BMP testingGrasses, green forage and fodder plants

Clover-grass silage — BMP: 2 studies, weighted mean 296 m³ CH₄/t VS

Clover-grass silage is a mixture of legumes and grasses from grassland or fodder fields. It usually has a better protein and fibre balance than silage from grasses alone.

CH₄ ≈ 52 % (estimate)

Published 3 August 2024Updated 29 October 20263 min readFeedstockData Editorial Team

Photo Bob Harvey / Wikimedia Commons, licence CC BY-SA 2.0, source file; changes: cropping

Weighted mean: 296m³ CH₄/t VS
Range: 256–335m³ CH₄/t VS
Records: 2studies
Publications: 2sources
Temperatures: 37 °C, 38 °Cof the test

Methane in biogas

≈51–54% CH₄

Medium methane content · ≈ 5.2 kWh/m³ of biogas

estimate from the substrate type The collected studies contain no measurement. The interval follows from the approximate composition (carbohydrate-dominated (no protein or fat characteristics)) and the yields of compound groups according to FNR.

How we calculate it and what the zones mean

Rating for biogas plants

not rated — editorial rating for biogas plants

Not rated: no dry matter or volatile solids in the data. We calculate the rating when the dry matter and volatile solids of the material are known.

Editorial rating per the methodology

Add to the co-digestion calculator CSV data

BMP curves from 2 studies

Weighted mean Individual records

Dashed lines are a model curve (first-order kinetics, B(t) = B₀·(1 − e^−kt)); the end point comes from the publication. All curves are model curves.

Cumulative methane production over the test [m³ CH₄/t VS]. The thick line is the weighted mean of all records (296); dots mark the end values given in the publications. Point at the chart to read values; click a legend item to hide or show a record.

Records and sources

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Source	BMP m³ CH₄/t VS	Temperature	Sample	Curve
Amon et al. (2007) DOI	335	38 °C · mesophilic	Clover Grass	end point (model curve)
Pyykkönen et al. (2023) DOI	256	37 °C · mesophilic	Clover-grass silage (CGS)	end point (model curve)
Weighted mean	296	arithmetic mean 296, SD 56

Process properties

seasonal
fibrous
editorial classification

Advantages

Methane potential in the database: weighted mean 296 m³ CH₄/t VS (median 296; range 256-335; 2 records from 2 sources).
In full-scale experiments, mixtures of clover-grass silage with poultry and horse manure gave 100-108% of the methane potential at a digestion time of 117-185 days (Pyykkönen 2023).

Drawbacks and risks

Spread of results in the database: standard deviation 56 at a mean of 296 m³ CH₄/t VS (2 records from 2 sources).

Co-digestion — what to combine it with, what to watch out for

combines well
Horse manure
A mixture of silage with horse and poultry manure achieved 100-108% of the potential at farm scale (Pyykkönen 2023).^[1]

Dosing

No source max share; Pyykkönen 2023 used mixtures of silage with poultry and horse manure at farm scale.

Practical requirements

Store the digestate so as to limit nitrogen losses: at farm scale, high N losses were recorded with storage under a tarpaulin in summer (Pyykkönen et al. 2023).

Sources of the process data

The co-digestion calculator will assess a specific mixture containing this substrate.

What it is and where it comes from

Mixtures of clover with grasses are a classic element of crop rotation and fodder cultivation in a temperate climate. Green forage from several cuts in the season is ensiled (or harvested as haylage) and can reach a biogas plant as a supplement. The compilation contains several records: a clover-grass mixture and silage from Finland. Both concern mesophilic conditions.

Properties relevant to digestion

Clover-grass silage combines the characteristics of two components: the grasses supply fibre and carbohydrates, the clover protein. As a result the ratio of C to N is usually more favourable than in pure grass. Digestibility depends strongly on the development stage at the time of cutting: younger green forage is more readily degradable, older more woody. Proper ensiling requires a suitable dry matter and sugar content, and clover, as a legume, has a high buffering capacity, so ensiling is more difficult than for grasses. Too large a share of protein can increase ammonia release.

How to read the spread of results

The compilation has records that differ in country, the proportion of clover to grass, the cutting date and the ensiling technique. Such mixtures are by nature heterogeneous: the botanical composition changes over time during use of the meadow or field and between cuts. Chop length, the storage period of the silage, sample preparation, and also the inoculum and test length have a further effect. The result of a single study therefore describes a specific mixture and not the whole category.

In biogas plant practice

Clover-grass silage is a useful supplement on farms that have fodder grassland and want to use surpluses. It can be mixed with maize silage or slurry. In storage, the standard rules of good ensiling are applied: rapid compaction, a tight cover, silage additives. In the mix, ammonia must be controlled, and the quality of the silage (butyric acid, soil contamination) assessed before feeding. Your own BMP test makes it possible to match the dose to the clover content in a given year. It is also good to keep silages from individual cuts separate in the store, because they differ in quality and digestibility. This allows the ration to be composed consciously and sudden changes in digester operation to be avoided when switching to a new batch.

Methodology in brief

BMP is the amount of methane obtained from volatile solids (VS) in a batch test, given in m³ CH₄ per tonne of VS (numerically the same as NmL CH₄/g VS in the publications) and converted to normal conditions (0 °C, 101.325 kPa). The weighted mean gives each publication a total weight of 1, split equally between its results for the substrate — a paper testing twenty varieties does not outweigh a paper with a single sample. In inter-laboratory programmes the unit is the laboratory (total weight 1), and within it the weight of a test grows with the number of bottles (replicates). Results after pretreatment (e.g. thermal, chemical, enzymatic) are shown separately and are not included in the mean.

Solid lines are curves measured in the tests. Where a publication gives only the final result, we draw a first-order kinetic model, B(t) = B₀·(1 − e^−kt): only the end point comes from the publication, and the shape of the curve serves purely as visualisation — do not read the production on a particular day from it. For this substrate k = 0.091 d⁻¹ was assumed (no kinetic data for the group – the median of all available rate constants k (93 values) was used).

The statistics describe the spread of published results — a descriptive compilation, not a meta-analysis or a guarantee of plant yield.

Full methodology and correction policy

Bibliography

All sources of the site: bibliography.