Tackling herbicide-resistant summer fallow weeds

It’s spring, and summer weed seeds are waking up. In the northern cropping region, the traditional glyphosate paraquat double-knock tactic is struggling to control many of these summer-growing weeds.

Mark Congreve, senior consultant with Independent Consultants Australia Network (ICAN), says managing these weeds begins with a good understanding of their life cycle and responses to environmental triggers.

“Understanding a weed’s seed dormancy, response to small rainfall events, effect of seed burial and annual versus short-lived perennial growth cycle traits is central to building a robust and effective management strategy,” says Mark. “Being ready to implement the most appropriate tactic at the right time has a long-lasting impact on the weed seed bank.”

In the northern region, the most challenging fallow weeds are awnless barnyard grass, feathertop Rhodes grass, windmill grass, flaxleaf fleabane and common sowthistle. Once established, these weeds are all difficult to kill with herbicides.

“Because mature plants are very hard to kill with herbicides, being able to anticipate germination events and respond quickly is the most effective strategy,” he says. “Even when using a robust second knock, the first tactic must be able to achieve at least 70 per cent control by itself, for the double knock to deliver the required level of control. And this needs to occur after every germination event.”

Feathertop Rhodes grass, awnless barnyard grass and windmill grass all have shallow root systems, so they will stress easily, and glyphosate resistance is widespread in these weeds.

“When soil moisture is limited, these grasses respond by modifying their leaf cuticle and closing stomata to reduce transpiration losses, and quickly moving from vegetative to reproductive growth,” says Mark. “Aim to apply herbicide as soon as possible after rain, as this stress response reduces herbicide uptake and increases the chance of survival.”

Many of these fallow weeds can germinate before the winter crop is harvested. Being able to follow the harvester with the sprayer is often the key to fallow weed management strategy. Some growers are now using robotic platforms with optical spot spraying to follow their harvester and kill summer weeds at a very busy time of year.

Speaking at WeedSmart Week in Griffith recently, Mark said the WeedSmart Big 6 strategies are just as important in the fallow as they are in-crop.

Anticipating target weed germination

Feathertop Rhodes grass seed has minimal dormancy, with germination occurring from early spring to early summer. This weed thrives in zero-till systems, so cultivation can be a useful tactic for controlling mature plants. Seed survives in the seedbank for only one or two years, so consistent efforts to stop seed set will pay off quickly.

Awnless barnyard grass seed has strong dormancy, with germination primarily from October through December. Seed can persist in the seedbank for at least two to three years, particularly if buried. It can proliferate in both zero-till and cultivated paddocks.

Windmill grass is a short-lived perennial, so mature plants will live through winter and set seed again the following year. It is often an indicator of poorer soil nutrition and does best in zero-till systems. Herbicides are generally ineffective on mature plants and especially if moisture-stressed.

The two main broadleaf weeds with widespread resistance to glyphosate are fleabane and sowthistle. Fleabane has two main emergence times – autumn and spring. It is more common in situations without crop competition, such as zero-till fallows. Fleabane has a deep taproot, which draws a lot of soil moisture from the soil profile. Once stem elongation has commenced, the large taproot helps the plant to survive herbicide applications. Seed persists in the soil for one to three years. Glyphosate resistance is widespread, and paraquat resistance is emerging in some fleabane populations.

Sowthistle germinates year-round, and its seed persists in the soil for one to three years. Once elongation begins, sowthistle can often survive herbicide applications. Glyphosate resistance is becoming more common in sowthistle populations.

Response to rainfall events

Mark says that feathertop Rhodes grass stands out in its rapid response to small rainfall events. Trials have shown that a small but problematic portion of the feathertop Rhodes grass seedbank will germinate in response to a rainfall event that delivers 10 mm in one or two days.

“Responding to every one of these small germinations is costly, but waiting for follow-up rain to germinate more seed before spraying may allow the first flush to survive and set seed. Feathertop Rhodes grass is a good candidate for spot-spraying with a robotic sprayer on a continuous loop to detect and spray small weeds as they emerge,” he says. “Alternatively, a residual herbicide applied prior to the first spring rainfall can be very effective.”

In contrast, awnless barnyard grass, fleabane and sowthistle require a cumulation of 20 mm or more over several consecutive days, keeping the soil surface wet, to initiate germination. All four weeds will germinate in high numbers after such an event, making it easier to time an effective double-knock herbicide application with either a blanket spray or optical spot-spray.

What’s working well

Implementing an effective double-knock program remains the optimal tactic for keeping fallow weed numbers low. Automated spot-spraying removes much of the timing and operator pressure and is becoming a cost-effective option for more cropping businesses.

Double knocks can’t deliver the high level of final control required if the efficacy of the first application is poor. Target small, actively growing weeds, and optimise for glyphosate efficacy. This means using a high-quality formulation at robust rates, coarse to very coarse spray quality, add ammonium sulfate, avoid mixing with other herbicides and apply when the air temperature is cooler and humidity is above Delta T 10.

“For best double knock results, wait at least three to seven days before applying the second pass. Ensure there is sufficient spray capacity to treat the whole fallow area twice within about 14 days from the germinating rainfall event,” says Mark. “Keeping in mind that within any seven-day window, there may be as little as 30 hours available when conditions are suitable for spraying.”

Double knocks are the time to use paraquat, glufosinate, Group 14 and Group 4 herbicides alone or in a mixture. Optical spot spraying is a good way to maximise impact while reducing the herbicide volume required. If applying the double knock as a blanket spray, it can be useful to include a residual herbicide in the second-pass tank mix.

If mature plants have established, cultivation may be the only effective control option. In this case, try to take the opportunity to address soil constraints through deep incorporation of nutrients or lime and breaking compacted layers, while removing the large plants and burying some of the weed seed bank. Be ready to follow-up with another weed control tactic after cultivation, such as a blanket spray or establishing a competitive crop or cover crop.

The new Hayes Weed Chipper with Weed-It targeted tillage machine can detect and chip out mature and herbicide-resistant weeds without disturbing the whole paddock. It can also be integrated with autonomous platforms.