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Paul Slack, Moree NSW

Bringing pre-emergent herbicides back into cotton systems

The new rules for managing Bollgard 3 cotton include changes to the requirements surrounding pupae busting. Dryland cotton farmer Paul Slack is considering his options but is cautious about the ramifications of a fully no-till cropping rotation on his farms east of Moree, NSW.

“Weed management in cotton has seen several transitions in the last 20 years,” he said. “Before Roundup Ready cotton we applied pre-emergent herbicide in a band behind the planter and relied on chippers or shielded sprayers to remove any survivors or late emerging weeds. With the adoption of Roundup Ready Ingard, then Bollgard II cotton across the industry, growers started using less and less pre-emergent herbicide and in the last eight to ten years or so we have been heavily reliant on glyphosate.”

Dryland cotton and grain grower Paul Slack (right) and his agronomist Tony Lockrey (AMPS) have been working on crop rotation options to deal with herbicide resistance in weeds.


With the only real cultural control measure being used being cultivation for the purpose of pupae busting, which also helped reduce the weed burden, there has been an over reliance on just one herbicide group. Although there is less cultivation required in Bollgard 3 production, there is still a recommendation to kill the cotton crop and any green bridge weeds or volunteers.

“Cultivation may still feature in our weed management program. We would never cultivate after corn or wheat but cultivation after cotton or chickpea is a good option to fix wash-outs or deep wheel tracks while there are lower levels of stubble in the paddocks. Uneven landform presents significant challenges for spraying, especially with optical sprayers, so it is important to repair damage as soon as possible,” said Paul.

“We have grown cotton every year since 1989 and know that no-till and stubble conservation in the grain phase of our dryland system is worth an additional half to one bale of cotton per hectare,” he said.

Longer rotation

“We got into trouble with herbicide resistance as a result of the short wheat / chickpea / wheat rotation that we used for 15 years or more, like many others in the district,” Paul said. “The legacy of this routine is our ongoing battle with Group A resistant black oats.”

One strategy Paul and his agronomist Tony Lockrey, AMPS Moree have put in place is a longer rotation with summer and winter crop options across the 4850 ha of cropping land. Cotton and corn are the best fit for summer with wheat and chickpea in winter.

Growing cotton and corn in summer and wheat and chickpea in winter has lengthened the crop rotation and added more options to diversify herbicide use.

“Usually we can double crop from corn into chickpea and possibly also from cotton to chickpea,” said Paul. “Wheat follows chickpea before returning to cotton or corn, providing a profitable mix of commodities.”

“Currently we are using a 3-year rotation and could consider lengthening this more,” said Paul. “Disease management and nematode levels influence crop rotation choices and then we plan our use of residual herbicides keeping plant-back restrictions and chemical MOA rotation in mind.”

Canola and barley are both options to lengthen the winter rotation but can only slot into the rotation under certain circumstances. Barley stubble lasts longer than wheat stubble, providing better moisture conservation for the next cotton crop however there are fewer herbicide options available for grass control, putting additional pressure on Group A chemistry and risking a blow-out in black oats. Canola offers more options for nematode and black oats control.

Protecting knock-downs

Herbicide resistance is evident in populations of annual ryegrass, liverseed grass, barnyard grass and black oats on the property. Depending on the size of the patches Paul’s strategy includes optical spray technology and or cultivation to stop seed set.

Paul is looking at ways to diversify his herbicide and non-herbicide tactics to regain control of herbicide resistance in weeds.

Summer fallow sprays targeting grass weeds are usually double knocked with paraquat, and Paul rotates fops and dims (between Verdict and Select) within Group A because these active ingredients have slightly different resistance profiles.

“It can take as few as seven applications of Group A herbicide to initiate resistance so this chemistry is a very short lived option if efforts are not made to control survivors,” said Tony. “To reduce this risk we have tested the use of Paratrooper (paraquat + amitrole) and paraquat + Group G herbicides (Sharpen and Sledge) this summer for ‘one shot’ control of glyphosate resistant grasses and rosette or early elongation fleabane. In some cases this was applied with other fallow residual herbicides to again take pressure off glyphosate and group A chemistry. With this approach it is essential to achieve excellent coverage so we used water rates in the range of 100–200 L/ha.”

Results were very encouraging so Paul and Tony have confidence that this brings another option, albeit another herbicide option, that does not involve glyphosate and Group As.

Pre-emergent resurgence

In fallow prior to the summer crop, Paul is re-introducing the pre-emergent use of Terbyne (Group C). Once the crop is sown he then applies a PSPE diuron + metolachlor (Group K) mix in cotton and atrazine + metolachlor in corn for grass and broad-leaf weed control. A recent registration also allows in-crop application of Bouncer (metolachlor) in cotton.

Flame is sometimes used in the fallow following chickpea and prior to the wheat crop. Balance (Group H) applied to chickpea provides effective pre-emergent grass control into the following summer as well, complementing the Flame. Balance also provides good control of volunteer cotton plants.

Metolachlor, one of the pre-emergent herbicides, requires incorporation with sufficient rainfall within two weeks of application. To hedge the risk, Paul applies one shot in August prior to planting rain for cotton and corn then follows up with a second shot applied PSPE in fields with problem summer grasses.

Paul currently grows conventional corn varieties however he is considering the use of imi-tolerant corn, and the soon-to-be-released IT sorghum or sunflower varieties, in the future as this would enable him to plant these crops after using Flame in the fallow without causing crop damage due to residual activity.

“The release of these crops will heighten the need for growers to carefully implement the imi herbicide stewardship measures,” said Tony. “With imi tolerant options for several winter and summer crops it is essential to observe the requirement to limit Group B (imi) applications to two sprays in a 4-year period.”

Crop competition

Using the crop to compete with weeds is often difficult in this drier cropping zone where much of the crop growth relies on limited stored soil moisture and row spacings are often wide.

Corn sown on 1-metre rows provides good shade and an opportunity to plant before the summer grasses germinate. “Corn is planted and a pre-emergent herbicide applied in August when the soil temperature hits 12 degrees C,” said Paul. “This gives the crop a competitive advantage over the summer weeds, which don’t germinate until the soil temperature reaches 14–15 degrees C. Corn also provides good canopy shade to suppress weed germination and growth as the season progresses.”

“Corn also leaves more residual soil moisture for the following chickpea crop,” he said. “This is a good combination for us, generating a yield benefit of 0.4 t/ha in the chickpea crop.”

In cotton, Paul is considering a change from a double skip planting configuration to a single 2.4 m wide row to better suit picking and the 3 m controlled traffic farming configuration, giving 5 rows in 12 m. Although he knows this will open the crop up to increased weed pressure the single row will favour better quality cotton and the picker will operate more efficiently.

Paul is considering a change from the double skip planting configuration to a single 2.4 m wide row to better suit picking and the 3 m controlled traffic farming configuration. Although Paul knows this may open the crop up to increased weed pressure the single row will favour better quality cotton and the picker will operate more efficiently.

Paul is a member of the local Dryland Cotton Group, and is hosting the group’s trial looking at the benefits of using cover crops to improve moisture conservation across the rotation. This is really a test of whether a cover crop is a viable way to improve the fallow efficiency across 5 to 7-year rotations. The group is trying different summer and winter cereals and legume species and species mixes to investigate the potential benefits of cover cropping. They are just part-way through a 3-year trial but the field pea is showing the most promise for retaining stored moisture and boosting yield in the following crop. Weed management is not the focus of this trial however there may be implications from this trial that Paul may use as he continues to fine-tune their production system.

“In our hot, dry environment, organic matter in standing stubble is mostly lost to the atmosphere rather than contributing to soil organic carbon,” he says. “More nutrients and carbon return to the soil when stubble is incorporated. We will need to determine when to leave stubble standing and when to do light incorporation of the cover crop residues to maximise the benefits of both.”

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