Managing weeds in dry seasons can be challenging for farmers, especially with a late season start. At our recent ‘Navigating the dry start ‘webinar, we were joined by Dr Chris Preston, Professor of Weed Management at Adelaide University who provided some key strategies and insights for effective weed management under these conditions.

Understanding the Season’s Challenges

Weed management becomes a crucial task in a dry season start. The dry surface conditions lead to weeds germinating closer together, making a well-planned herbicide strategy essential. When the weeds come up in a less extended germination period, they tend to emerge closer together. This can be advantageous if the pre-emergent herbicide strategy is well-executed, allowing these herbicides to perform most of the heavy lifting.

Pre-emergent Herbicide Strategies

Pre-emergent herbicides are vital in managing weed growth during dry seasons. If applied correctly, they can significantly reduce the weed burden.

• Ryegrass: A proper pre-emergent herbicide strategy can prevent major issues with ryegrass. However, pre-emergent herbicides must be carefully chosen and correctly applied to be effective.
• Brome and Barley grass: These weeds might pose more challenges. The strategy for brome, particularly, involves using Clearfield® crops with herbicides like Intervix®, though resistance can be an issue. For barley grass, there are more effective pre-emergent options, but monitoring and timely application are critical.

If the pre-emergent herbicide hasn’t effectively controlled these weeds, in-crop control becomes more challenging due to staggered emergence and variations in weed growth stages. Crop competition will also be reduced with a dry, late season break.

Post-emergent Herbicide Application

Timing is critical for post-emergent herbicides to ensure crop safety while managing weeds effectively. Even if crops are at varying growth stages, it might be beneficial to delay application slightly to allow the crops to catch up, especially legumes. This ensures that herbicides are applied at the safest time for the crop.

Monitoring and Prioritizing

Continuous monitoring of weed growth and crop stages is essential.

• Prioritizing Paddocks: Prioritize paddocks based on weed burden and crop stages. This helps in organizing and timing herbicide applications more effectively.
• Knowing Weed and Crop Stages: Understanding the growth stages of crops and weeds in various soil types within paddocks is crucial. This involves regular field inspections and sometimes even digging to check for weed germination below the surface.

Herbicide Resistance

Herbicide resistance is a growing concern. Understanding the resistance levels of different herbicides can aid in planning effective weed management strategies. As part of the GRDC RiskWi$e project, 9 paddocks in the Millewa had ryegrass tested for herbicide resistance. The results have been summarised below:

• Clethodim Resistance: Tests show no resistance to clethodim at 500 ml, which allows for flexible application timings and reliable weed control even at later stages.
• Intervix® Resistance: High resistance levels to Intervix® in ryegrass mean that alternative control methods must be used in Clearfield crops. This underscores the importance of a robust pre-emergent strategy.
• Glyphosate Resistance: There are significant levels of resistance to glyphosate, even at high concentrations. This necessitates alternative strategies such as using Paraquat for crop topping in pulse phases and considering early sowing with effective pre-emergent herbicides.

Future Planning

Planning for future seasons is crucial, especially if current strategies are not fully effective.

• Pre-emergent Herbicide Strategy: A good pre-emergent herbicide strategy is essential for managing resistant ryegrass in Clearfield crops. Products like Sakura combined with Treflan or Avadex can be effective, especially in dry sowing situations.
• Crop Sequences: Switching from legumes to cereals may miss some grass control opportunities, necessitating careful planning for future crop sequences to manage weed populations effectively. Clearfield barley and crop topping can help manage grass weeds even when the crop sequence changes.
• Long-term Control: Two consecutive years of good weed control are necessary for long-term management, especially for weeds like brome and ryegrass. Barley grass might need only one good year followed by an acceptable control year.

In Summary

Effective weed management in dry seasons requires a combination of well-planned pre-emergent herbicide strategies, timely post-emergent applications, and continuous monitoring of weed growth and crop stages. Understanding herbicide resistance levels and planning for future seasons are also crucial components. By staying informed and adapting strategies as needed, farmers can better manage their weed burdens and protect their crops.

Stay tuned for more updates and insights on weed management and other farming challenges. Join our events and keep in touch with experts to stay ahead in your farming practices.

The ‘Navigating the dry start’ webinar was brought to you by the Mallee Catchment Management Authority and Mallee Sustainable Farming with funding from the Australian Government’s Future Drought Fund.

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