What Is The Biomass Production Of Sorghum?

Biomass production refers to the amount of biological material or biomass that is generated by a plant through photosynthesis. It is an important characteristic when evaluating crops for bioenergy use. Bioenergy crops like sorghum are grown specifically for the purpose of producing large amounts of biomass that can be converted into biofuels, biopower or bioproducts. Sorghum is considered one of the most promising bioenergy crops due to its high biomass yields, stress tolerance, and wide adaptability.

Sorghum is a C4 grass species that can efficiently convert solar energy and CO2 into high biomass yields. It requires less water and fertilizer compared to other crops like corn. Certain sorghum hybrids, called energy sorghums, are optimized for maximum biomass production. These types can produce over 30 tons of dry biomass per acre under ideal growing conditions. The high biomass yields, combined with desirable chemical composition and agronomic traits like drought tolerance, make sorghum an ideal candidate for sustainable bioenergy production.

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Sorghum Biology

Sorghum is a C4 plant that utilizes the C4 photosynthetic pathway which enables efficient conversion of sunlight into biomass. It has a tall erect stem with thick stalks and broad leaves. Sorghum goes through distinct growth stages during its lifecycle which influences biomass production.

The germination stage begins when the seed starts to take up water and the radicle emerges. Optimal soil temperature for germination is between 70-95°F. The seedling growth stage follows, where seedlings develop 3-5 leaves and a nodal root system. This stage lasts around 30 days. Next is the grand growth stage, characterized by rapid stem elongation as the plant grows taller. This lasts 40-55 days.

During the reproductive stage, the panicle emerges from the flag leaf sheath. Flowering occurs over 4-8 days, beginning at the top of the panicle. This is followed by grain filling which occurs over 30-40 days. Physiological maturity is reached when the black layer forms near the point of attachment of the kernel, stopping the flow of nutrients to the grain. At maturity, the lower leaves start to senesce while the stalk remains green.

Sorghum’s C4 photosynthesis, erect growth habit, and rapid growth rate contribute to its ability to produce substantial biomass yields.

Factors Influencing Biomass Yield

There are several key factors that influence sorghum biomass yield including climate, soil conditions, water availability, pests, and fertilizer.

Climate factors like sunlight, temperature, and rainfall impact sorghum growth and biomass production. Sorghum grows best in warm, sunny climates with adequate rainfall during the growing season. Excessive heat or drought can limit yields.

Soil conditions like texture, structure, pH, and nutrient levels also affect sorghum biomass. Sorghum grows well in fertile, well-drained soils. Heavy clay or compacted soils can restrict root growth and reduce yields. Low soil fertility may require additional fertilizer inputs.

Sufficient water availability through rainfall or irrigation is critical for sorghum growth. Moisture stress during flowering and grain fill stages can significantly reduce biomass and grain yields. Supplemental irrigation may be needed in drier areas.

Pests like insects, diseases, and weeds can damage sorghum and require monitoring and management. Key pests include stem borers, shoot flies, downy mildew, anthracnose, and striga weed. Control measures may include insecticides, fungicides, herbicides, and integrated pest management.

Fertilizer application provides essential nutrients like nitrogen, phosphorus, and potassium for sorghum growth. Fertilizer rates should be based on soil test results and yield goals. Excess fertilizer can lead to lodging and pollution risks.

Sorghum Biomass Composition

The biomass composition of sorghum varies depending on the plant parts. The main components of interest for bioenergy applications are the stalks, leaves, and grain.[1]

The stalks of sorghum contain high levels of structural carbohydrates like cellulose and hemicellulose that can be converted into biofuels. The cellulose content ranges from 32-48% while hemicelluloses range from 22-29%. [2]

The leaves of sorghum also contain cellulose (17-21%), hemicelluloses (14-16%), and lignin (9-13%). [3] The grain contains starch that can be used for ethanol production.

There is genetic variation in the biomass composition of different sorghum hybrids. Breeding sorghum hybrids with optimal cellulose and hemicellulose levels is an active area of research for improving bioenergy feedstocks.

Harvesting Sorghum Biomass

The optimal time to harvest sorghum for biomass is when the plants reach physiological maturity, which occurs after the grain has reached maximum dry weight (Bendorf 2021, https://dr.lib.iastate.edu/bitstreams/c6cac458-58e4-4a31-803c-95a28f96dfb5/download). At this stage, the moisture content of the biomass is around 70%, requiring drying before storage and processing (Yang 2023, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10573072/).

Harvesting methods for sorghum biomass depend on the desired components. For grain, combines are typically used to separate the grain from the stalks. To collect stalks and leaves, specialized harvesters like forage choppers are needed, which cut and chop the biomass into smaller pieces for collection. Self-propelled mower-conditioners can also be used to cut and crush the stalks (Bendorf 2021).

Proper moisture levels, typically 60% or below, are necessary when baling and storing biomass to prevent spoilage. Field drying requires 5-7 days before baling, or artificial drying may be used. Round or square balers are common to collect and compress the stalks and leaves into transportable bales (Yang 2023).

Processing and Storage of Sorghum Biomass

After harvesting, sorghum biomass must be properly dried and stored to maintain quality before it is utilized. Sorghum should be dried to a moisture content below 15% to prevent spoilage and maintain stable composition for conversion processes (University of Georgia Extension, 2012).

Drying methods include in-field windrowing, use of on-farm grain dryers, and commercial rotary drum dryers. Densification into bales, pellets, or briquettes can also assist in drying by reducing volume for storage and transport. Proper storage facilities are needed to protect biomass from moisture, UV light, oxygen, and microbial degradation. Common storage methods include covered outdoor stacks, indoor warehouses, gas-tight silo bags, or plastic wrapped bales (Genera, 2020).

Overall, rapid drying, densification, and storage with minimized exposure are key to preserve the composition and energy content of sorghum biomass for later use.

Biomass Conversion

Sorghum biomass can be converted into usable forms of energy through various processes:

Combustion involves burning the biomass directly to produce heat and electricity. This is done by combusting the biomass in a boiler to generate high-pressure steam that spins a turbine and generator (https://www.sciencedirect.com/science/article/abs/pii/S1364032120300654). Sorghum biomass has a heat value of 16-17 MJ/kg, making it suitable for combustion (https://biofuels.ifas.ufl.edu/biomass-sorghum.shtml).

Gasification converts the biomass into a gas mixture called syngas through partial oxidation at high temperatures. The syngas can then be used to generate electricity or produce fuels and chemicals. Sorghum’s high cellulose and hemicellulose content make it favorable for gasification (https://www.sciencedirect.com/science/article/abs/pii/S1364032120300654).

Fermentation utilizes microorganisms to anaerobically break down biomass sugars into alcohols like ethanol or butanol. Sorghum juice, grain, or bagasse can be used as feedstocks for fermentation. Sweet sorghum varieties have sucrose-rich stalks well-suited for ethanol production, yielding up to 8,000 liters of ethanol per hectare (https://www.sciencedirect.com/science/article/abs/pii/S1364032120300654).

Integrated systems can combine multiple conversion processes, like using bagasse left over from ethanol production for combustion or gasification to improve overall energy and material recovery.

Sorghum Biomass Yield

Typical dry matter yields for biomass sorghum range from 8 to 30 tons per acre, with an average of 15 tons per acre [1]. Biomass yield is dependent on several factors:

Genetics – Hybrids developed specifically for biomass production generally have higher yields than traditional sorghum varieties.

Environment – Biomass yield is heavily influenced by soil fertility, water availability, and length of growing season.
Agronomic practices – Factors like plant population density, fertilization, and pest control impact sorghum biomass yields.

With optimal genetics and management, irrigated sorghum can produce over 30 tons per acre of dry biomass [2]. However, average rainfed yields under commercial production are lower, often ranging from 8-12 tons per acre depending on seasonal rainfall.

Advantages of Sorghum Biomass

Sorghum has several advantages that make it an excellent crop for biomass production:

High yields – Sorghum can produce high biomass yields, with dry matter yields ranging from 8 to 22 tons per acre depending on the hybrid and growing conditions (https://biofuels.ifas.ufl.edu/biomass-sorghum.shtml). The high carbohydrate content makes sorghum an efficient feedstock for biofuel production.
Low inputs – Sorghum requires lower inputs compared to other crops like corn. It has lower fertilizer requirements and is drought tolerant, so irrigation costs are reduced (https://www.nrel.gov/docs/fy04osti/35378.pdf).

Drought tolerance – Sorghum is naturally drought tolerant due to its ability to produce a wax layer on the leaves to reduce water loss. It can produce reasonable yields under limited rainfall or dry conditions (https://biotechnologyforbiofuels.biomedcentral.com/articles/10.1186/s13068-022-02170-6).
Flexibility – Sorghum can be used to produce food, feed, fiber and fuel. The same crop can be diverted to different markets as needed (https://biofuels.ifas.ufl.edu/biomass-sorghum.shtml).

With its high biomass yields, low inputs, and drought tolerance, sorghum is well-suited as an energy crop for biofuel production.

Conclusion

In summary, sorghum has emerged as a promising bioenergy crop due to its high biomass yield potential, tolerance to drought and other stresses, wide adaptability, and suitability for cultivation on marginal lands. Advances in sorghum breeding and biotechnology have resulted in improved hybrids and varieties tailored for biomass production. The composition of sorghum biomass makes it amenable for conversion to various biofuels through biochemical and thermochemical processes. While grain sorghum and sweet sorghum have been more extensively researched, biomass sorghum offers significant potential as a dedicated energy crop.

Going forward, continued research and advancements in agronomic practices, breeding, and conversion processes will help further unlock the biomass production potential of sorghum. With the drive for renewable alternatives to fossil fuels, sorghum is poised to play an important role as a bioenergy feedstock into the future. However, for sorghum biomass production to reach its full potential, further work is needed to develop high-yielding hybrids adapted to diverse environments and robust, cost-effective conversion technologies optimized for sorghum biomass.