We are excited to introduce our new GRDC funded project titled “Optimising Weed Management Efficiency: Exploring Residual-Based Herbicides for Summer Fallow.” This project is located at two sites in Wentworth, NSW.

The project will explore the use of residual-based herbicides, such as Overwatch® Herbicide, applied as a pre-emergent to aid with summer weed management. This innovative approach aims to provide farmers with effective tools to control weeds during the challenging summer months and the growing season, ultimately improving crop yields and sustainability.

Reflecting on a Long Dry Summer and Very Dry Autumn

The SA/VIC/NSW Mallee region has experienced an exceptionally dry summer and autumn, receiving very minimal rainfall since January’s storm activity. This prolonged dry spell has significantly impacted summer weed management plans, prompting farmers and agronomists to reassess their strategies.

The spring of 2023 saw average to below-average rainfall in many areas. However, the brief rain in January, combined with generally mild to warm temperatures, created short-lived ideal conditions for weed germination, including new weeds and mature ones such as fleabane, which had germinated in late winter/early spring.

Strategic Approach to Weed Management

Given the challenging conditions, effective weed management requires a strategic and adaptable approach. It is crucial to balance the benefits and drawbacks of different weed control methods to optimise outcomes and ensure sustainability.


  • Soil Moisture Conservation: Effective weed control helps conserve soil moisture for the upcoming planting season.
  • Weed Seed Control: Reducing the weed seed bank can minimise weed pressure in the future.
  • Effective Double Knock Strategy: Using a combination of herbicides to increase efficacy and manage resistance.
  • Aiding Seeding Season: Minimises challenges with machinery by controlling large, cumbersome weeds like paddy melons.


  • Soil Erosion Risk: Disturbed soil can lead to erosion, especially in low rainfall areas.
  • Expense: Costs associated with chemicals, equipment, and labour can be significant.
  • Herbicide Resistance: Risk of spraying stressed or mature weeds ineffectively, contributing to resistance.
  • Impact on Subsequent Crops: Potential residual effects of herbicides on future crops and non-target damage, especially if temperature inversions are present.

Herbicide Behaviour and Breakdown

Understanding the behaviour and breakdown of herbicides over the summer is crucial for effective weed management. In dry conditions, herbicides may persist longer on the soil surface, potentially affecting subsequent crops. Factors to consider include:

  • Degradation Rates: Herbicides break down at different rates depending on temperature, moisture, and microbial activity. Dry conditions can slow down this process.
  • Volatility: Some herbicides are prone to volatilisation, especially in hot and dry conditions, which can reduce their efficacy and increase the risk of non-target damage.
  • Soil Binding: In dry soils, certain herbicides may bind more tightly to soil particles, reducing their availability for weed control.

Challenges with Spraying Mature, Stressed, and Dust-Covered Weeds

Spraying mature, stressed, and dust-covered weeds presents several challenges:

  • Reduced Efficacy: Stressed weeds may not absorb herbicides effectively, reducing the overall efficacy of the treatment.
  • Dust Interference: Dust on weed surfaces can act as a barrier, preventing herbicides from making proper contact with the plants.
  • Herbicide Resistance: Ineffective spraying can contribute to the development of herbicide-resistant weed populations, complicating future management efforts.

Integrated Weed Management

Effective weed management is a complex process involving various strategies. The principles of integrated weed management (IWM) are increasingly being adopted, emphasising a holistic and sustainable approach. IWM incorporates multiple tactics to manage weeds effectively while ensuring the long-term productivity and sustainability of agricultural systems.

Further Learning

For more insights into effective pre-emergent weed control, consider watching this recent webinar facilitated by WeedSmart:

Achieve effective pre-em weed control while protecting the crop at seeding


Mallee Sustainable Farming (MSF) plays a vital role in connecting research, extension, and farmers. MSF promotes economically viable, socially responsible, and environmentally sustainable agriculture. By leveraging MSF’s expertise and support, farmers can work towards a more prosperous and resilient future in agriculture.

More about the Wentworth project

For additional information about the GRDC project focusing on summer fallow weed management strategies

Click Here

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