Silk forage sorghum (Sorghum species hybrid cultivar "Silk")

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Silk forage sorghum is a tall clumping grass. It can compete with crops and hybridise with cultivated sorghums.

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How does this weed affect you?

Silk forage sorghum:

invades dryland and irrigated cropping areas and competes with crops
harbours disease and insect pests of crops especially the sorghum midge Contarinia sorghicola
can be toxic to livestock
cross with crop sorghums, contaminating seed and reducing productivity.

Livestock poisoning

All sorghums can accumulate cyanogenic glycosides and have the potential to poison stock.

Where is it found?

Silk forage sorgum grows in subtropical and semi-arid climates, usually in areas with 500–700 mm of annual rainfall. In grows best in fertile heavy clay soils.

How does it spread?

By seed

Seeds can be spread:

in water
by sticking to animal wool or fur
by animals that eat the seeds
in cotaminated hay or grain.

By plant parts

Plants can grow from rhizomes which can be spread by machinery, including road graders and cultivation equipment.

What does it look like?

Silk forage sorghum is an annual grass that grows in clumps up to 4 m tall.

Leaves are:

up to 50 cm long and up to 2 cm wide
smooth
slightly-toothed along the margins

Ligules (small structures where the leaf sheath meets the blade of the leaf) are membranous and have short hairs on older plants.

Flowerheads are:

pale green when young
dark red-brown when older
branching to form a pyramid-shape
up to 30 cm long.

Seeds:

shiny dark red brown to black
oval
up to 4 mm long.
sometimes have a bristle (awn) at one end.

Stems are:

upright
up to 3.5 m tall
oval in cross section.

Silk forage sorghum has short rhizomes (underground stems)

Roots are:

fibrous, branched
up to 1 m long.

References

Parsons, W.T., & Cuthbertson, E. G. (2001). Noxious weeds of Australia. 2^nd edition. CSIRO publishing

Queensland Government. (2025). Cyanide poisoning from sorghum Business Queensland. Retrieved 30 September 2026 from: https://www.business.qld.gov.au/industries/farms-fishing-forestry/agriculture/animal/health/contamination/sorghum/cyanide

Control

Prevention

Use quality assured seed sources for grain and forage sorghums.

Slashing and mowing

Repeated slashing or mowing can prevent seed set. Do not slash plants with seedheads.

Herbicide control

Spray actively growing plants. If possible spray when plants they are seedling.

Herbicide options

WARNING - ALWAYS READ THE LABEL
Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website www.apvma.gov.au

See Using herbicides for more information.

Flupropanate 745 g/L (Various products)
Rate: 200 mL per 15 L of water
Comments: Knapsack spray when plants are actively growing and at least 30 cm tall. Spray to the point of run-off. Follow label directions as per Johnson grass.
Withholding period: Spot spray: Do NOT graze or cut for stock feed for at least 14 days. Blanket spray: Do NOT graze, or cut for stock feed for at least 4 months. If stock are grazed in treated areas after required time has passed, remove stock from treated areas and do NOT slaughter or milk for human consumption until they have been on clean feed for at least 14 days.
Herbicide group: 0 , Unknown mode of action.
Resistance risk: Moderate

Flupropanate 745 g/L (Various products)
Rate: 12 L/ha
Comments: Boom spray. Follow label directions as per Johnson grass.
Withholding period: Spot spray: Do NOT graze or cut for stock feed for at least 14 days. Blanket spray: Do NOT graze, or cut for stock feed for at least 4 months. If stock are grazed in treated areas after required time has passed, remove stock from treated areas and do NOT slaughter or milk for human consumption until they have been on clean feed for at least 14 days.
Herbicide group: 0 , Unknown mode of action.
Resistance risk: Moderate

Flupropanate 745 g/L (Various products)
Rate: 1 L per 100 L of water.
Comments: High volume spray to actively growing plants at least 30 cm tall. Spray to the point of run-off. Follow label directions as per Johnson grass.
Withholding period: Spot spray: Do NOT graze or cut for stock feed for at least 14 days. Blanket spray: Do NOT graze, or cut for stock feed for at least 4 months. If stock are grazed in treated areas after required time has passed, remove stock from treated areas and do NOT slaughter or milk for human consumption until they have been on clean feed for at least 14 days.
Herbicide group: 0 , Unknown mode of action.
Resistance risk: Moderate

Glyphosate 360 g/L (Various products)
Rate: 150 mL in 15 L of water.
Comments: Spot spray actively growing plants at the early head stage. Follow label directions as per Johnson grass.
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

Glyphosate 360 g/L (Various products)
Rate: 6.0 L per Ha
Comments: Boom spray actively growing plants at early head stage. Follow label directions as per Johnson grass.
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

Glyphosate 360 g/L (Various products)
Rate: 1.0 L per 2.0 L of water
Comments: Wiper control. Follow label directions as per Johnson grass.
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

Biosecurity duty

The content provided here is for information purposes only and is taken from the NSW Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.

For technical advice and assistance with identification please contact your local council weeds officer.

Reviewed 2026