Irrigated wheat study yields valuable tips for northern growers
Author: Toni Somes | Date: 04 Apr 2018
Northern growers looking to irrigate wheat have been advised that the most profitable irrigation strategy depends on how much extra water is available to the crop as stored soil water, and how much rainfall the crop receives.
The finding is one of several conclusions from the Grains Research and Development Corporation investment into ‘Better Irrigated Wheat Agronomy’, a project led by CSIRO in response to the widespread failure of large areas of fully irrigated wheat in the northern region in 2008.
The five year research project assessed a range of factors to determine their impact on irrigated wheat in vertosol soils in Queensland and northern New South Wales, including the risk of lodging, applying nitrogen (N) for improved protein and yields, the use of plant growth regulators (PGRs) and the selection of lodging resistant wheat varieties.
CSIRO Project Team Leader Allan Peake said that the most important decision for irrigated wheat growers was whether the crop needed to be fully irrigated, particularly when they have several paddocks available for irrigated wheat but not enough water to fully irrigate them all.
He said deficit irrigation – a strategy where a larger area of crop is grown and less irrigation water is applied per hectare – is comparatively less likely to cause lodging and can be more profitable in regions or years where in-crop rainfall is more than 150mm, and when significant amounts of stored water are available in the soil profile before sowing. In drier conditions, they found that applying more irrigation to a smaller area was likely to be the better option.
However he warned that both deficit and fully irrigated paddocks can be susceptible to lodging, and growers should use a range of agronomic tools to reduce lodging risk including variety choice, irrigation strategy, N application strategy, plant population, and PGRs.
The two most lodging resistant varieties available and tested as part of this research for Queensland and northern NSW are LRPB Cobra and Dart, but these can still lodge under extreme conditions.
Researchers also found:
- Varieties respond differently to in-crop N application. Suntop, Wallup, Kennedy and LRPB Cobra often had higher yields when N was applied in-crop but Mitch and LRPB Lancer did not. In-crop N application was found to increase grain protein by 0.4% for most varieties and locations.
- The response to PGRs was influenced by N and irrigation strategy. When lodging was severe, PGRs gave the biggest yield response (0.6 t/ha) on well irrigated paddocks with more than 120 kg/ha of N available at sowing. Even when there was little or no lodging, PGRs improved yield by 0.32 t/ha on average in paddocks with more than 120 kg/ha of N at sowing. However, PGRs had a negative effect on yield for some varieties in an experiment that was only partially irrigated and experienced lower yields in the region of 5.5 t/ha.
- Growers may achieve improved yield by using row spacings as narrow as 19cm compared to 28 or 38cm, but the results were not consistent across varieties and locations. Achieving a yield benefit from narrow row spacing was more likely when lodging was avoided.
A study published by CSIRO in 2016 investigated whether full irrigation or deficit irrigation was more profitable for northern region growers.
Using the APSIM crop model, a range of whole-farm irrigation scenarios were investigated, with access to the same amount of irrigation water – 1300 megalitres (ML) – that was assumed to be in storage at sowing, and 1000 hectares of land were available to be irrigated.
The water was used to fully irrigate a smaller area, or partially irrigate increasingly larger areas.
A long-term climate data set was used to see if a particular strategy worked for different seasons (i.e. wet, dry or average), for three locations: Emerald, Goondiwindi and Gunnedah. A wheat price of $250 per tonne at the farm gate was assumed and two different water cost scenarios compared, where low cost water was $40/ML and expensive water was $120/ML.
The simulations were also conducted for two different amounts of stored soil water at sowing, either zero or 100mm. The average growing season rainfall was 100mm at Emerald, 174mm at Goondiwindi, and 212mm at Gunnedah.
The study used the concept of ‘risk efficiency' – the balance between risk and potential profit – to determine the best strategy, rather than using a long-term average gross margin.
Generally, it showed that in a dry, warm environment (Emerald), the most risk-efficient strategy was to apply more irrigation water to a smaller area of land.
At Gunnedah, a cooler environment with higher and more reliable winter rainfall, the most risk-efficient strategy was to deficit irrigate, spreading water over a wider area.
When water became more expensive, risk-efficency was improved by applying more water to a smaller crop area.
Dr Peake explained it could seem counter-intuitive to apply more irrigations per hectare to a smaller crop area to produce better profits when the price of water is more expensive.
“Full irrigation is more likely to be the most risk-efficient option when rainfall and stored soil water are limited and the cost of water is high, because larger areas of partially irrigated wheat have greater amounts of ‘wasted’ water through evaporation,” he said.
“Smaller areas of fully irrigated wheat are also more likely to conserve water in the soil which is then available for the next crop, and we valued this remaining stored water at the same price as irrigation water in the study. But at the end of the day the price of water was less important than the amount of in-crop rainfall and stored water at sowing, when it comes to deciding on the best irrigation strategy.”
Dr Peake said it was important for irrigated wheat growers and agronomists to read the full project results booklet, and to test new techniques and varieties on a small scale first.
“Unfortunately there are no risk-free options in farming, so it is important for growers and agronomists to familiarise themselves with all of the issues before they grow an irrigated wheat crop.”
Dr Allan Peake, CSIRO
07 4571 3212
Toni Somes, Cox Inall Communications
0427 878 387
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