Key points

• Missed winter cropping opportunities – or lost crops – have increased the focus on summer cropping
• Research has shown sorghum can be planted early, but patience is required
• Soil temperatures should reach at least 12°C and be on a warming front

As spring approached this year, summer grains specialist Loretta Serafin began fielding a lot of calls. The wet conditions at sowing in 2022 meant many northern growers had missed winter cropping opportunities or lost the crops they had managed to plant with burst seed. So, summer options were top of mind. The phone calls all had a similar query – how early can we plant?

Ms Serafin, a summer grains research agronomist at the NSW Department of Primary Industries (DPI), found the questions a little ironic: “I’ve been telling growers for a while we can plant earlier to avoid heat stress, but this year many have been wanting to push even earlier, which is too risky and will likely reduce plant establishment.”

For the past four years, Ms Serafin has been a part of a team led by the University of Queensland in collaboration with the Queensland Department of Agriculture and Fisheries (DAF) and NSW DPI working on a GRDC-invested project on early sorghum planting.

Traditionally, sorghum is planted when soil temperatures reach 16°C to 18°C at sowing depth for three or four consecutive days, and when frost risks have mostly passed. However, planting earlier – at 12°C and into a rising plane of soil temperatures – can be successful and reduce the risk of heat stress during key flowering periods.

Despite the eager mood this year, her message is for patience. “Everyone should remember that early sowing is one opportunity to establish sorghum, but there is a wide planting window. I have been encouraging people to consider their risk and wait until soil temperatures warm up sufficiently. Planting into temperatures below 12°C risks a patchy establishment, which can mean a replant that no one wants to see when soil moisture profiles are so full.

We cannot really plant earlier than 12°C. There will always be a trade-off between getting the crop out of the ground and later heat, but planting too early risks the seed sitting in the ground for weeks. This increases disease exposure risks and seed rot.

The research has tested the boundaries, planting into cold soils and experiencing
some light frosts. “We’ve seen losses of around 13 per cent of the plants as a result of frost. We also commonly see cold symptoms, including purpling and leaf bleaching in the early growth stages.”

As well as patience, Ms Serafin suggests using a soil temperature probe to check conditions. “Soil temperatures can fluctuate quite wildly in the late winter and early spring period. The absolute minimum is 12°C and rising, planted on a warming front.”

Ms Serafin recently spoke at GRDC events across the region, outlining key considerations for summer cropping.

Sowing temperatures

Early sowing can be successful if a few guidelines are adhered to:

• Measure soil temperature and only plant when above 12°C and rising.
• Use only high-quality seed (for both germination and vigour).
• Ensure sufficient seedbed moisture to allow germination and emergence over about two weeks.
• Be prepared to increase planting rates or accept a lower establishment percentage.
• Select paddocks with low weed burdens.

Widening the planting window beneficially brings the flowering window forward. This avoids the peak heat and moisture stress periods of late December and early January. Earlier harvests also bring the benefit of additional time for a fallow refill, increasing the chances of double cropping back into a winter crop.

She says there is no difference in the cold tolerance of hybrids.

In-season rain

Although most will not struggle for a full starting profile this season, in-crop rain is still needed.

Planting

Ms Serafin encourages using a precision planter if possible. “A precision planter gives you more-even plant spacing and a more-even water use across the paddock. This improved uniformity at important growth stages, like at flowering, can help when looking for Heliothis and midge. At physiological maturity, it can help with desiccation timing.”

Plant below three centimetres. A shallower sowing depth means less insulation against temperature changes and a higher risk of the seedbed drying out. “Deeper sowing, below 6 to 7cm, means reduced establishment but gives more stable temperatures,” she says. “The trade-off is maybe about a 10 per cent reduction in establishment, but more-stable conditions. So the best range for planting depth is typically 4 to 5cm.”

Aim for four to six plants per square metre. “This will provide plenty of top-end yield potential with most hybrids and is more than enough plants, even if it doesn’t look like it. We estimate 80 per cent establishment in ideal situations.”

Ms Serafin suggests selecting at least two high-yielding hybrids that have desired characteristics, such as maturity or standability, to spread production risk. “Regardless of the planter, focus on seedbed moisture, sowing depth and seedbed placement.”

Row spacing will depend on target yield and water availability. “Sorghum will throw more or less tillers depending on row spacing.”

As a rule of thumb, for target yields greater than 3.5 tonnes per hectare, use solid plant row spacings. For yields between 3 and 4t/ha, use 100cm row spacing. For less than 3t/ha use 100cm solid or consider skip or wide-row configurations.

“The advantage of wide or skip-row spacing is the ability to conserve water in skip areas for flowering and grain fill as the plant roots do not generally explore this area fully before flowering. Weed control is more critical in these wide-row configurations.”

More information: GRDC Grains Research Update paper and GRDC Grains Research Update video.