Silage pH: What’s Ideal and Why It Really Matters for Irish Farmers

When it comes to making high-quality silage, there are lots of numbers you can track — dry matter, crude protein, metabolisable energy, ash. But there’s one number that tells you almost everything you need to know about how well your silage fermented: pH.

In this edition of Silage Sessions, Ryan Duffy from Precision Microbes breaks down why a pH of between 3.8-4.2 is the ideal level for grass silage — and what it tells us about the success (or failure) of microbial fermentation.

What Is Silage pH?

pH is a measure of how acidic or alkaline something is. On a scale from 0 to 14:

• 1 = very acidic
• 7 = neutral
• 12 = very alkaline

In silage, we want an acidic environment — because acid preserves the grass, kills bad microbes (such as clostridial and enterobacterial species), and protects your valuable nutrients (such as crude protein and metabolisable energy).

What’s the Ideal pH for Grass Silage?

The ideal silage pH is around:

🟢 3.8 to 4.2

If your pH is in this range, it likely signals:

• A successful microbial fermentation
• Good microbial activity (e.g., Lactic acid bacteria)
• Minimal spoilage (indicated by high NH3-N and butyric acid levels)
• Nutrient preservation

But if the pH is outside that range — either too high or too low — problems can arise. It is also important to consider the duration of time it took for this pH level to be achieved.

What Happens if the pH is Too High?

A high pH (5 or above) means that fermentation was weak, restricted, or too slow, and bad bugs got a foothold.

What causes high pH in silage?

• Lack of homo-fermentative lactic acid bacteria
• Presence of bad microbes like Clostridia and Enterobacteria
• Poor oxygen exclusion (air = spoilage)
• Low sugar content in the grass (no simple sugars = no fermentation fuel)
• High moisture content (harder to acidify)

Signs in your silage analysis:

• pH of 5–6 or higher
• High ammonia nitrogen (NH3-N >10%)
• High butyric acid (+/- acetic acid)
• Low lactic acid (<75% of total fermentation acids)

What this means on-farm:

• Nutrients lost during fermentation
• Reduced palatability (bad smell, sour or rancid taste)
• Reduced animal intake
• Risk of poor animal performance, especially in dairy or finishing stock

What If the pH is Too Low?

While rare, overly acidic silage (pH <3.8) can also cause issues — especially in early spring.

What can it lead to?

• Higher rumen acid loading
• Risk of sub-acute ruminal acidosis (SARA) (+/- ruminal acidosis)
• Reduced intakes and digestive upset, especially if animals are on low-fibre diets

Precision Microbes Silage Additive is a liquid, ready-to-use (RTU) formulation containing live bacterial agents, specifically lactic acid forming bacteria (LAB) and homofermentative bacterial populations. Designed for efficient crop preservation, it promotes a rapid pH drop in both clamps and bales, ensuring high-quality silage. The product is organic certified and can be applied using either ultra-low dosing or standard dosing methods, offering flexibility and ease of use for farmers aiming to enhance silage fermentation and nutrient retention.

What Influences Silage pH?

Several factors impact how low your silage pH can go — and how fast it gets there.

1. Dry Matter (DM%)

• Wet grass takes more acid to reach the target pH
• Ideal: ~30% DM

2. Sugar Content

• More sugars = more food for lactic acid bacteria
• Young, leafy grass cut in the afternoon is ideal

3. Oxygen Exclusion

• Oxygen limits bacterial fermentation and promotes fungal spoilage
• Compact and seal tightly to prevent aerobic conditions

4. Microbial Population

• Homo-fermentative LAB produce only lactic acid → fast pH drop
• Hetero-fermentative LAB produce multiple acids → slower pH drop
• Bad bugs (e.g., Clostridia) keep pH high and cause spoilage by either competing with beneficial microbes for simple sugars and consuming the valuable lactic acid produced by the beneficial microbes

Why pH Is One of the Most Valuable Metrics

When you send off a forage sample and receive your silage analysis, pH is one of the first things to look at. It gives a clear indication of:

• How successful the fermentation was
• Whether nutrients were preserved or lost
• If the silage is safe and palatable for your livestock

💡 Combine pH data with NH3-N, butyric acid, DM%, and lactic acid levels for a more complete picture. 

How to Achieve Ideal Silage pH

Here’s your action plan to hit the target pH of 4 and keep bad bugs out:

1. Harvest at the Right Stage

• Cut grass before heading
• Aim for high sugar and moderate DM

2. Wilt for ~24 Hours

• Aim for ~30% dry matter
• Use tedders to speed up drying
• Avoid overly wet or overly dry silage

3. Compact and Seal Tightly

• Oxygen exclusion is non-negotiable
• Wrap bales well and seal pits as swiftly as possible

4. Use a Silage Additive

• Choose one containing homo-fermentative lactic acid bacteria
• These produce copious quantities of lactic acid efficiently and drop the pH fast
• The Precision Microbes Silage Additive is formulated specifically for this purpose

In Ryan’s words:

“Its unique formulation promotes the production of lactic acid via homo-fermentation, which drops the pH of silage rapidly and helps establish the microbial balance we all want.

The Bottom Line

If your pH is wrong, the quality of your silage is questionable.

Getting the pH to around 4.0 as quickly as possible:

• Preserves more nutrients
• Stops harmful bacteria
• Improves smell, taste, and intake
• Supports better milk yield and live weight gain

Tracking pH — and understanding what affects it — gives you real control over your silage quality year after year.