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Keeping a lid on weeds growing in your crops

In-crop weed control is particularly difficult in some years. Even after you have done all you possibly can to get your crop off to a competitive start the growing season can throw up some major challenges.

In this article we explore key principles that impact on the efficacy of in-crop ‘selective’ herbicides. You will most likely have to make some compromises and it is almost impossible to implement every tactic perfectly every time.

Herbicide mixes help to preserve the effectiveness of each mode of action by avoiding unnecessary usage.

Over the last few years WeedSmart has collected and promoted great advice from seasoned agronomists, wise researchers and crafty farmers on all aspects of weed control. To save you some time we have collected the resources that we think can be of assistance as you make the hard decisions about what to apply, when to apply and how to apply the herbicide and non-herbicide weed control tools at your disposal.

Post-emergent herbicides have been widely used in Australian crops because they are generally highly effective and easy to use. Unfortunately, their popularity has led to widespread resistance and most farms will have at least one weed species that is resistant to at least one post-emergent herbicide mode of action.

Despite recent increases in resistance, post-emergent herbicides remain an integral component of weed control strategies in many production systems.

Key messages:

  • Avoid the routine use of any weed control tactic – mix, rotate and keep changing.
  • Know what modes of action still work – test for susceptibility.
  • Have a plan for dealing with a weed blow-out.
  • Right Product, Right Time, Right Application.

Planning your in-crop herbicide use

Widespread and increasing herbicide resistance demands a planned approach to herbicide use throughout the crop sequence. If you have been using a particular herbicide or group of herbicides routinely, it is probably because they work well. To ensure these effective products remain an option into the future, it is necessary to use them less often!

Testing weeds for their susceptibility to a range of herbicides is cheap compared to applying a herbicide that has limited or no effect. Resistance to one or more herbicide does not mean you have no options.

We now know that mixing and rotating herbicides is an effective strategy to prolong the effective life of each mode of action. But even these mixes and rotations need to be change.

There are currently very few in-crop herbicides available for grass control. There are more options for broadleaf control. Plan a herbicide use program that spans your crop sequence so you can ‘save’ particular herbicides for use in crops where there might be limited alternatives, while using a range of other modes of action in other crops.

All herbicides applied in crop will have some impact on crop safety. Herbicides must be applied according to the correct crop growth stage for each herbicide.

Shielded spraying opens up the possibility of using other chemistry in-crop that would otherwise not be an option. Some growers are also looking for ways to include non-herbicide in-crop tactics such as inter-row cultivation or scuffling in wider-row cropping situations.<

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For planning herbicide use through the season and the crop sequence the NSW DPI Weed Control in Winter Crops booklet is full of useful tables of selective herbicides for each crop type. Click to download a copy.

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If you don’t have a copy of Mark Congreve and John Cameron’s ‘Understanding Post-emergent Herbicide Weed Control in Australian Farming Systems’ GRDC technical manual, you really need to download this resource.

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The GRDC Integrated Weed Management in Australian Cropping Systems manual provides a comprehensive guide to IWM, including the use of in-crop herbicides.

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This table shows the in-crop herbicides and their application timing for cereals. It is an extract from the NSW DPI Weed Control in Winter Crops booklet. You can download a copy here.



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– Achieving maximum impact

Herbicide application decisions are complex and it is almost impossible to have all the important factors in place every time. Start with the product label and follow the instructions on rate, nozzle use, boom height, speed, adjuvants, optimal environmental conditions and so on.

When mixing, the order is important to avoid precipitation in the tank but it is also important to ensure that the tank mix partners each retain their efficacy. Some mixes are antagonistic and should be avoided because this will likely result in a significant reduction (up to 50% in some products) in efficacy. Adding an adjuvant can reduce herbicide selectivity and thereby increase crop damage.

The GRDC GrowNote Spray Application manual provides detailed information on:

  • Planning your spray operations – things to think about
  • Preparing for spraying – checks, accuracy and efficiency
  • Spraying system – major components and set-up considerations
  • Selecting a spraying system options available and operational considerations
  • Review and planning for future needs

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The manual comprises 23 modules and a ‘water flush for residuals’ calculator. Each module includes a series of videos (see playlist below).

The video playlist is a great place to start:



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Right time – Manage multiple germinations

Timing is one of the hardest things to get right. Large spray programs and wet weather can make it very difficult to apply herbicides at the optimal time in every paddock for every weed.

The guiding principle is to always target small weeds, even if this means multiple applications. In wet years, multiple germinations will occur and waiting for the next germination invariably means the first flush will be much more difficult to kill and more likely to generate lots of seed for next year.
All herbicides applied in crop will have some impact on crop safety. Herbicides must be applied according to the correct crop growth stage for each herbicide. Also consider the effect of environmental conditions, particularly frost, on crop safety.
Environmental conditions at the time of spraying can make a huge difference to herbicide update in weeds. Look for the directions on the label for optimal conditions as spraying outside these parameters will result in less effective weed control.
avoiding chemical residues in grain.

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Be ready for weed blow-outs

Wet years are the classic weed blow-out situation. Pre-emergent herbicides breakdown faster, spray applications can’t be applied on time and weeds will take advantage of any gaps that might develop in the crop.

Monitoring the weed pressure and having a plan to limit the potential damage might help.

The aim is to stop a massive seed set event using tactics such as – crop topping, hay or patching out. And be ready to implement harvest weed seed control – all the methods work well.

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Never cut the herbicide application rate

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Spray well – correct nozzles, adjuvants and water rates

Spray application is a technical field and growers need to make sure their equipment and application techniques are spot-on. The GRDC Spray Application GrowNote provides detailed information and about 80 videos to demonstrate key skills. Prevent spray-drift The focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum. Bill Gordon’s 10 Tips for Reducing Spray Drift Choose all products in the tank mix carefully. Understand the product mode of action and coverage requirements. Select (and check) the coarsest spray quality that will provide effective control. Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY. Use weather forecasts to inform your spray decisions. Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas. Set the boom height to achieve a double overlap of the spray patterns. Avoid higher spraying speeds. Leave buffers unsprayed if necessary and come back. Continue to monitor conditions, particularly wind speed, at the site during the spray operation High water rates don’t have to slow you down Some growers are concerned that increasing the water rate when applying herbicide will slow down their spray operation and cost them money. However, the biggest financial loss during spraying usually comes from a failed spray job. To keep your spray operation as time efficient as possible when using more effective and reliable application volumes, you can: Use nurse tanks around the farm to reduce the time spent travelling back to a central re-fill point. Use a larger pump, e.g. 2.5 inch, to make re-filling quicker. Pre-mix the batch while the sprayer is operating. Many mixes can be held in the mixing tank for up to 6 hours. However, wettable granules and suspension concentrates will need agitation to keep them in solution. For pre-emergent herbicides in high stubble situations, carrier volume has a large effect on the level of control achieved. Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high Water quality and mixing order Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property. Water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Getting the mixing order right is essential for effective spray results. Don’t start mixing until the water quality is right Podcast – Mixing herbicides Adjuvants Sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives. When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. Correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.
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Clean borders – avoid evolving resistance on the fence line

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