Early summer weed control brings greatest benefits

GRDC research has shown that every dollar invested in summer weed control generates an average return of $5, with early action enhancing the prospective benefits of control. ​

Early summer weed management can boost water use efficiency, crop establishment and yields, herbicide efficacy, pest and disease control, nutrient availability and overall grower profitability.

The GRDC’s landmark National Water Use Efficiency Initiative concluded that complete weed control, involving spraying 10 days after a significant rain event, resulted in the greatest subsequent winter crop yield across 300 millimetre to 700mm annual rainfall areas when compared with late summer weed control and no control at all.

Summer weed control will be important for growers in those parts of the southern region where ground cover protecting topsoils is adequate. I

n areas where ground cover is scant following significant rainfall deficits and failed crops, growers will need to consider the risks of eliminating any existing vegetation too early, including weeds and volunteer crop plants, which can assist with erosion prevention.

WUE Initiative researcher James Hunt, La Trobe University, is encouraging growers to manage summer weeds at the three to five leaf stage using herbicides at registered label rates, as herbicide efficacy is generally highest when summer weeds are young and actively growing.

Dr Hunt says summer weed control replicated experiments conducted during the WUE Initiative demonstrated average yield improvements in winter crop performance of 60 per cent.

“This clearly shows the benefit of controlling summer weeds for improving yields of winter crops,” he said.

“In seasons with high growing season rainfall, yield improvement was driven by nitrogen availability, in seasons with low growing season rainfall it was driven by water availability alone, and in average seasons it was driven by both nitrogen and water availability.” 

Dr Hunt said a further benefit of summer weed control was the destruction of the green bridge (weeds and volunteer cereals) that could harbour pests and diseases between seasons, potentially reducing winter crop performance.

“Additionally, as we see rainfall patterns changing it is important to conserve any out-of-season rainfall and a major part of achieving this is through summer weed control,” Dr Hunt said.

Growers are reminded to adhere to best practice when spraying summer weeds to reduce the risk of off-target spray drift and to be aware of new restrictions to the use of 2,4-D.

The Australian Pesticides and Veterinary Medicines Authority has suspended the labels of all products containing the active ingredient 2,4-D from October 4, replacing them with a permit.

Key changes for using 2,4-D under the permit include:

•    Applicators must now use at least a Very Coarse (VC) spray quality;

•    When using a boom sprayer, boom heights must be 0.5 metres (or lower) above the target canopy;

•    Downwind buffers now apply (typically less than 50 metres, subject to rate and product being applied) between application sites, downwind sensitive crops and environmentally sensitive aquatic areas.

In areas where minimising topsoil loss will be a priority in the coming months, the GRDC encourages growers to consult with their advisers about land management strategies.