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IMI-tolerant crops – use sparingly and to best effect

The much-anticipated release of imi-tolerant sunflower and sorghum hybrids over the next few years has the potential to increase the options open to growers to better utilise summer rainfall.

This new technology also increases the potential for over-use of the broad-spectrum imidazolinone (or imi) herbicides, along with an increased risk of escalating weed resistance. To manage this risk, it is essential that growers use a calculated approach to crop and herbicide rotation, and use multiple weed control tools in the year of imi herbicide application to prevent seed set – as recommended in the Weedsmart 10-Point Plan.

Since the Clearfield technology was commercialised in 1992, plant breeders in Australia have developed the range of imi-tolerant summer (maize, and soon sorghum and sunflower) and winter (wheat, barley and canola) crops, along with lentils with improved tolerance of imi herbicides.

Imi-tolerant sorghum hybrids (centre) currently in the development phase are showing promise for improved control of grass weeds in-crop. (Photo: Rob Crothers, DuPont Pioneer)

Imi-tolerant sorghum hybrids (centre) currently in the development phase are showing promise for improved control of grass weeds in-crop. (Photo: Rob Crothers, DuPont Pioneer)

Understanding the risk

Plant breeders use conventional methods to breed the single gene imi-tolerance trait into a range of crops. Unfortunately, weeds can also naturally evolve resistance to Group B herbicides, all of which work by targeting the function of the ALS enzyme required to produce essential plant proteins.

Weeds that are resistant to one Group B herbicide will often be resistant to others in the group, even if there has been no exposure to the other sub-groups. This propensity for cross-resistance is a considerable risk and strategies must be in place to minimise exposure across the whole Group B MOA, not only the imidazolinone sub-group.

To protect the imi-tolerant crop technology it is essential to implement the stewardship guidelines for these crops. Richard Holzknecht, BASF technical services manager for Queensland and NSW, says growers will see optimal results in imi-tolerant summer crops when they use post-emergent imi herbicide products to control weeds.

“Pre-emergent imi products are generally less effective in controlling germinating weeds compared with well-timed post-emergent products,” he says. “Some pre-emergent imi herbicides, such as imazapic, have longer soil half-life and can restrict crop rotational choice or cause adverse plant-back effects.”

“The residual use of imazapic products also exposes new weed germinations to ever-reducing sub-lethal doses of product, which increases the risk of weed resistance.”

There are limited post-emergent grass weed options available in summer cropping due to increasing incidence of weed resistance to Group A and B (SU) and potential crop damage. One solution is the use of the post-emergent imi herbicide, imazamox (e.g. Intervix®) to provide broadleaf and grass weed control in the one product.

Stewardship recommendations

“Post-emergent application targets a known weed population and takes advantage of crop canopy closure to suppress late germinations,” says Mr Holzknecht. “Growers also have the option to use an alternate MOA for their pre-emergent and fallow applications to layer control measures to achieve better season-long weed control.”

Imi herbicides can then be used to control a broad spectrum of grass and broadleaf weeds, including weeds that are closely related to the imi-tolerant crop. Crops with imi-tolerance are also a useful option if residues are present in the soil from previous applications that would otherwise prevent the production of conventional varieties or hybrids.

The over-arching recommendation for imidazolinone resistance management is to limit using Group B herbicides to twice in every four-year period per paddock and to thoroughly control any surviving weeds.

Mr Holzknecht says it is necessary to understand and utilise each herbicide’s strengths and to use each product to its best advantage. “Weed species biology, crop rotation and herbicide choice all need to be taken into account when planning an integrated weed management program,” he says. “Growers must avoid over-use of imidazolinone products in cropping systems, as the risk of resistance to these herbicides is high.”

“Across the rotation, look for ways to layer control measures by using alternate modes of action for fallow, pre-emergent and post-emergent applications in different crops,” he says. “And consider a tank mix with a non-ALS mode of action herbicide at full label rates for in-crop weed control.”

Imi-tolerant sorghum and sunflower hybrid releases

Nuseed’s program manager for summer crops, Chris Haire has been involved in the development of imidazole (imi) tolerant sunflower hybrids that will increase the opportunities to safely plant sunflower into situations that would be untenable with current hybrids.

“With an imi-tolerant hybrid, growers will be able to safely plant into paddocks where imidazole (Group B) chemistry has been recently applied, say after an IT-maize, IT-wheat or pulse crop, negating the current 12+ month plant-back requirement, which restricts the choices that growers have in summer,” he says.

“The second major benefit will be that an imi-tolerant sunflower hybrid could be planted into a paddock that had a broadleaf weed burden—a situation that would currently not be advisable.”

Nuseed expect to be conducting in-country trials required to gain chemical registrations for the new use pattern in 2017–18. Mr Haire is hopeful that a small amount of seed may be commercially available in 2018–19, opening up more cropping options for growers in Central Queensland and the western Darling Downs particularly.

Imi-tolerant sunflower trial – 28 days after application after planting into a weedy situation (0 days after application inset).

Imi-tolerant sunflower trial – 28 days after application after planting into a weedy situation (0 days after application inset).

Similarly, Pioneer Seeds have sorghum hybrids with Group B tolerance coming through their plant breeding program, adding to their suite of IT canola and corn hybrids. Rob Crothers, Australian sorghum and northern corn product manager said that INZEN hybrids will be evaluated in field trials again this coming summer.

“When we are successful in bringing this new technology to the market place it will enable growers to spray grass weeds with a post-emergent herbicide in crop for the first time,” he says.

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Never cut the herbicide application rate

Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations. Full rates when mixing herbicides too! When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality. Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL. Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.   Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice. Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.  

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.

Clean borders – avoid evolving resistance on the fence line

About one-quarter of glyphosate-resistant populations within broadacre cropping situations across Australia come from fencelines and other non-cropping areas of the farm. Along paddock borders, where there is no crop competition, weeds can flourish and, if not controlled, set lots of seed. The traditional approach has been to treat these weeds with glyphosate to keep borders clean but after 20-odd years this option is now failing and paddock borders are becoming a significant source of glyphosate-resistant weed seed. Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems. In some situations, cultivation can be used to kill the weeds and provide a firebreak, but on light soils this may pose an erosion risk and mowing or slashing may be safer options. Another possible tactic is to continue using herbicides but to ensure that a clean-up operation is carried out before any survivors can set seed. Some growers are choosing to increase the heat on weeds along the borders by planting the crop right to the fence and then baling the outside lap and spraying with a knockdown herbicide to kill any weeds and provide a firebreak. Another good option in some situations is to maintain a healthy border of vegetation using non-invasive grasses. In Queensland, buffel grass is a good example of a grass that can outcompete other weeds while not invading crop lands. If only herbicides are used on fencelines, resistance is inevitable. Surviving weeds on fencelines have no competition and access to plenty of soil moisture, so they set a lot of seed and resistance can easily flow into neighbouring paddocks. Other resources It’s time for a glyphosate intervention Farm hygiene cottons on – Cleave Rogan, St George What’s new in management of herbicide resistant weeds on fencelines? Keeping the farm clean – Graham Clapham, Norwin Don’t jeopardise glyphosate for clean fencelines Keeping fencelines clean Resistance risk to knock-down herbicides on irrigated cotton farms

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