Hybrids have replaced open-pollinated varieties. Choose an early maturing hybrid with good vigour and drought tolerance If sowing on very wide row spacings (double skip rows) a medium or late variety may be an option. Check with seed companies for the most suitable variety for your conditions.
Sowing time:
Seeding can begin once the soil temperature reaches 15o C at seeding depth at 9.00 am (although emergence will be quicker if the soil is warmer at 18o C).
This normally occurs between mid-September and mid-October. Soil temperature can be tested using a standard thermometer, purchased at any chemist or white goods outlet. Grain sorghum does not tolerate frost so wait until the risk of frost is over before planting.
Sowing depth:
Sow into moisture at approximately 5 to 7 cm (only deep enough for good moisture and to allow the roots to grow down ahead of the drying front). Sow deeper if the top 7 cm is dry, but remember that the deeper you go the colder the soil and the longer for the plants to emerge.
Emergence will be reduced if seeds are sown too deep. In some situations the top few centimetres may be scalped off in the seeding operation (sowing in a furrow) to reach good soil moisture levels while still only having 5 to 7 cm of soil over the seed.
Establishment levels will vary depending on the type of seeding system. Press wheels set at 2 to 4 kg/cm width will ensure good seed to soil contact without compacting the soil.
Row spacing:
Row spacings of 1 m have been used with some success in low rainfall areas.
Plant density:
Optimum plant density is 30,000 to 40,000 plants/ha (3-4 per square metre or per metre row on 1 m row spacings).
Seeds/kg and germination percentage may be branded on the side of the bag, but if not, assume 25,000 to 30,000 seeds/kg. Note that the germination percentage is normally lower in sorghum than corn and sunflower.
The seeding rate required can be calculated using the following formula:
Seeding rate = Target plant density per ha
Seeds/kg x Germination % x Expected establishment %
Example:
- Target plant density 35,000
- Seeds/kg 28,000
- Germination % 70%
- Establishment rate (tynes with press wheels) 60%
- Seeding rate =
Row spacing and the distance between plants within rows are not as critical in grain sorghum as maize and sunflower. At low plant densities or with uneven plant distribution, grain sorghum will tiller to compensate.
Low plant populations can be targeted in marginal dryland areas to minimise moisture stress at flowering (and associated reduced yield). Higher populations should be targeted if sowing with less accurate equipment such as combines and airseeders that reduce germination percentage, or in areas with high levels of stored soil moisture and/or summer growing season rainfall.
Fertiliser and nutrition:
Grain sorghum has a similar nutrient requirement to wheat. Adequate levels of nitrogen (N), phosphorus (P) and potassium (K) are important, with a 1 t/ha crop needing about 30 kg/ha N, 8 kg/ha P and 30 kg/ha K. It is sensitive to zinc deficiency and to some extent manganese and sulphur deficiencies. Trace elements should be applied three to five weeks after emergence as a foliar treatment, based on plant tissue test results, to prevent micro-nutrient deficiencies.
Grain sorghum can access nitrogen from depth, so application of nitrogen at seeding may be all that is required. Cropping rotation and soil type will impact on the amount of available deep nitrogen. Nitrogen reserves can be determined by using pre-plant deep soil tests.
Do not place more than 5 kg/ha N with the seed when sowing at 1 m row spacing. Ideally, the remaining requirement for nitrogen (20 to 50 kg) should be applied before planting. (Rain from mid-August onwards will enable the nitrogen to be washed into the moist root zone increasing availability through the growing period) or banded at seeding into moisture (below or to the side of the seed).
Banded nitrogen should be isolated from the seed to prevent toxicity. When press wheels are used on medium to heavy-textured soils, banding 5 cm below and to the side of the seed should be adequate, however this may need to be increased to 10 cm on lighter soils or if press wheels are not used.
In high yielding situations consider topdressing additional nitrogen at the last opportunity before the crop gets too tall (depending on soil moisture and likelihood of rain).
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