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Key takeaway:

The flux tower saw that over summer, weeds used 2mm/day of water. This means there is a 2mm loss of water in the soil per day that weeds will use if they cannot  be sprayed in time. This is critical information with lack of rainfall, and when soil moisture is key for the following year’s sowing. (See Figure 1 below.)

 

Long-Term Trials For Drought Resilient Farming Practices

 

Water use by summer weeds quantified

 

Long-term field trials usually take years to begin producing useful findings. However after one season, the long-term trials run at the University of Melbourne’s Dookie Campus, funded by the Australian Government’s Future Drought Fund, are already producing information that is helpful for crop and livestock farmers to manage drought.

 

Learn more about the long-term trials in this announcement.

 

A feature of droughts since 1990 is that the rainfall decline has occurred during the growing season, but that there has also been an increase in summer fallow rain that falls in between cropping seasons. Grain farmers in southern Australia have made important adaptations to this emerging pattern of drought. One key management change has been strict control of summer fallow weeds that establish in response to summer storms to store the water in the soil for subsequent crops to use.

While most growers are aware of the benefits of controlling summer weeds, the size of modern cropping programs, labour and machinery constraints, and lack of safe spray conditions during summer can make it difficult to control weeds as quickly as most farmers would like.

Until now it has not been possible to estimate the cost of delayed summer weed control. However, an eddy covariance flux tower installed at the Dookie Long-Term Trial site (see photo) has been able to do this. This cutting-edge technology measures evapotranspiration over large footprints, encompassing whole paddocks and was operational in November and December 2024 when 178 mm of summer rain disrupted harvest, downgraded grain and germinated summer weeds.

Ideally, the weeds would have been controlled in mid-December, but as the delayed harvest ran into Christmas, it was 14 January 2025 before weeds were controlled with a tank mix of glyphosate and 2,4-D amine and then double-knocked with paraquat. The flux tower showed that from the time when weeds began using significant amounts of water until they were controlled, they had transpired 121mm of water, or 2 mm of water a day. Interestingly, the data also showed that weeds stopped using water within two days of the first herbicides being applied (see Figure 1).

For the first time, this enables growers to put a value on the cost of delayed summer weed control. Allowing for 2mm of water a day used by weeds as a rough rule of thumb, this translates into a potential loss of 50kg per hectare of wheat grain, assuming transpiration efficiency for grain of 25kg/ha and mm. At a wheat price of $300 per tonne, this represents a loss of $15/ha per day,  which can be offset as a return against the potential costs of controlling weeds earlier by investing in additional machinery or labour or engaging a contractor to get the job done faster.

 

Contact for further information:

 

Prof James Hunt, Professor & William Farrer Lectureship in Agronomy,
School of Agriculture, Food and Ecosystem Sciences

email: james.hunt@unimelb.edu.au

or

Prof Michael Tausz, School of Agriculture, Food and Ecosystem Sciences

email: michael.tausz@unimelb.edu.au

 

This project received funding by the Australian Government’s Future Drought Fund. Fluxtower expertise is supported through collaboration with OzFlux and the Ecosystem Processes platform of the Australian Terrestrial Ecosystem Research Network.