Sorghum - Nutrition, irrigation and harvest

Harvesting a sorghum field

Quick links to topics on this page:

• Nutrition
• Irrigation
• Harvesting
• Grain yield

Nutrition

Soils should be tested every 3-5 years as part of a nutrient-monitoring programme. Actual fertiliser usage should now be determined using soil testing, in conjunction records of grain production and grain quality for individual paddocks. Soil test results should be interpreted by experienced accredited personnel.

Nitrogen (N)

Nitrogen fertiliser application rates should be based on your target yield, seasonal expectations and previous paddock history. Application rates will vary considerably from area to area. On the Darling Downs, higher rates are generally used than on the western Downs or in central Queensland because the country has been farmed considerably longer, the soils are capable of storing more plant-available moisture and yield expectations are generally higher. However the same principles apply in all areas - if the available nitrogen is low at planting then adequate nitrogen fertiliser must be applied to achieve your yield goal.

Low grain protein, the signal of nitrogen deficiency

The grain protein content of sorghum can be used as a reliable indicator of the nitrogen supply available for that particular sorghum crop. Table 1 summarises the relationship between grain protein and nitrogen supply.

Table 1.  Using sorghum grain protein as an indicator of season nitrogen supply.

Sorghum grain protein (@13.5% moisture)	Indicated nitrogen supply
Less than 9.0%	Acute nitrogen deficiency Grain yield would almost certainly increase with increased nitrogen supply (e.g. nitrogen fertiliser). Protein % will also increase if nitrogen supply is adequate for optimum season yield.
9.0% - 10.0%	Marginal nitrogen deficiency Grain yield may increase and protein will increase with increasing nitrogen supply.
Greater than 10.0%	Nitrogen not limiting yield this season Higher nitrogen supply may increase grain protein.  Producing higher protein by adding extra nitrogen is only economical if high protein premiums exist.

Crop nitrogen requirements

The amount of nitrogen removed from the paddock in harvested grain can be calculated as:

N removed (kgN/ha) = Yield (tonnes/ha) x Protein (%) x 1.6

As a general rule, twice this amount of nitrogen is required to grow the crop, giving the follow formula that can be used to estimate the nitrogen requirement of a sorghum crop:

N Required (kgN/ha) = Yield Goal (tonnes/ha) x Protein (%) x 3.2

The nitrogen supply required to produce a range of yield and protein levels is given in Table 2. 

Table 2. Available soil nitrogen needed for expected yield and grain protein levels.

Yield (t/ha)	Grain protein
	8%	9%	10%	11%	12%
2.0	51	58	64	70	77
2.5	64	72	80	88	96
3.0	77	86	96	106	115
3.5	90	101	112	123	134
4.0	102	115	128	141	154
4.5	115	130	144	158	173
5.0	128	144	160	176	192
6.0	154	172	192	212	230
7.0	180	202	224	246	268
8.0	204	230	256	282	308

Nitrogen supply

The crop nitrogen requirement can be supplied from the following sources:

1. N in the soil as nitrate
2. N mineralised through the growing season
3. N applied as fertiliser.

Soil nitrogen levels can be determined using soil testing. Always carry out nitrogen tests to the estimated rooting depth of the crop in question.

Mineralisation rates will depend on age of cultivation, soil organic matter status and seasonal conditions. Consult your advisor to gain a mineralisation estimate.

The crop nitrogen requirement not supplied by the soil from either soil nitrate reserves or mineralisation should be applied as fertiliser.

Where a grain legume was the previous summer crop

The contribution of previous summer grain legumes to soil N levels is very erratic - ranging from actual depletion to an increase of up to 40 kg N/ha. Growers are advised to adopt a conservative approach and treat as for other summer crops unless previous experience dictates otherwise. With good plant growth, but little removal of nitrogen in grain (i.e. with low yield), the loss of soil nitrogen under cropping will be less with a grain legume than other crops.

Nitrogen fertiliser application

Nitrogen fertiliser can be applied in a number of forms, the most common being anhydrous ammonia (82% N) and urea (46% N). The choice is usually dependent on price, availability, and ease and convenience of use.

Care must be taken when applying nitrogenous fertilisers at planting. Release of ammonia from the fertiliser can damage the germinating seedling if applied with the seed at planting. Table 3 details the safe rates for application with the sorghum seed at planting.

Table 3.  Safe rates of some nitrogen fertiliser products sown with sorghum seed at planting.

Row spacing (cm)	N kg/ha	Urea kg/ha	DAP kg/ha	MAP Starterfos kg/ha
18	25	54	130	200
25	18	39	90	138
50	9	20	45	69
75	6	13	30	46
100	4.5	10	23	35

Rates should be reduced by 50% for very sandy soil sand may be increased by 30% for heavy textured soils or where soil moisture conditions at planting are excellent.

Rates should be reduced by 50% when planting equipment with narrow slit openers is used (the fertiliser concentration is increased around the seed).

Rates may be increased by 50% when airseeders are used operating at high pressures with wide openers. Air seeders spread the fertiliser bands when operating at high pressures reducing the fertiliser concentration around the seed.

Phosphorus (P)

Summer cereal crops generally are not as responsive to phosphorus as wheat and barley. Soil levels need to be quite low (below 15 mg/kg bicarb P on the Darling Downs, below 10 mg/kg on the western Downs and central Queensland) before consistent responses to phosphorus fertiliser occur. Deficiency symptoms include stunted plants and reddening of lower stems.

Fertiliser placement in a band with the seed is important as phosphorus movement in the soil is very limited. Application rates vary from 5-10 kgP/ha depending on soil type and district.

Applying phosphate fertiliser can induce zinc deficiency either by interference with zinc uptake or by relative dilution of zinc concentration in the plant by the large increase in production caused by phosphate application. A small amount of zinc applied with the phosphate overcomes the problem.

Phosphorus deficiency is more likely to occur after a long fallow due to low numbers of vesicular arbuscular mycorrhiza (VAM) fungi in the soil. VAM are the beneficial soil fungi which help plant roots take up both phosphorus and zinc.

Zinc (Zn)

Yield responses to zinc from trial work and grower experience are common in many areas (i.e. the Darling Downs). Zinc plays a vital role in a plant's ability to use nitrogen and transform it into yield and protein.  Zinc is therefore a vital element to the plant and should not be overlooked in a balanced crop nutrition program.

Detection of zinc deficiency is not easy but response to zinc fertiliser occurs frequently on old cultivation on heavy clay soils with high soil pH levels.  Soil erosion, soil structural problems (e.g. hard pans) and root diseases can all increase the likelihood of zinc deficiency. The availability of zinc to many crops is increased by the presence of mycorrhiza in the soil. Crops grown after long fallows or other events that deplete soil mycorrhiza population will be most at risk of suffering zinc deficiency.

Critical levels for zinc

Soil pH less than 7.0 - 0.4 mg/kg;
Soil pH greater than 7.0 - 0.8 mg/kg.

On the western Downs the deficiency is usually associated with low soil zinc test (less than 0.4 mg/kg), high soil pH (above 8) and low organic carbon (less than 0.7%).

Zinc can be applied directly to the soil (zinc sulfate monohydrate), as a component of a starter fertiliser, as a foliar spray (zinc sulfate heptahydrate) or as a seed dressing.  Zinc sulfate monohydrate should be applied at least four months before planting at 10-20 kg/ha, which will provide enough zinc for 5-8 years.

Sulfur (S)

Sulfur responses are widespread on the eastern and southern Darling Downs. Deficiencies have also occurred on the Anchorfield and Haselmere soil types of the central Darling Downs and in areas of the Jimbour plain.  It is prevalent on basaltic black earth soils that have been intensively farmed for 20 years or more, particularly if they have been eroded, waterlogged or irrigated, especially where double cropping is practiced. Soil sulfur levels in the intensively farmed districts east of the range tend to be low, especially where gypsum and/or sulfur containing fertilisers have not been used regularly.

Soil test levels of less than 4 mg/kg sulfur (0 to 10 cm) is indicative of likely sulfur response. A rate of 8-10 kg/ha of sulfur is normally adequate. A deep soil test to 120 cm, may give a better indication of profile sulfur supply.

Gypsum at the rate of 200-400 kg/ha every three years is the cheapest source of sulfur.

Potassium (K)

Potassium deficiency rarely occurs in the sorghum growing areas of Queensland except in the South Burnett.  However, there is the potential for the deficiency to occur on some of the older farming soils particularly on the Darling Downs.

Due to the gradual decline in soil potassium levels with crop removal and historically low fertiliser application rates, some situations (particularly red soils) require K fertiliser applications.  However, crops also vary in their response to improved soil K levels. Generally winter cereal responses have been low to moderate unless gross deficiencies occur. Yields of rain grown cereals like corn and sorghum are less likely to respond to K applications than yields of grain legumes (soybeans and navybeans) and peanuts under conditions of marginal soil K status.

Potassium fertilisers cannot be placed in direct contact with seed at rates required. Fertilisers should be applied by side banding at planting, combine drill preplant in fallow or broadcast and cultivated in fallow or prior to preceding crop.

Summer cereal crops generally are not as responsive to phosphorus as wheat and barley. Soil levels need to be quite low (below 15 mg/kg bicarb P on the Darling Downs, below 10 mg/kg on the western Downs and central Queensland) before consistent responses to phosphorus fertiliser occur. Deficiency symptoms include stunted plants and reddening of lower stems.

Fertiliser placement in a band with the seed is important as phosphorus movement in the soil is very limited. Application rates vary from 5-10 kgP/ha depending on soil type and district.

Applying phosphate fertiliser can induce zinc deficiency either by interference with zinc uptake or by relative dilution of zinc concentration in the plant by the large increase in production caused by phosphate application. A small amount of zinc applied with the phosphate overcomes the problem.

Phosphorus deficiency is more likely to occur after a long fallow due to low numbers of VAM fungi in the soil. VAM are the beneficial soil fungi which help plant roots take up both phosphorus and zinc.

Zinc (Zn)

Yield responses to zinc from trial work and grower experience are common in many areas (i.e. the Darling Downs). Zinc plays a vital role in a plant's ability to use nitrogen and transform it into yield and protein. Zinc is therefore a vital element to the plant and should not be overlooked in a balanced crop nutrition program.

Detection of zinc deficiency is not easy but response to zinc fertiliser occurs frequently on old cultivation on heavy clay soils with high soil pH levels. Soil erosion, soil structural problems (e.g. hard pans) and root diseases can all increase the likelihood of zinc deficiency. The availability of zinc to many crops is increased by the presence of mycorrhiza in the soil. Crops grown after long fallows or other events that deplete soil mycorrhiza population will be most at risk of suffering zinc deficiency.

Critical levels for zinc

Soil pH less than 7.0 - 0.4 mg/kg;
Soil pH greater than 7.0 - 0.8 mg/kg.

On the western Downs the deficiency is usually associated with low soil zinc test (less than 0.4 mg/kg), high soil pH (above 8) and low organic carbon (less than 0.7%).

Zinc can be applied directly to the soil (zinc sulfate monohydrate), as a component of a starter fertiliser, as a foliar spray (zinc sulfate heptahydrate) or as a seed dressing.  Zinc sulfate monohydrate should be applied at least four months before planting at 10-20 kg/ha, which will provide enough zinc for 5-8 years.

Irrigation

Total water requirement for grain sorghum is 6-7 ML, with average irrigation requirement of 4 ML per hectare.

Harvesting

The crop requires a warm, summer growing period of 4-5 months. As a general guide, medium to medium-quick varieties will flower in 60-65 days when planted in October, and 50-55 days when planted in late December.

If drying facilities are available, harvesting can commence at 25% grain moisture followed by drying to 13-14% moisture. Grain moisture: 13.5% delivered into storage. For long-term storage, aim for moisture content of 12%. Heads harvested at various stages of maturity can complicate harvesting and storage.

Header settings

Drum speed: 750-900 rpm for conventional header and 550-650 rpm for rotary header.

• new threshing bars, use slower drum speed
• old threshing bars, use standard speed
• use sorghum extension fingers.

Pre-harvest spraying with glyphosate

Grain sorghum is a perennial plant capable of continued growth beyond physiological maturity of its initial grain crop. Preharvest treatment with glyphosate is a useful harvest aid, terminating crop growth by killing the development of immature tillers, desiccating green foliage and accelerating the natural dry-down of mature grain. It improves timeliness of harvest and prevents unnecessary plant growth and wasting of accumulated soil moisture.

Apply glyphosate when the crop is physiologically mature at the 'dough' stage - approximately 25-30% grain moisture, when a small black layer appears at the base of the seed. Plants will need to have sufficient unstressed green leaf at the top of the canopy to absorb glyphosate. Treatments may be applied by ground rig with suitable clearance or by aircraft, utilising a minimum spray volume of 20 L/ha.

Harvest should not be delayed after glyphosate application as dead stalks are more likely to lodge especially in hybrids with only fair to moderate standability. Moisture stress during grain fill or strong wind after treatment can aggravate the problem.

If preharvest spraying is to become a regular management practice, choose grain sorghum hybrids with a four or five star lodging rating. Also, sorghum hybrids that produce a set number of tillers of uniform maturity (determinant) will respond better to treatment than those with tillers of varying maturity (indeterminant). Application rates are described on registered glyphosate product labels.

Ergot in the crop?

If ergot is present at harvest, prepare in advance for delays due to clogging of machinery by honeydew on the heads. Be prepared to sacrifice some lighter grain in order to send as much ergot as possible out the back of the header - ergots are lighter than grain. Ergots are toxic to livestock, honeydew is not. Queensland and New South Wales grain receival depots allow for 0.3% ergot content by weight.

To estimate levels of ergot contamination a grain sample, spread half a cupful (about 100 g) of grain on to a light coloured background and separate the different colours and shapes. After removing sound grains, more than 20 ergots may constitute a problem.

The following laboratories accept grain for ergot contamination estimation (costs vary from $20 to $40 per sample):

SGS - Agritech Laboratory Services
214 McDougall Street
Toowoomba
Ph: +61 7 4633 0599
Fax: +61 7 4633 0711

Queensland Seed Testing Laboratory
c/- University of Queensland - Gatton
Ph: +61 7 5460 1487
Fax: +61 7 5460 1486

Seed Testing Laboratories of Australia Pty Ltd
Mansfield Qld
Ph: +61 7 3849 2744
Fax: +61 7 3849 2704

Grain sorghum yield

• Dryland yield: 2-6 t/ha.
• Irrigated yield: 7-10 t/ha.

With good soils and management, long-term average yields above 2.5 t/ha could be expected for dryland cropping.

Data provided by the APSIMSORG computer program shows median yield of 4.6t/ha can be achieved for sorghum sown between September to January (yield ranges 2 - 6 t/ha). This relates to a soil of 245 mm available water holding capacity full at planting. The median yield reduces by half if the soil profile is only half full.