Keeping a lid on weeds growing in your crops

By Cindy Benjamin - Weedsmart

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.

Right product – 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.

• 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.  

• 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.  

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

• 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.  

  WeedSmart resources:

• What are the ‘mix and rotate’ options for in-crop herbicides?
• Testing for susceptibility and resistance
• How can I implement the mix and rotate strategy to combat herbicide resistance?
• How can I avoid getting stuck in an imi herbicide cycle?
• Can we grow broadleaf crops without clethodim?
• What can be done to regain control of herbicide resistant sowthistle?
• Grower case study: Tim Rethus, Horsham Vic
• Grower case study: Leigh Bryan, Swan Hill Vic
• Grower case study: Phil and Brad Jackson, Westmar Qld

https://weedsmart.org.au/