 15% | Excessive protein breakdown, likely clostridial activity |
High pH combined with high ammonia points to the same root problem: fermentation was too slow, and undesirable bacteria took over before lactic acid bacteria could acidify the material. If you are seeing this pattern, our guide on diagnosing bad quality silage pH explains how to identify the cause and what it means for your livestock.
The Bacteria That Drive the pH Drop
The fall in pH is the work of lactic acid bacteria (LAB), which ferment plant sugars into lactic acid under anaerobic conditions. The species that matter most in silage include lactic acid bacteria such as Lactobacillus plantarum and Pediococcus pentosaceus, which acidify rapidly in the early stage, and Lactobacillus buchneri, which produces acetic acid and improves stability when the silo is opened. Other species commonly present include Lactococcus lactis and Leuconostoc mesenteroides, which are typically active in the earliest hours of fermentation before the more acid-tolerant lactobacilli take over.
These bacteria only work in an oxygen-free environment. That single fact drives almost everything else about how silage is made.
Two Things That Determine Whether pH Drops Properly
1. Seal the Silo Completely
Lactic acid bacteria need anaerobic conditions. If air keeps entering, aerobic bacteria consume the very sugars the LAB need, fermentation stalls, and pH never reaches target.
This means packing the material tightly to expel trapped air, sealing immediately after filling rather than leaving the silo open overnight, and keeping the seal intact throughout storage. It also means resisting the temptation to open the silo repeatedly out of curiosity. Every opening admits oxygen and invites putrefactive bacteria, and the result is decay rather than preservation. Maintaining that seal for months of outdoor storage is exactly what quality LLDPE silage film is designed to do, resisting punctures and UV degradation that would otherwise break the anaerobic barrier.
2. Get the Additive Dosage Right
Fermentation depends on having enough lactic acid bacteria and enough fermentable sugar for them to work with. Additives support both. A bacterial inoculant supplies the LAB directly, while a carbohydrate source such as molasses provides sugar for crops that are naturally low in it, such as legumes.
Dosage matters in both directions. Underdosing means the additive cannot do its job, and you have spent money for no benefit. Overdosing wastes money without a proportional gain and, in the case of sugar sources, can leave residual substrate behind. Follow the manufacturer’s stated rate and apply it evenly through the material rather than concentrating it in one place.
Reading the Result
When fermentation has succeeded, the signs are consistent. The silage is yellowish-green to golden-brown, holds its structure without becoming slimy, smells cleanly sour rather than rotten, and tests within the pH range appropriate to its dry matter. When it has failed, the material is dark or black, smells putrid, and the pH sits well above target.
Well-made silage, stored under an intact seal, reliably keeps for 12 to 18 months, which is what allows farmers to carry feed across seasons rather than scrambling for forage when pasture runs short.
Frequently Asked Questions about Determining pH in Silage
Q: How do I measure the pH of silage on my farm?
The most accurate method is a calibrated digital pH meter. Take a representative sample from several points, mix roughly 20 to 25 grams of chopped silage with about 100 ml of distilled water, let it stand for 15 to 30 minutes so the acids dissolve, then measure with a meter calibrated using pH 4.0 and 7.0 buffer solutions. For a quicker but less precise check, squeeze juice from fresh silage directly onto a pH test strip and compare against the color chart. Strips are useful for spotting an obvious problem at the feed face, but a meter is necessary for a reliable figure.
Q: When should I test silage pH?
Test after fermentation is complete, which is typically 21 to 30 days after ensiling. Testing earlier gives a misleadingly high reading because the pH is still falling. Test again at feed-out if you suspect aerobic spoilage, since pH can rise once oxygen re-enters the silage and yeasts and molds become active.
Q: What pH is considered good for silage?
It depends on the dry matter content. Very wet silage below 25% DM should be below pH 4.0. Grass silage at 25 to 30% DM targets 4.0 to 4.3. Whole-crop maize at 30 to 40% DM targets 3.8 to 4.2. Well-wilted material above 40% DM can be stable at 4.3 to 4.8. A single figure quoted without reference to dry matter is not meaningful, which is why pH and DM should always be assessed together.
Q: Why did my silage pH not drop far enough?
The most common causes are oxygen ingress, insufficient fermentable sugar, and material that was too wet at ensiling. If air keeps entering through a poor seal or a damaged film, aerobic bacteria consume the sugars the lactic acid bacteria need, and fermentation stalls. Low-sugar crops such as legumes may simply lack the substrate to fuel a strong pH drop, which is where a molasses or inoculant additive helps. And very wet material has a high buffering capacity that resists acidification. Correcting these means wilting to the right DM, packing and sealing thoroughly, and using an appropriate additive.
Q: How long can properly fermented silage be stored?
Well-made silage with an intact airtight seal reliably keeps for 12 to 18 months without significant nutritional loss. The limiting factor is not time but the integrity of the seal. Once oxygen enters through a puncture or a damaged cover, aerobic spoilage begins regardless of how well the original fermentation went. Inspect stored bales and silos regularly and repair any damage to the film promptly.
Reviewed and updated by the Silopak Editorial Team on 14 July 2026. The pH targets in this guide are given in relation to dry matter content, and the measurement procedures reflect standard on-farm practice.
