Summer Forage Crops -
Sorghum

Forage Sorghums

Main Types

Sudan x Sudan grass hybrids

• Suitable for repeated, heavy grazing with excellent recovery between grazing
• Feed quality is often higher than the Sudan grass x grain sorghum hybrids
• Suited to hay making because it tillers heavily and has finer stems than other sorghums
• Lower prussic acid levels than other forage sorghums

Grain sorghum x Sudan grass hybrids

• Suitable for repeated, heavy grazing with good recovery between grazing
• Potential for a high dry matter yield
• Best grazed at around one metre in height
• Early maturing types require strict grazing management in order to maximise both their fodder yield and feed quality

Sweet sorghum x Sweet sorghum hybrids

• Slow recovery from grazing (poor regrowth)
• Thick stems with high sugar content
• Mainly used for standover feed into autumn-winter, or silage
• Preferred planting time December to January

Sweet sorghum x Sudan grass hybrids

• Similar to sweet sorghums with a high sugar content, but a faster regrowth

Grain sorghum x Grain sorghum hybrids

• Limited recovery from grazing
• High forage yield combined with high grain content
• Developed specifically for the silage industry

Perennial forage sorghums

Silk sorghum is a short lived perennial more suited to short term pasture rotations of 2-3 years duration.

Maturity ratings - Forage sorghums

These maturity ratings are based on the time taken to reach the flowering stage in an ungrazed situation:

Maturity ratings 

Quick - 50 to 75 days
Medium - 75 to 100 days
Medium to Late - 100 to 125 days
Late - 125 to 150 days

This is intended to provide a guide to the relative maturity of the forage sorghum varieties and serves to identify those early maturing varieties which require careful grazing management in order to prolong length of growing season and maximise fodder yields.

Planting information - Forage sorghums

Sowing time
The main planting time for forage sorghum is early October through to late January. Soil temperatures should be 15° to 16° C and rising for quick, even germination.

Row spacing
25 to 50 cm

Opinions vary considerably as to preferred row spacings. Narrow rows will produce finer stems with a higher leaf to stem ratio, and are more suited to hay production. Under favourable conditions fodder yields may also be higher with narrower rows. Wide rows may provide easier access to grazing animals, and less wastage from trampling.

Seed placement depth
50 to 75 mm into moisture

Sowing rate

Recommended sowing rates (kg/ha)

Forage sorghum type	Dryland		Irrigated
	Marginal	Favourable	Grazing	Hay	Silage
Sudan grass x Sudan grass	2-4	4-8	10-15	15-20	15-20
Grain sorghum x Sudan grass	3-5	5-10	10-20	20-30	10-20
Sweet sorghum x Sweet sorghum	3-5	5-10	10-20	-	15-20
Grain sorghum x Grain sorghum	2-4	4-8	-	-	6-8

Weed control - Forage sorghums

Similar to weed control in grain sorghum.

Crop nutrition - Forage sorghums

A high level of nitrogen fertility is required for forage sorghum production.

As a guide:
Dryland 80 to 100 kg N/ha preplant
Irrigated 100 kg N/ha preplant + 50 kg N/ha after grazing.

These rates may need to be adjusted upwards in a double crop situation, or down if the sorghum is preceded by a long fallow. Heavier rates may be justified when growing irrigated crops where main requirement is for hay or silage with a relatively higher protein content.

Maximum rates of nitrogen fertiliser that can be sown with forage sorghum seed. 

Row spacing (cm)	Max. N in seed furrow  (kg N/ha)	Maximum rate of product (kg/ha)
		Urea	CK 700	CK 600S	MAP/Starter 12
18	25	54	75	125	200
36	13	27	35	62	100
53	8	18	25	40	66
70	6	13	18	30	50
90	5	11	15	25	40

Note:

• Rates should be reduced by 50% for very sandy soils.
• Rates may be increased by up to 30% for heavy clay soils and where soil moisture levels at planting are high.
• Rates should be reduced by 50% when planting with narrow slit openers (spear points) which concentrate the fertiliser around the seed.
• Rates may be increased by 50% when air seeders are operated at high pressures with wide openers (30 to 40 cm sweeps).

Phosphorus and zinc
Similar to that used for grain sorghum - see Sorghum - Fertiliser.

Grazing management - Forage Sorghums

Forage sorghums are ideally grazed when they are around 80 to 100 cm in height. Any younger than this and they may contain toxic levels of prussic acid. Once above 100 cm in height, the protein level and feed value declines rapidly.

For best regrowth, do not graze lower than 15 cm or recovery will be slow. Rotational grazing is preferred for better utilisation or the forage.

Ergot

Grazing crops need to be carefully managed to avoid problems with the development of the sorghum ergot fungus in flowering crops.

Forage sorghums should be grazed or cut before flowering in order to prevent the production of fungus spores. This will minimise the risk of infecting adjoining grain crops, and will avoid toxicity problems in grazing animals. More information on ergot and feed thresholds here.

Carrying capacity

These vary considerably. The following rates are provided as an approximate guide for planting purposes:
Dryland 2-4 steers/ ha
Irrigated 4-7 steers/ha.

As an example, plant 0.4 ha of one of the Sudan grass crosses for every five head during October. Do the same on the next rain, and then sow 0.4 ha of one of the sweet sorghum crosses for every three head during January.

Weight gains

Liveweight gains of cattle on forage sorghum vary with the age and weight of cattle, the condition of the stock, and the stage of growth and type of sorghum. Generally, liveweight gains on forage sorghum vary between 0.5 to 1 kg per head per day.

Forage sorghums often contain very low levels of sulphur with this deficiency being further aggravated by high HCN levels in the sorghum.

Use of sulphur blocks (10 to 12% S) is highly recommended when grazing forage sorghums. These can substantially improve weight gains, and also help reduce the risk of prussic acid poisoning.

Hay production - Forage sorghums

Forage sorghums make excellent quality hay provided their stems are not thick and woody. Cut when the crop reaches 90 to 100 cm. If growing the crop specifically for hay production, select fine stemmed varieties and increase the seeding rate.

Prussic acid poisoning

Forage sorghums can release the toxic compound hydrogen cyanide (HCN) when leaf material is consumed by the animal. This compound is a derivative of cyanide and is responsible for the conditions commonly referred to as prussic acid poisoning.

HCN levels are most potent in:

• young plants under 6 weeks old
• young regrowth
• plants stressed from lack of moisture, cold weather, or nutrient deficiency.

While all sorghums contain some HCN, levels vary significantly between different varieties. Grain sorghums and sweet sorghums often tend to have higher HCN levels than other varieties.

Common varieties and relative HCN content are shown in the following table.

Relative HCN content 

Variety	Prussic acid risk
Super Sudax Sudan Speedfeed Zulu Cow Chow Jumbo Sugardrip Sugargraze	Very low High

Strict grazing management during the early growth stage is required with varieties which may contain relatively high levels of HCN

Prussic acid poisoning - chronic

In most cases the economic impact of high HCN levels is depressed livestock weight gains or milk production. These sub-lethal effects tend to be masked by seasonal conditions and often go undetected.

Sulfur supplements can help minimise the problems associated with low weight gains in forage sorghum.

Prussic acid poisoning - acute

Although stock losses do occur at times, these often tend to be overstated. The numbers lost are usually very low, especially considering the area of forage sorghum planted each year.

Deaths from prussic acid poisoning usually result from grazing hungry stock on stressed crops. This situation could be avoided by feeding hay to take the edge off their appetite to prevent stock eating a large quantity of forage sorghum in a short time. Monitor the situation closely.

Symptoms of acute poisoning

Stock should be closely watched when first introduced to sorghum suspected of being poisonous. Watch stock continuously for the first hour and then intermittently over the next few days. Affected stock often die within minutes of eating a lethal dose of forage.

Poisoned animals that are found alive can be saved if promptly treated.

Symptoms include:

• rapid heavy breathing
• frothing at the mouth
• muscular twitching/convulsions
• staggering
• coma

Treatment of poisoning

Under field conditions, oral drench treatments are usually the most practical. Treatment consists of:

• Cattle 55 g hypo* in 500 mL water

• Sheep 15 g hypo* in 500 mL water

* Hypo refers to photographic sodium thiosulphate

These rates can also be injected directly into the rumen (through the top left flank) using a syringe.

Intravenous and subcutaneous injection techniques can also be used but are best administered by qualified vet.

HCN levels in hay, silage

Although some reduction in HCN potential occurs during hay making, it does not necessarily render the sorghum safe for livestock.

Sorghum silage is much safer than hay, as the acid fermentation process normally releases any HCN present.

Pulpy kidney

Vaccination of cattle for pulpy kidney with '5 in 1' is recommended for stock placed on lush forage crops.

Nitrate poisoning

Another problem that can be encountered when grazing sorghum and millets is nitrate poisoning which can be fatal.

Treatment: Mix up 4 grams of methylene blue in 100 mL of distilled water/or saline solution. Inject subcutaneously a the rate of 10 mL/45 kg body weight

Grazing grain sorghum stubble

Grain sorghum stubble is a useful roughage, particularly during the autumn/winter period. Its value as a feed, however, needs to be weighed up against possible disadvantages of feeding it off, including:

• delayed ground preparation for the next crop
• further depletion of soil moisture and nitrogen levels if existing plants regrow
• soil compaction from stock
• may forgo the opportunity to spray out the sorghum with glyphosate

Liveweight gains average around 0.4 kg/head/day, but can vary widely between 0.2 to 0.8 kg. Stubble with some grain residue initially provide weight gains up to 0.8 kg/day, while palatability and feed value drop right away after heavy frosting.

Grazing young regrowth, particularly after rain, should be avoided. Refer to section on prussic acid poisoning above.

Publications

Forage sorghum and millets:

· The Forage Book, Stuart, P.N., (Ed) (1993), Pacific Seeds.

Forage legumes:

• Koala lablab, Agnote NSW Agriculture/DPI, DPI/132, 1995.
• Darling Downs & Moreton Summer Crop Management Notes, 2000-2001, chapters on Cowpea, Grain lablab, Soybean (this publication).
• Summer Legume Forage Crops: Cowpeas, Lablab, Soybeans, NSW Agriculture, Agfact P4.2.16, 1989.

Acknowledgment

This DPI Note is based on the corresponding chapter from the Darling Downs Summer Crop Management Notes book, a publication of DPI Farming Systems.

More information

For more information on this topic, please see the following document:

• Summer Forage Crops - Millets and Legume

Further information

• DPI Business Information Centre 8.00 a.m. to 6.00 p.m. weekdays: Phone 13 25 23 (Queensland residents); non-Queensland residents phone (07) 3404 6999
• DPI's website at www.qld.gov.au

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Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

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File No: fs0263a
Replaces: fs98263
Last updated 24 December 2003