BMP testingGrasses, green forage and fodder plants

Proso millet, whole crop — BMP: 2 studies, weighted mean 260 m³ CH₄/t VS

Common millet as a whole plant is a thermophilic cereal considered as an energy crop on lighter soils. The compilation describes it with a small number of records from different studies.

rating: good CH₄ ≈ 52 % (estimate)

Published 30 January 2025Updated 29 October 20263 min readFeedstockData Editorial Team

Photo Roger Culos / Wikimedia Commons, licence CC BY-SA 4.0, source file; changes: cropping

Weighted mean: 260m³ CH₄/t VS
Range: 253–268m³ CH₄/t VS
Records: 2studies
Publications: 2sources
Temperatures: 38 °Cof the test

Methane in biogas

≈51–53% CH₄

Low 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 (protein from total nitrogen (N 0.8 % DM × 6.25)) and the yields of compound groups according to FNR.

How we calculate it and what the zones mean

Rating for biogas plants

good — editorial rating for biogas plantsStable, usually accepted without a fee

Potential per tonne of fresh matter: high (53 m³ CH₄/t); no particular threats to stability. Handling costs: fibres, floating layer.

Why? Rating factors

Potential per tonne +2

+2potential 53 m³ CH₄/t fresh matter (BMP 260 × DM 22.4 % × VS 91.5 %)

Process stability 0

0no factors

Handling and costs −0.5

−0.5fibres, floating layer — shredding and intensive mixing

Total points: 1.5 · thresholds: ≥ 2.5 very good, ≥ 1 good, below — neutral; negative with cumulative process risk.

Editorial rating per the methodology · confidence of the rating: low · excluding market prices

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 (260); 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

Download CSV

Source	BMP m³ CH₄/t VS	Temperature	Sample	Curve
Garcia et al. (2019) DOI	253	—	millet - Panicum Miliaceum L.	end point (model curve)
Lallement et al. (2023) DOI	268	38 °C · mesophilic	millet	end point (model curve)
Weighted mean	260	arithmetic mean 260, SD 10

Typical composition

Dry matter (DM): 22.4% (range 21.8–22.9)
Volatile solids (VS): 91.5 % DM (range 91–92)
Nitrogen: 0.8 % DM
C/N ratio: 53.4

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: MY-D); dry matter: 2 determinations, volatile solids: 2 determinations. C/N was calculated as the ratio of C and N content (% DM) from the same source.

Sources: Garcia et al. (2019); Lallement et al. (2023).

The composition of the raw material varies between batches — for plant calculations use an analysis of your own sample. The calculator inserts typical values automatically.

Process properties

Low confidence: the description is based on general characteristics of the substrate group, not on studies of this material. Treat it as a hint to be verified.

No published process studies

The literature we use does not describe the advantages or drawbacks of digesting this substrate beyond the BMP test results. Below are process characteristics assigned on the basis of the type of material — an editorial classification, not a study result.

seasonal: seasonal availability — requires storage
fibrous: lignocellulose, long fibres → floating layer, harder mixing, slow degradation

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

combines well
group: Animal excreta (slurry, manure, poultry litter)
Co-digestion with excreta (slurry/solid manure) is the standard arrangement for plant biomass: they supply alkalinity and microflora.^[1][2]

Dosing

Introduce gradually, starting with a small dose and monitoring the process (e.g. FOS/TAC, pH, gas composition).

Practical requirements

Ensiling makes it possible to store seasonal biomass; properly ensiled plants (in the study maize, sorghum, forage rye, triticale) can be stored for up to a year without loss of methane production (Herrmann et al. 2011).

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

Common millet (Panicum miliaceum) is an annual grass grown for grain and fodder, with a short growing period and good drought tolerance. In Poland it is grown on a small scale, but because of climate change interest in plants resistant to water shortage is growing. In the biogas context it is studied as a whole plant harvested for green forage or silage. The compilation contains several records from several studies.

Properties relevant to digestion

The green biomass of millet is similar in character to other cultivated grasses: it supplies structural carbohydrates and small amounts of protein. Digestibility depends strongly on the harvest date: before grain formation the biomass is more readily degradable, later the fibre share rises. Ensiling is possible with sufficient dry matter and sugar content. Because of a C-to-N ratio higher than in legumes, mixing with slurry or another nitrogen-rich substrate is beneficial.

How to read the spread of results

The compilation has few records: some from a study in which the test temperature is not given, some from mesophilic conditions. They differ in variety, harvest date, form of the material, sample preparation and test parameters. The year of cultivation and the course of the weather, which determines the proportion of leaves, stems and ears, also have an effect. The small number of measurements does not allow the full variability of the material to be assessed, so the result is treated as indicative.

In biogas plant practice

Millet as a whole plant can be an addition to the mix on farms on poorer soils, as a crop more drought-resistant than maize. It requires harvesting with a forage harvester, ensiling in the manner typical of grasses and quality control. In the ration it behaves like other grass silages. Cost-effectiveness depends on the green mass yield, which should be assessed locally, and on the price of competing substrates. A BMP test is recommended before introduction on a wider scale. In choosing a species for energy cropping it is good to consider not only the gas potential but also yield stability in dry years, cultivation costs and the availability of harvesting equipment, because these determine the final cost of a unit of energy from biogas.

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.