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Stop and think about any prior soil works before you spray for weeds

After addressing soil constraints with a range of tactics, herbicides can be more risky to your crop and may require careful planning.
Photo: GRDC

If you have recently undertaken soil renovation, such as mouldboarding, spading or deep ripping, you may need to rethink your herbicide strategy.

Reorganising the soil surface can change how pre-emergent herbicides behave in the soil.

Less organic matter

Soil amelioration, particularly inversion, reduces the organic matter (OM) content of the topsoil.

OM intercepts and binds a proportion of herbicide applied to the soil.

By having less OM for the herbicides to bind to, the herbicides are 'hotter' (more biologically available).

Less binding also means the herbicide is more mobile, which can increase the risk of herbicide moving from the inter-row into the furrow and coming into contact with your crop.

Softer soil and herbicide movement

Because ameliorated soils are soft, it is harder to control seeding depth. With greater variability in seeding depth it is likely more seeds will be closer to the herbicide.

Softer soils can also be less stable, particularly in the first season after renovation when there is little stubble cover.

This increases the chance of soil containing herbicide getting blown or washed into furrows.

In some cases, the furrow walls can collapse because the soil is loose, moving the herbicides closer to the crop seed.

A softer soil surface also means more soil throw and increased likelihood of soil from one row being thrown into adjacent rows, increasing the risk of crop injury.

Other considerations

Amelioration changes where weed seeds are in the soil.

Fully inverting the topsoil can bury weeds to 20 to 40 centimetres, which is too deep for most species to emerge from.

Other methods that mix or disturb the topsoil more evenly distribute the weed seeds between three to 35cm.

This buries some seeds too deep to emerge, but can also stimulate a germination event (an autumn tickle effect).

The good news is that more-uniform weed emergence combined with the increased bioavailability of herbicides will still make weeds easier to control.

Herbicides can persist for much longer after soil amelioration. Research is underway, but the current thinking is that reduced soil OM in the topsoil also reduces the microbial activity, which means fewer microbes to break down herbicide residues.

Some research suggests that soil amelioration can change the weed populations in two main ways.

Lighter seeds

First, soil renovation favours species with lighter seeds (less likely to be buried) and species more commonly dispersed by birds, animals and vehicles.

From field observations, wild oats tend to get effectively buried, while capeweed is less likely to be buried.

Soil properties

Second, changes to soil physical and chemical properties, such as pH and nutrition, can favour some weeds. For example, some research shows increasing pH reduces wild radish, but favours ryegrass.

As research is in early days, and because weed populations are unique to a farm, there is no specific advice yet on how soil amelioration changes weed populations.

However, this means that the herbicides you previously used might not be the right choice anymore.

It is important to monitor what weeds are in the paddock and adapt your management practices accordingly.

What you can do

  1. Closely read and follow the herbicide labels. Some herbicides have clear advice to not use in light soils with low OM levels and some recommend lower rates in those scenarios. For example, most application rates for Trifluralin (with the widespread adoption of no-till) are now recommended at 1.5 to 3.0 litres per hectare. There is also a recommended rate of 0.8L/ha with tillage on light soils and this is a much more appropriate rate after soil renovation. Don't reduce rates of newer herbicides unless there is research to support it. Lower rates may be sub-lethal for weeds and therefore ineffective, or even induce metabolic resistance in recurrent generations.
  2. Consider not using pre-emergent herbicides in the first year. The risk in the first year is particularly acute with severely reduced soil structure and no stubble cover. This option is more suitable after complete soil inversion (rather than ripping or mixing), as burying the weed seeds can give excellent weed control.
  3. Use an early post-emergent herbicide. This gives you a chance to assess the size of the weed burden and what the most frequent weed species are. These herbicides are mostly taken up through contact on the leaf (not through the soil) so there is no additional risk of crop damage from changes to the topsoil.
  4. Choose your next crop with care. IMI-tolerant crops, particularly barley, are commonly sown. Hay is an option if the surface is flat and devoid of rocks, clumps and sticks, but soil re-compaction is a risk. Canola has many herbicide options and is more crop-safe for both pre and post-emergent herbicides, but it is trickier to sow after soil amelioration than a cereal. It is often sown too deep because of the soft surface. It is susceptible to poor establishment, especially when sown dry, because soil and post-emergent herbicide can cover it in furrows.
  5. Apply herbicide when the soil has higher moisture as this gives the herbicide a better chance of binding to the topsoil. Applying to dry soil has an increased risk of herbicide movement, especially if you get rain soon after application.
  6. Some growers, especially those with fallow in their rotation, don't sow a crop after amelioration (but might tickle in some oats for ground cover) letting the weeds come up (assisting with erosion prevention) and just dealing with the weeds.

NOTE: This article was produced as part of the GRDC 'Maintain the longevity of soils constraints investments and increase grower adoption through extension - western region' investment.

GRDC Research Code: PLT1909-001SAX

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