Dairy & Slurry
How Many Cows Do You Need?
Herd size is only a starting point. What matters more is how much slurry can actually be collected, how consistently, and the farm's energy demand.
9 min readRead Guide
Dairy & Slurry
How Much Slurry Does a Dairy Cow Produce and What Does It Mean for AD?
There are indicative figures for slurry per cow, but they're a rough starting point, not a design number. Here's why the gap between the two matters.
6 min read · Reviewed September 2026
Quick Answer
AHDB's Slurry Wizard guidance uses parlour washings of around 20 to 30 litres per cow per day for conventional milking systems (typically 5 to 15 litres per day for robotic milking), added on top of excreta volume, when working out total slurry storage requirements. More importantly for anaerobic digestion, only the slurry that can actually be collected is available as feedstock — farm-specific measurements are more useful than a generic litres-per-cow figure.
Key Points
- Indicative slurry figures are a rough guide, not a guaranteed AD feedstock rate
- Parlour washings add significantly to total slurry volume beyond excreta alone
- Water increases hydraulic volume without adding corresponding biogas energy
- Actual measurement and farm records beat generic per-cow figures for AD planning
What is a useful indicative figure?
AHDB's Slurry Wizard tool — used to work out slurry storage requirements — adds parlour washings of typically 20 to 30 litres per cow per day for conventional systems on top of excreta volume, when calculating total slurry production for storage planning. For robotic milking systems, AHDB advises checking with the machine manufacturer and typically using a lower washings figure, often in the range of 5 to 15 litres per cow per day. These figures illustrate why any single number should be treated as an estimate, not a guaranteed AD feedstock rate.
Why can farm slurry volumes be much higher than excreta alone?
Because the store may also receive parlour wash water, robot wash water, rain from dirty yards, and roof water where drainage isn't properly separated from dirty areas.
Why does this matter for AD?
Because water increases hydraulic volume but doesn't provide corresponding biogas energy. Imagine 60 litres of relatively concentrated slurry versus 90 litres consisting of the same slurry plus 30 litres of wash water — the second system has more liquid to pump, heat, store and process, without gaining equivalent additional organic matter.
The real number required
For AD planning, try to establish collectable slurry per day for your farm specifically, rather than simply multiplying cows by a generic slurry figure. Actual flow measurement, tank levels and reliable farm records can substantially improve the quality of early plant sizing — the same principle covered in our guide on digester sizing.
Frequently Asked Questions
Sources
Related AD Guides
Equipment
What Size Digester Do I Need?
Digester size can't be calculated from cow numbers alone — it depends on feedstock volume, dry matter, retention time and the farm's actual energy demand.
9 min readRead Guide
Dairy & Slurry
Can AD Run on Slurry Alone?
Yes — slurry-only AD is already operating on UK dairy farms. Here's what affects gas production, how slurry actually moves through the system, and what it needs to work well.
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Dairy & Slurry
What Happens to Slurry in AD?
Follow cattle slurry from the yard through the digester to biogas and digestate, and see where odour actually changes along the way.
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PLANNING AN AD INSTALLATION?
Need help with the practical side of an AD project?
Charlie Bell Building supports farm-scale anaerobic digestion installations from site preparation and groundworks through to equipment installation and commissioning support.