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Beating glyphosate-resistant grasses in summer fallow

In the face of widespread glyphosate resistance in their summer grasses, Condamine growers Jake and Felicity Hamilton of Krui Pastoral needed to change their herbicide use significantly.

They have since adopted many of the WeedSmart Big 6 strategies to regain control of glyphosate-resistant weeds on their 5000 ha summer fallow area.

In the late 2000s, it was clear that even high glyphosate rates were no longer affecting their summer grasses – particularly button grass, urochloa (liverseed grass), barnyard grass and feathertop Rhodes grass.

“We had generated this situation in the 1990s when we changed to no-till farming and stubble retention,” says Jake. “The removal of cultivation and stubble burning favoured summer grasses that can germinate on minimal rainfall events and quickly set seed. Additionally, we relied almost exclusively on glyphosate for summer weed control, particularly in years that were too dry for summer cropping.”

They decided to try Group 1 [A] herbicides in fallow and double-knocked each application with two successive applications of paraquat. Jake was very conscious of the short route to resistance for Group 1 chemistry and that ‘you can’t spray your way out of resistance’.

“What we hoped to achieve was to halt seed set for a few years and to introduce extra tactics to keep putting pressure on the weed seed bank,” he says. “We followed up the Group 1 blanket spray with spot spraying using our ‘Gator side-by-side to deal with any individual weed survivors, and if there were weedy patches, we used the 6 m boom on the ‘Gator.”

Spot spraying summer grasses is time-consuming, but Jake found it incredibly effective and cost-efficient. In the first summer, they spent 400 hours spot spraying, mainly chasing feathertop Rhodes grass seedings.

Although quite expensive, the Group 1 herbicides had a dramatic knockdown effect on the glyphosate-resistant grass weed populations, and Jake knew their use was worthwhile.

“We were looking for other chemistries to introduce to the program, and an agronomist friend mentioned that Flame (imazapic) had been widely used in the past,” says Jake. “At $12/ha, it was cheaper than the Group 1 products, and being a different mode of action group was a distinct advantage.”

They introduced imazapic (Group 2 [B]) to their summer spray program and gained residual control on grasses in fallow without causing any crop safety issues in the following wheat crop.

“We were confident to apply imazapic across the whole cropping area to keep the summer fallow clean, and it really paid off in 2011 when the farm was flooded over summer, and we emerged with our fallows still clean,” he says.

Since then, the Hamiltons have introduced five or six residual herbicides to avoid generating resistance. They are careful with their residual chemistry choices in each paddock to keep their options open for summer cropping and are aware of the behaviour of the products they use in the soil.

“Imazapic is a good example of a residual that requires rain to activate,” says Jake. “We haven’t run into any issues with it to date, but the potential is there for it to activate later in the summer and not break down before winter sowing time. We are also mindful of the potential for increased microbial degradation of the residual actives if these herbicides are used too frequently.”

They have also increased their spray capacity to get across the whole cropping area within seven days. Using two self-propelled sprayers fitted with John Deere See and Spray Select optical weed detection units, they have achieved their goal of spraying on time and spraying less overall. The optical sprayer is treating just 2 to 5 per cent of their area for grasses and up to 10 per cent of the area for fleabane. Jake says the See and Spray optical sprayers are easy to use and operate effectively at higher speeds.

“The forward-facing cameras also provide excellent weed detection accuracy, and we’ve seen very few misses,” he says. “We hope that green-on-green capability will also be available in the future.”

These days, the Hamilton’s summer spray program usually involves only three passes – the first to apply the residual herbicides straight after harvest, then glufosinate at 10 L/ha with 150 L/ha water rate with the optical sprayer for grass escapes and lastly, a heavy rate of paraquat for fleabane control.

“We have since introduced imi-tolerant wheat to our rotation, and that allows us to apply OnDuty* in winter before canopy closure in spring. We have noticed a reduction in early germinating feathertop Rhodes grass, particularly on our lighter soil types,” says Jake. *OnDuty is a Group 2 [B] pre-emergent herbicide (imazapic + imazapyr) registered for use in Clearfield canola and wheat production.

Following two reasonable winter seasons, the 2016–17 summer was too hot for summer cropping, with no rain falling between September and February. Jake took this opportunity to do more laser levelling to remove the melon holes characteristic of brigalow scrub soils. Levelling brings about an immediate increase in yield and more even crops. It also helps in the fallow by preventing waterlogged patches where grass weeds can evade residual herbicides and create a weed hot-spot.

“We purchased a second-hand Caterpillar D11R and fitted it with TopCon GPS,” says Jake. “With the dozer, we can cut 10 cm below grade on our first pass, which creates a good blend of topsoil with any exposed subsoil to avoid ‘scalping’ the paddock.”

The Hamiltons have effectively regained control of glyphosate-resistant grasses by consistently applying the WeedSmart Big 6 tactics of rotating herbicide modes of action and improving spray efficacy in their summer fallow.

Jake outlined his experiences with glyphosate-resistant summer weeds at WeedSmart Week 2023 in Dalby. Growers and agronomists are encouraged to join the waiting list and make plans to attend WeedSmart Week 2024 in Port Lincoln, SA on 29 and 30 July to hear how growers are solving weed problems in that region.

Hear more about Jake’s experience with the John Deere See and Spray technology on the podcast.

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