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Gearing up to use pre-emergent herbicides

Pre-emergent herbicides are not without their challenges. Working out the best pre-emergent herbicide choice for a particular situation requires a thorough knowledge of how the herbicide works.

Dr Chris Preston, University of Adelaide (UA) Associate Professor—Weed Management says some of the earlier practices regarding pre-emergent herbicides need to be reconsidered in the light of greater experience under different field conditions and changes in sowing technology.

Competitive (right) v non-competitive canola — growers can take advantage of the canola varieties with greater competitive ability.

Competitive (right) v non-competitive canola — growers can take advantage of the canola varieties with greater competitive ability.

The latest recommendations from research in the high rainfall zones of southern Australia can be summed up as: sow a competitive crop early, on the first opening rain, with pre-emergent herbicide; sow the cleanest paddocks last and implement harvest weed seed control.The latest recommendations from research in the high rainfall zones of southern Australia can be summed up as: sow a competitive crop early, on the first opening rain, with pre-emergent herbicide; sow the cleanest paddocks last and implement harvest weed seed control.

“Pre-emergent herbicides on the market vary in their water solubility, ability to bind to soil components, behaviour under different soil moisture conditions and rate of degradation over time,” he says. “Unfortunately, the seasonal conditions that unfold can have a significant effect on the efficacy of any product applied. For example, some products are not well suited to higher rainfall zones because multiple weed germinations are more likely and by later in the season the herbicide has dissipated or moved too far down the soil profile to have any effect on later germinations.”

One solution to this problem is the pre-emergent application of trifluralin followed with the recently registered use of Boxer Gold applied post-emergent. This relies on rainfall to incorporate the Boxer Gold and has provided excellent control, even of trifluralin-resistant weeds.

The method that provided the highest level of control in the high rainfall zone was pre-emergent Sakura + Boxer Gold post-emergent.

A third option for the high rainfall areas is Sakura + Avadex. This is also expensive but provided a high level of control, and avoids using Boxer Gold in wheat, leaving it to be used in other crops in the rotation.

“Keep in mind that Sakura can only be used in wheat crops so it is good to avoid using Boxer Gold in both wheat and barley crops. Sakura + Avadex may be a better choice in the wheat crop rather than Sakura + Boxer Gold,” he says. “There is a very high risk of losing Boxer Gold as an effective herbicide if it is used frequently in a rotation, so it is essential to plan herbicide use across the crop rotation and use different chemicals in break crops.”

Early (left) v late sown cereals — ryegrass head count was lower in early sown crop.

Early (left) v late sown cereals — ryegrass head count was lower in early sown crop.

These combinations of herbicides aim to reduce seed head production through season-long control but rely on harvest weed seed control to manage any survivors.

“Herbicide resistance has been shown to occur rapidly if these new chemistries are used unwisely,” says Dr Preston. “They can be part of a weed management plan but must not be relied on without the implementation of supporting non-chemical tactics, including harvest weed seed control and competitive cropping.”

For pre-emergent herbicides to be effective the chemical needs to be in the right place at the right time—beginning with the right stubble management, sowing equipment and sowing depth. “The guiding principle is that the pre-emergent herbicide must be in contact with the soil to have any effect. Some products wash off stubble better than others and so stubble load and whether it is standing or laying flat will influence the efficacy of the pre-emergent,” says Dr Preston. “Large droplets and high water rates are generally required when stubble is present to ensure the herbicide reaches the soil.”

Dr Preston’s trial work clearly demonstrated that planting equipment, such as single disc seeders, which do not remove soil from on top of the crop row, causes an increased incidence of crop damage from pre-emergent herbicides. He says there is a need to make some compromise in a zero till system to ensure soil containing the herbicide was thrown to one side as the crop seed is sown. “It is also important that the planter closes the seeding slot, particularly if a product such as Boxer Gold is to be applied post-emergent,” he says.

Sowing date is also important. Initial research suggests that sowing early, while the temperatures are still warm, with a fast growing crop and pre-emergent herbicide will suppress early weed germinations and any later germinations will occur under the crop canopy and be less likely to out-compete the crop.

“What we saw with later sown crops was that ryegrass was able to grow above the crop canopy and competition from the crop did not affect ryegrass seed head production,” he says. “This suggests that sowing weedy paddocks as early as possible with a pre-emergent herbicide could be a very useful tactic in helping to drive down weed seed production, particularly when harvest weed seed control is added to the system.”

To hear more from Dr Preston on his trial work with pre-emergent herbicides in the high rainfall zones of southern Australia, watch the ‘Setting crops up for success’ webinar recording available here.

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