Sorghum (annual)


Scientific name

Sorghum spp. annual forage hybrids
Sorghum vulgare Pers. (sweet sorghum)
Sorghum spp. hybrid (sweet Sudan grass hybrids)

Synonyms

  

Family/tribe

Family: Poaceae (alt. Gramineae) subfamily: Panicoideae tribe: Andropogoneae.

Common names

There have been numerous hybrid cultivars bred by institutions and commercial seed companies in America, Australia and Africa.
These hybrids can be classified as:
Sudan x Sudan grass hybrids, e.g. Superdan, PAC8288;
Sweet sorghum x sweet sorghum hybrids;
Sweet sorghum x Sudan grass hybrids;
Sorghum vulgare (Open pollinated sweet sorghum x sweet sorghum hybrids), e.g. Early Orange, Saccaline, Sumac, Sugardrip, Tracy, White African;
Sorghum spp. hybrid (sweet sorghum x sweet sorghum hybrids), e.g. Lahoma, Piper, SS.6, Sucro, Sugargraze, Hunnigreen, Megasweet;
Sorghum spp. hybrid (Sweet sorghum x Sudan grass hybrids), e.g. Bantu, Zulu, FS-22A, Sudax, Speedfeed, Jumbo, Lush.

Morphological description

These hybrids are vigorously-growing, erect (culms 2–3 m in height) annuals with branched tillers.  Leaves 2.5–4.0 cm wide.  Inflorescences are large pyramidal panicles with secondary and tertiary branches.

Distribution

Man-made hybrids confined to locations where purposefully sown.  Widely sown on fertile soils in semi-arid northern Australia.

Uses/applications

The various types of hybrids have different agronomic features and uses.
Sweet sorghum hybrids tiller well, are tall with fine stems, flower early and have low prussic acid.
Sweet sorghum x Sudan grass hybrids are more compact and leafy with more juicy and sweet stems.  Prussic acid levels are low.  They grow rapidly but some cultivars selected for later flowering are easier to manage as they remain leafy for longer.
Sweet sorghum x sweet sorghum hybrids are tall, late flowering with high sugar levels in their stems.  Prussic acid levels are high.  More suitable as stand-over feed into autumn.

Ecology

Soil requirements

Adapted to fertile soils from clay to loam with neutral to high pH (pH 5–8.5).  Some tolerance of salinity but not waterlogging .

Moisture

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Good drought tolerance and best in semi-arid conditions with 500–800 mm rainfall.  Poor tolerance of flooding and have a greater incidence of leaf diseases in high rainfall districts.

Temperature

Summer growth;  seed planted when soil temperatures are above 15°C. 
As very fast growing annuals, they can be grown wherever the growing season is long enough – from adequate soil temperature to end of wet season.

Light

Full sunlight.

Reproductive development

Short-day response, beginning to flower 7–8 weeks after planting.  Reliable seed producers.

Defoliation

Tolerant of heavy grazing with coarse stems remaining unless grazed early.  Should be grazed heavily when it reaches 50 cm high to prevent development of coarse stems and early flowering.  Should not be grazed below 15 cm if good regrowth is expected.

Fire

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Not normally subjected to fires.

Agronomy

Establishment

Planted 2–5 cm deep into arable seedbed at 4–10 kg/ha when soil temperature exceeds about 15°C.  Large seed germinates and establishes vigorously.

Fertiliser

Should be grown only on fertile soils and so would need additional nitrogen on soils of marginal soil fertility .  N can be applied where irrigation or adequate soil moisture is available, but the plants can quickly grow past the optimum stage without careful grazing management.

Compatibility (with other species)

May be sown with summer-growing legumes, but as these generally grow more slowly than the sorghum, it is difficult to find the optimal stage or frequency of grazing for the mixture.

Companion species

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Legumes:  Lablab purpureus and cowpeas (Vigna unguiculata ).

Pests and diseases

Planting sorghums near sugar cane is generally discouraged for fear of cross transmission of disease.

Ability to spread

  

Weed potential

Can be difficult to eradicate from subsequent grain crops.

Feeding value

Nutritive value

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Nutritive value greatly depends on soil fertility .  It provides good feed only on good soils.

Palatability/acceptability

Moderately palatable and digestible when young.  The sweet sorghum hybrids keep a higher free sugar content in the stem and are more suitable as stand-over feed into winter or the dry season.

Toxicity

Leaves can be poisonous to grazing livestock due to hydrogen cyanide (prussic acid), especially in young dark-blue coloured regrowth after a dry spell.
Having supplement blocks containing sulphur available will reduce the risk of prussic acid poisoning.

Production potential

Dry matter

DM yields can be up to 30 t/ha/yr.

Animal production

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Genetics/breeding

  

Seed production

Seed does not shatter readily and can be harvested easily.  No dormancy .
Since seed of ‘Silk’ (perennial Sorghum spp. hybrid ) and that of the noxious weed S. halepense are of similar colour, care must be taken with the purity of seed crops.

Herbicide effects

Seedlings and young plants are easily eradicated with non-residual contact herbicides such as glyphosate.

Strengths

  • Easy establishment and rapid growth.
  • Very productive on fertile soils.
  • Pioneer species with other perennial grasses or legumes.

Limitations

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  • Demands high soil fertility .
  • Prussic acid poisoning.
  • Seed cannot be distinguished from that of S. halepense.

Other comments

Will quickly strip nitrogen from marginal soils making it difficult to plant following permanent pasture .  Difficult to eradicate from subsequent grain crops.

Selected references

Catalogues from commercial seed companies such as:
The Forage Book (ISBN 0 9594231 1 7). (Pacific Seeds, Toowoomba, Queensland, Australia).

Internet links

Cultivars

Cultivars

Country/date released

Details

Numerous bred commercial cultivars.      

Promising accessions

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Promising accessions

Country

Details