Read time: 5 minutes

How can I use the WeedSmart Big 6 to keep old chemistry working?

with Andrew McMahen, agronomist, Nutrien Ag Solutions, Manangatang/Ouyen

In low and medium rainfall cropping regions there is often resistance to introducing more expensive chemistry to the herbicide program due to highly variable seasons.

Andrew McMahen, Nutrien Ag Solutions Manangatang/Ouyen agronomist, says this puts additional pressure on older, cheaper chemistries that have been in use for many decades, and increases the risk of herbicide resistance and weed blow-outs.

“Here in the northern Victorian Mallee the change from mixed farming based on wheat and medic pastures to continuous cropping in the last 20 or 30 years took away the non-selective spray-out of medic that killed survivor weeds from the cereal phase,” Andrew says. “We are seeing more escape weeds in the legume phase now and need to refocus attention on stopping seed set to preserve the efficacy of the herbicides that still provide good control at relatively low cost.”

While imi-tolerant crops in the rotation have been very valuable in keeping control of brome grass there is a high risk of herbicide resistance to the Group 2 [B] mode of action in brome grass unless other strategies are implemented.

“Protecting the imi-tolerance technology for brome and testing for susceptibility in annual ryegrass are high priorities when we are planning weed management programs with clients,” he says. “The WeedSmart Big 6 is a useful tool to use in these discussions to make sure we have included as many tactics as possible to keep weed numbers low. Nutrien Ag Solutions recently became a financial sponsor of WeedSmart to add our support to this practical science-based weed management program.” 

How can I mix and rotate without blowing the budget?

In brief: Any herbicide program that relies heavily on one or two herbicide modes of action for a target weed is bound to fail eventually.

The details: In the Mallee, trifluralin (Group 3 [D]) has been routinely applied for many years due in part to its lower cost compared to alternatives. There are indications that trifluralin efficacy on annual ryegrass is declining so now is the time to include alternative modes of action.  

Since the price of Boxer Gold (Group 15 [J/K]) has reduced, more growers are adding it to their pre-emergent program for annual ryegrass to help extend the ‘life’ of trifluralin and for added logistic benefits.

For brome grass control, back-to-back applications of Group 2 [B] herbicides in cereal and legume phases is also risky practice. Imi-tolerant cereals and legumes have proven very beneficial for brome grass control, but again, alternatives must be incorporated in the program. This could be in the form of a non-herbicide tactic such as haymaking to remove survivors and stop seed set every few years.

Testing for herbicide susceptibility is money well-spent to ensure the products used are effective.

How can I increase crop competition in a low rainfall environment?

In brief: Invest in improving soil nutrition, build soil organic matter through the crop rotation and choose the most competitive varieties.

The details: Legumes are essential to the rotation in a low rainfall zone, particularly on sandy soils. In a cereal dominated rotation, growing bigger crops can result in nutrient tie-up due to the higher stubble load. Including a legume adds nitrogen and aids in mineralisation to feed the next crop.

Paddocks with improved soil nutrition will enable crops to take advantage of favourable seasonal conditions, and also suppress weeds.

How do I develop an integrated weed management plan?

In brief: There is a lot to consider in the planning for weed management, and dealing with herbicide resistance can seem overwhelming. Getting good advice and using the WeedSmart Big 6 checklist provides a firm foundation for an integrated plan to keep weed numbers low.

The details: The WeedSmart Big 6 integrated weed management program starts with maximising crop competition to suppress weed seed production. Then add the other tactics, such as having a diverse farming system, mixing and rotating herbicide modes of action, preventing seed set and implementing harvest weed seed control, to reduce the impact of herbicide resistance.

Events like WeedSmart Week (in Dalby and Dubbo in 2023) and local trials provide great opportunities to learn and exchange information and ideas. For example, this year the Nutrien Ag Solutions annual trial site in Ouyen investigated the new post-emergence use pattern for Mateno in barley, compared 22 wheat and 8 barley cultivars and compared nitrogen application methods (urea and liquid injection), along with several other replicated trials of local interest.

Other resources:

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