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Water-nitrogen interactions could lessen late-season weather risks

To improve the understanding of nitrogen and water use efficiency of canola and the implications for the critical growth period of canola, a team of NSW Department of Primary Industries researchers are using semi-controlled environments – rain-out shelters and drip irrigation.
Photo: NSW DPI

Sowing decisions for canola need to take into account the risks of abiotic stresses such as drought, heat and frost later in the season. To better inform these decisions, attention is being focused on how water and nitrogen interactions affect canola development, particularly through the critical reproductive growth periods.

Many canola growers are locked into crop variety choice and therefore recommended sowing period well before sowing. Consequently, decisions on whether to continue with the planned canola variety or change to a less-risky crop option such as barley are based on summer rainfall and estimated plant-available water.

The risks associated with sowing canola into variable soil water availability scenarios and the implications of nitrogen management in these situations are not well understood.

Previous research supported by GRDC, CSIRO and the New South Wales Department of Primary Industries (DPI) in the ‘Optimised Canola Profitability’ project has shown that matching cultivar phenology with sowing date to allow varieties to flower within their optimal flowering period, to minimise the risk of exposure to stress events (frost, heat and drought), was a critical factor to maximise yield potential.

However, for the longer-season hybrid canola varieties with vigorous deep rooting ability, there is a perception that increased soil water extraction earlier in the season may leave these cultivars susceptible to yield loss during dry springs, when starting subsoil moisture is low.

This assumption leads to growers taking a conservative nitrogen management approach aiming to limit early biomass accumulation and soil water extraction. This results, potentially, in more soil water being available later in the season to safeguard grain fill.

Balancing nitrogen and water use

This project aims to examine the available soil water and nitrogen interactions to identify the management decisions most conducive to canola profitability in Australia’s variable climate.

Under the Grains Agronomy & Pathology Partnership, which has co-investment from NSW DPI and GRDC, this project is utilising rain-out shelters combined with in-crop drip irrigation to implement multiple in-crop soil water availability. This is being done at two contrasting locations – the NSW DPI Wagga Wagga Agricultural Research Institute, which is in a medium-rainfall environment, and the NSW DPI Yanco Agricultural Institute, which is in a low-rainfall environment.

All treatments started with a full soil water profile from good summer rainfall at both locations. Three water profiles were set up, one which remained full, one which was topped up at flowering and one which was not topped up at all. The soil water treatments are overlaid with a range of nitrogen rates and timings to assess the interaction between these two management factors. One mid-season hybrid cultivar was sown in April to flower within the optimal flowering time at both locations. The results from this experiment, combined with the findings from the ‘Optimised Canola Profitability’ project, will provide valuable insight into the response of canola to nitrogen and soil water through the critical growth period (100 to 500 degree days after the start of flowering). The effect of different nitrogen treatments on biomass accumulation and subsequent conversion into grain yield will be determined across a range of soil water availabilities.

Visual assessment throughout the season indicated the flowering period was hastened by up to two weeks in plots with a depleted soil water profile compared to well-watered plots.

With increasing fertiliser prices these results will guide growers to better manage their in-season nitrogen management based on profile water and seasonal conditions. Refining nitrogen management strategies will also likely increase confidence to grow canola in low-rainfall regions and provide indirect benefits of a disease and weed break for the cereal dominant rotations of these environments.

More information: Danielle Malcolm, 0429 171 337,

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