What is dry matter (DM) in silage?

Dry matter is what remains after all the water is removed from the fresh crop. A grass silage at 25% DM contains 25 kg of actual nutritional material per 100 kg of fresh silage. Feed budgets are always planned on a DM basis because the water content varies between batches.

How is clamp density calculated?

Clamp density is the fresh weight per cubic metre of compacted silage, typically 650-750 kg/m3 for grass and 700-750 kg/m3 for maize. The calculator multiplies your clamp volume (L x W x H) by this density to give fresh weight, then applies the DM percentage to derive dry-matter tonnes.

What shrinkage figure should I use?

Shrinkage covers fermentation gas losses (4-6%), effluent run-off (2-5%) and face/shoulder losses during feedout (2-4%). Grass silage at 25% DM typically loses 10-15% overall; maize at 30% DM loses 8-12%; wholecrop loses 6-10%. Well-sealed, well-compacted clamps sit at the lower end of these ranges.

How many tonnes of silage does a hectare of grass produce?

A productive first-cut grass silage field in the UK typically yields 20-30 fresh tonnes per hectare (5-7.5 t DM/ha). Second-cut yields are lower (15-20 t fresh/ha). Maize yields are higher, commonly 35-45 fresh tonnes per hectare (10-14 t DM/ha). Enter your own figures for accurate results.

How do I estimate feeding days for a dairy herd?

A high-yielding dairy cow eats roughly 14-18 kg of DM per day, which equates to 50-70 kg of fresh grass silage at 25% DM. The calculator divides your store DM (after shrinkage losses) by total herd DM demand to give feeding days. Always keep a buffer of at least 2-3 weeks in case of a late spring or poor second cut.

What is a good dry matter for ensiling?

Grass should be wilted to 25-30% DM before ensiling for an optimal balance between fermentation quality and effluent risk. Below 20% DM generates large volumes of effluent and nutrient loss; above 35% DM is difficult to consolidate and risks aerobic instability (heating). Maize is usually harvested at 28-35% DM at the whole-plant stage.

Silage Calculator — Gera Tools

Silage is the backbone of year-round livestock feeding on most mixed and dairy farms. Getting the sums right — how much you cut, how much fits in the clamp, and how long it will last — is critical to winter feed planning and avoiding mid-winter forage shortfalls. This silage calculator handles all three core calculations: clamp volume to tonnage, field area to expected yield, and store size to feeding days.

How it works

Clamp volume to tonnage

The most direct calculation. If you know your clamp dimensions, the fresh weight is simply:

Fresh weight (kg) = Volume (m³) × Compacted density (kg/m³)

Density varies by crop and compaction effort — grass silage typically packs at 650–700 kg/m³, maize at 700–750 kg/m³. Dry-matter weight follows:

DM weight (kg) = Fresh weight × (DM% ÷ 100)

Finally, the available tonnage after fermentation and effluent losses:

Available (kg) = Fresh weight × (1 − Shrinkage% ÷ 100)

Field area to yield

Enter your field area and your expected (or past-season) yield in fresh tonnes per hectare. The tool converts area to hectares, multiplies by yield to get total fresh tonnes, applies the DM% for dry-matter yield, and divides by density to tell you how large a clamp you need:

Clamp volume (m³) = Total fresh yield (kg) ÷ Density (kg/m³)

It also shows three suggested L × W × H configurations at typical clamp widths.

Store to feeding days

This mode is your winter feed-budget check. Enter the total fresh weight in your store plus your livestock type and headcount. The tool looks up a typical DM intake per head per day (or accepts a custom figure), converts the store to available DM after shrinkage, and calculates:

Feeding days = Available DM (kg) ÷ (Head × DM intake per head per day)

A warning flag appears when the store will last fewer than 60 days.

Worked example

A dairy farmer harvests 10 ha of first-cut grass silage at a yield of 25 t/ha. The silage is ensiled at 25% DM, packed to 680 kg/m³, with estimated 12% shrinkage.

Total fresh yield: 10 × 25 = 250 t (250,000 kg)
DM yield: 250,000 × 0.25 = 62,500 kg DM (62.5 t DM)
Available after losses: 250,000 × 0.88 = 220,000 kg (220 t)
Clamp volume needed: 250,000 ÷ 680 = 368 m³ (e.g. 46 m × 8 m × 1 m, or 26 m × 8 m × 1.8 m)

That store will feed 100 mid-yield dairy cows (14 kg DM/head/day) for:

Available DM: 220,000 × 0.25 = 55,000 kg
Days: 55,000 ÷ (100 × 14) = ~39 days

This tells the farmer that one cut will not carry a 100-cow herd through a typical 140-day housing period — a second cut or alternative forage source is essential.

Formula note

All formulae used are standard agronomic conventions:

Fresh weight = volume (m³) × density (kg/m³)
DM weight = fresh weight × (DM% / 100)
Available weight = fresh weight × (1 − shrinkage% / 100)
Feeding days = available DM ÷ (head count × DM intake per head per day)
Fresh intake per head = DM intake ÷ (DM% / 100)

Density figures follow AHDB and SAC Consulting advisory ranges. DM intakes are based on AFRC/TCORN standards for the given production class. All calculations run entirely in your browser — no data is uploaded.