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Dry sow clean paddocks first

More and more growers are dry seeding wheat to avoid terminal drought at the end of the season. Research undertaken by CSIRO and WANTFA has shown that this can result in significant yield gains and larger whole farm grain production albeit with a small increase in frost risk.

However, WANTFA executive director Dr David Minkey said that there is a greater risk of weeds germinating alongside the crop. “Unless the weed seed bank is very low, the heavy reliance on in-crop selective herbicides can quickly lead to herbicide resistance,” he said.

The breakdown patterns of pre-emergent herbicides under a variety of seasonal conditions can affect the efficacy of early and staggered germinations of annual ryegrass. WANTFA executive director Dr David Minkey recommends dry sowing the cleanest paddocks first to minimise the risk of poor weed control results.

The breakdown patterns of pre-emergent herbicides under a variety of seasonal conditions can affect the efficacy of early and staggered germinations of annual ryegrass. WANTFA executive director Dr David Minkey recommends dry sowing the cleanest paddocks first to minimise the risk of poor weed control results.

With this risk in mind WANTFA has been investigating the behaviour of pre-emergent herbicides under early dry sowing conditions to determine the rate of breakdown under various seasonal conditions.

“While the first step is to sow the cleanest paddocks, the next important strategy is to lay down pre-emergent herbicides to take care of any early germinating weeds,” he said. “We need to determine the rate of decay of the pre-emergent herbicides so growers will have a good idea whether the pre-emergent herbicide will still be effective when the rain comes, and can take action early if necessary.”

“Our research is focusing on overlaying emergence patterns for annual ryegrass and the efficacy of different pre-emergent products in different seasonal scenarios,” said Dr Minkey. “When the pre-emergent herbicides are applied to dry soil there is usually very little decay of the product. The biggest risk of decay is in years where a small rain event occurs that is enough to activate the herbicide but is insufficient to initiate crop germination. Under these conditions the pre-emergent product will have reduced efficacy when good rains arrive.”

“Some annual ryegrass seeds can remain dormant through April and May, creating a situation where the early germinating ryegrass is controlled but later, staggered flushes occur after all the pre-emergent herbicide has decayed. In these situations the need for effective harvest weed seed control is important,” he said.

With the 2016 season being off to a hot and wet start in Western Australia Dr Minkey expects ryegrass dormancy to be broken down early, suggesting that the main flush of ryegrass germination could be early in the season when the pre-emergent herbicide is active if cool conditions are met.

“Where several light rainfall events occur after dry seeding the rate of pre-emergent herbicide breakdown is more rapid. If these conditions are forecast, applying a higher registered dose and mixing compatible products can extend the efficacy of the herbicide,” he says. “If the initial application decays before the weeds germinate it may be possible to apply early post-emergent residual product such as Boxer Gold, keeping in mind that the result may not be fully effective and the operation will be expensive.”

Pictured with Craig White, Bayer Crop Science Technical Advisor and Leader of Integrated Weed Management, Dr David Minkey is determining the decay curves for a range of pre-emergent herbicides under different season conditions.

Pictured with Craig White, Bayer Crop Science Technical Advisor and Leader of Integrated Weed Management, Dr David Minkey is determining the decay curves for a range of pre-emergent herbicides under different season conditions.

The potential for herbicide resistance is high if dry sowing is not managed well, and many supporting strategies must be in place to eliminate survivors. “Pre-emergent herbicides must be supported by other weed control practices,” says Dr Minkey.

“Clean paddocks are a must,” he says. “Then consider planting crops with early vigour or crops with in-crop herbicide options, such as RR and TT canola, include harvest weed seed control and invest in summer weed control to preserve moisture and nitrogen to support early crop growth.”

The WeedSmart 10 point plan provides information on a diverse range of crop management practices that can be used to eliminate weed survivors throughout the season.

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