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Using pasture phases to beat herbicide resistant weeds

with Tim Condon, Senior Consultant, Delta Agribusiness

Cropping a thousand hectares with a low weed seed bank is worth at least $20 thousand per year. Including a pasture or fodder phase can help achieve this while revving up the whole farming system.

Delta Agribusiness senior consultant, Tim Condon says the reason that including a pasture phase is one of the WeedSmart ‘Big 6’ tactics to manage herbicide resistance is the opportunity it provides to drive down weed seed numbers before returning to a cropping phase.

Tim Condon, Delta Agribusiness says adding a 2–3 year pasture phase provides growers with a number of additional weed control tactics to address herbicide resistance.

“The key is to always go into the crop phase with low weed numbers and also go into the pasture phase with low numbers,” he says. “The idea that pastures are an effective ‘reset’ after a weed blow out doesn’t always work.”

What does work is taking a planned approach right across the pasture phase and using a number of tactics known to be highly effective at preventing seed set. Several of the tactics available for use in a pasture phase can provide over 90 per cent control of the target weeds. The plan needs to outline how the pasture phase will fit into the crop rotation and what tactics will be used seasonally and rotationally to maximise the effect on weed numbers.

“The aim of the game is to establish dense, persistent and nodulating pastures,” says Tim. “Pastures offer the opportunity to have two to three consecutive years of no seed set. Go into the pasture phase with a clear plan for weed control and avoid the temptation in a tough winter to ‘drop the weed control program – because you need the feed’.”

Tim recommends incorporating fodder crops or short-term pastures for grazing as well as fodder conservation. The livestock options are flexible and can include running a livestock enterprise, buying in or agisting livestock for short-term grazing.

“Having livestock in the system can also influence your choice of other weed control tactics such as harvest weed seed control – where you can use the chaff dumps or chaff lines as an additional feed source,” he says. “Using heavy strategic grazing after applying glyphosate and prior to a double knock dose of gramoxone is also a very effective tool”

The weed control tactics that offer the greatest seed set prevention in pastures are hard winter clean, hay and silage. Using as many tactics as possible each year helps to target different weeds with both herbicide and non-herbicide control measures.

Two or three years of pasture also pays dividends with improvements in soil tilth, fertility and water infiltration as a result of growing persistent, nodulating, dense pastures.

What is the most effective winter clean tactic?

Short answer: A hard winter clean.

Longer answer: A hard winter clean is hard on the desirable perennials and annual legumes but very effective on weeds. If you choose to use a soft winter clean to preserve the legumes, be sure to follow up with another tactic.

For a hard winter clean to be most effective the pasture must be grazed very short prior to spraying. If there are patches of long vegetation or different weed species, consider spraying these areas separately, or twice, with the most effective chemicals. You can target both annual grasses and broadleaf weeds, but choose the herbicide mixing partners carefully.

A hard winter clean is the most effective way to end a pasture phase and to maximise the chances of re-entering the cropping phase with very low weed numbers.

What other tactics can be used?

Short answer: Spray topping.

Longer answer: Spray topping with either glyphosate or gramoxone early in the pasture phase can be used to start the process of driving down the weed seed bank. Timing is critical. For example, paraquat must be sprayed at flowering while glyphosate has a wider window of growth stages and can be more effective across a range of species at a single given application timing. Of course, a complete and timely spray fallow in the last year of the pasture phase is critical part of the process. Often a double knock with glyphosate followed by gramoxone can be employed to achieve 100% control.

What are the keys to growing dense, persistent and nodulating pastures?

Short answer: Attention to every detail!

Longer answer: A pasture phase is a good time to analyse the soil and take action to correct any acidity or other nutrient constraints. Zero or minimum tillage systems can lead to pH and nutrient stratification with sub-surface layers remaining acidic. Sampling and testing the 5–10 cm layer will identify if this is a problem. This is an ideal time to consider strategic tillage if this is an issue.

Decide on whether to establish the pasture with a cover crop or by direct seeding – both have pros and cons. Direct seeding is the best choice if the pasture mix includes a short term legume species or is grass based in drier environments.

If you choose to establish the pasture under a cover crop, the pasture must win this competition. Choose a crop that can be sown early and sow the crop in a north south direction at a low seeding rate. Remember that it is all about giving the pasture the competitive edge. Choose pasture species that are suited to your area and are weed suppressive.

For a pasture phase to be an effective weed control measure the pasture must be persistent, nodulating and dense.

What fodder conservation measures provide the best weed control?

Short answer: Both hay and silage are great for preventing seed set.

Longer answer: Using a combination of tactics works well. Silage is cut earlier in the season – so crash grazing after cutting followed by a spray fallow is highly effective. As is a pre-cutting application of glyphosate, particularly with hay making.

Other resources:

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What’s the benefit of a double paraquat knockdown?

The ‘double knock’ strategy has long been used and promoted as a valuable tool in the battle against glyphosate resistance in weeds, with paraquat typically applied to control weeds that survived the ‘first knock’ of glyphosate.  
James Jess, research and technical services manager, Western AG in Ballarat, says growers in his client group and beyond have used a double paraquat application to great effect this year, and avoided a very serious blow-out of glyphosate resistant annual ryegrass.
James Jess, research and technical services manager, Western AG in Ballarat. (Source: Syngenta)
“The 2020 season did not provide an opportunity for an effective pre-seeding knockdown and growers across the higher rainfall zones of Victoria found many large, well-tillered ryegrass plants flourishing in their crops,” he says. “We sent samples to Peter Boutsalis at Plant Science Consulting and the results of the Quick Test showed high levels of glyphosate resistance.”
The live plant samples sent to Plant Science Consulting were tested for their response to rates from 2 to 5 L/ha of 600 g active ingredient glyphosate, and many survived rates of 4 L/ha and above. Given the high level of resistance, Western AG put down a trial to compare a range of double knock options so they could give their clients more confidence going into the 2021 season.    
“We knew that the surviving plants growing in fields across the district would be setting a huge amount of seed and that growers would face a devastating situation unless we took decisive and effective action in 2021,” says James. “In the badly infested patches growers also took measures to reduce the amount of seed entering the seed bank at the end of the 2020 season using hay cutting or harvest weed seed control. In paddocks with mainly glyphosate susceptible ryegrass, desiccating feed barley is also a good way to drive down weed numbers.”
‘Double-knock to protect glyphosate’ is one of the WeedSmart Big 6 tactics, which will be the centre of discussion at WeedSmart Week in Esperance, WA in August this year. This flagship event always attracts growers from interstate keen to see how other farmers are keeping weed numbers low in different systems. Early bird registration is now open.
What double-knock options did you trial?
In brief: 1. Glyphosate followed by paraquat and 2. two sequential paraquat applications.
The details: A series of timings were also tested for both the double knock options. Each of the ‘second knocks’ were applied 3, 7 and 12 days after the first knock application.
The two paraquat applications either 3 or 7 days apart were clearly effective in the trial, with the first paraquat application providing 90 per cent control of the glyphosate resistant ryegrass. This means that in a year where it is not possible to implement a double knock, we know that a single application of paraquat at robust rates will still do a good job of reducing weed numbers.

Get more details from the Western AG trial site report.
What advice did you give your clients going in to the 2021 season?
In brief: Delay seeding and implement a double paraquat knockdown.
The details: It was essential to get on top of the glyphosate resistance in ryegrass and avoid a blow-out. Last season the resistance level was high but the plants were still mainly found in manageable patches. Given the amount of seed that was likely added to the seed bank and potentially spread during harvest, it was essential that extra emphasis was put on having a clean seedbed going in to the 2021 season.
Once an effective knockdown has been applied we then recommend growers use a pre-emergent herbicide to reduce weed emergence when the crop is young. Later germinations are then suppressed by the competitive crop. In our trial we used Sakura incorporated by sowing (IBS), which provided excellent early weed control.
Has the recommendation been adopted successfully?
In brief: Yes, rapid and widespread adoption.
The details: There was immediate adoption of this tactic in response to the situation that emerged last season in the high rainfall zone of Victoria. Over 70 per cent of the Western AG client base in the higher rainfall western districts of Victoria implemented a double-paraquat application pre-seeding to target glyphosate resistant ryegrass before seeding the 2021 winter crop. This high level of adoption was a result of the strategy being actively promoted to clients, with the trial results giving growers the confidence to implement the recommendation.
The Western AG double knock trial last year provided growers with the confidence they needed to take decisive action on glyphosate resistant ryegrass before seeding the 2021 crop.
The blow-out was a real eye-opener for growers about how important it is to keep weed numbers low and the resistance mechanisms in play for all agricultural chemicals – not just herbicides.
The double paraquat tactic is also a good knockdown prior to sowing Roundup Ready canola to meet the stewardship requirements for using the RR technology.
Although resistance to paraquat is currently quite rare in annual ryegrass, it has been found in situations where paraquat has been applied at sub-lethal rates over a long period of time. There have also been new cases of paraquat resistance confirmed, and identified as developing, in ryegrass populations in WA, SA, Victoria and NSW this year.
With this in mind, a simple switch to double paraquat as a pre-seeding knockdown is not recommended as a standard practice but rather as a strategic tactic to contain glyphosate resistance in ryegrass. Once that has been achieved, a set of diverse strategies, including herbicide mixes, must be implemented and any survivors must be removed before they set seed.
Resources

Western AG Double knock field demo results – 2020
Double paraquat and RR canola – Podcast with Mark Lawrence 

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How do you manage summer weeds without spraying at night?

Concerns are being raised about the practical implications of this for summer weed control programs.
Mary O’Brien, a private consultant with extensive experience in managing spray drift, is keen to see growers fully adopt spray application practices that maximise herbicide efficacy and minimise off-target drift.
 
Mary O’Brien says the ‘community drift’ that can occur when a number of applicators are each putting a small amount of product in the air at the same time can have very damaging effects on off-target sites.
“The bottom line is that allowing spray to drift is like burning money,” she says. “Any product that doesn’t hit the target is wasted and the efficacy of the spray job is reduced, mildly resistant biotypes may survive as a result of low dose application and there is potential damage to sensitive crops and the environment.”
“The difficulty is that many growers want to spray at night to cover more ground when conditions are cooler and potentially weeds are less stressed. Having a restriction on night spraying does restrict the time available to cover the areas required.”
Having heard these concerns from growers across the country Mary keeps coming back to the fact that if there was a limitation to capacity at planting or at harvest, growers would scale up to get the job done in a timely manner.
“Buying another spray rig or employing a contractor is an additional cost, especially after a couple of tough seasons, but I really think this is insignificant against the cost of losing key products and the resultant escalation in herbicide resistance to the remaining herbicides,” says Mary. “This problem is not confined to 2,4-D or even to herbicides. I recently spoke to a stone fruit grower who was forced to dump his whole crop after a positive MRL return for a fungicide he had never even heard of, let alone used.”
What about just slowing down and lowering the boom during night spraying?
Short answer: This, coupled with a good nozzle, will reduce drift but it will never eliminate it.
Longer answer: The correct ground speed and boom height will have a large effect on the amount of product that remains in the air. The problem is that it only takes 1 per cent of the product remaining in the air to cause off-target damage.
Once there are a few operators putting just 1 per cent of their product in the air at the same time, the amount of product quickly accumulates and can potentially be very damaging. Mary calls this ‘community drift’.
Isn’t it better to spray weeds at night when it’s cooler?
Short answer: Not really.
Longer answer: Research by Bill Gordon showed that even if you keep everything else the same, night spraying can put at least three times more product in the air than daytime application, even if weather conditions are similar and there is no temperature inversion in place. The main difference between day and night is how the wind is moving across the landscape, rather than the wind speed.
Under inversion conditions, the air moves parallel to the ground surface and this means that the product can move significant distances away from the target before coming to the ground.
To achieve the best results through daytime spraying, applicators should focus on treating small, actively growing weeds. When there is good soil moisture, weeds are unlikely to be stressed even when the temperature is quite high.
Temperature inversion conditions are more common at night and in the early morning. These conditions generate a laminar flow of air across the landscape allowing small droplets to travel many kilometres away from the target site before coming to ground.
Can I use other products at night and just avoid using 2,4-D?
Short answer: The current changes to 2,4-D labels has drawn a lot of attention but the problem is the same for all crop protection sprays – herbicides, fungicides and insecticides.
Longer answer: Different products have different properties and some may work better at night but the problem is the sensitivity of some crops to certain products, such as 2,4-D. All products are tested for their efficacy and the label provides detailed information about the required spray quality and spray application conditions. Many products have explicit label instructions regarding wind speed, temperature inversions (or laminar flow) and night spraying.
Given the high risk of drift at night, applicators need to be very confident that there is no inversion present, and weather conditions should be measured at least every 15 minutes to ensure wind speed remains above 11 kilometres per hour. An on-board weather station is the best way to monitor conditions.
A visual demonstration using smoke to simulate the the lateral movement of small spray droplets when a temperature inversion is in place.
What can I do to improve spray efficacy and avoid spray drift?
Short answer: If you do just one thing – change your nozzle.
Longer answer: All the factors that increase drift also reduce efficacy. To improve efficacy and reduce drift, use a better nozzle (larger spray quality) and appropriate water rates (matched to spray quality and stubble load), slow down and keep the boom low. Wind is required to push product downward and onto the target, and remember that the 3–15 km/h wind speed is for day time conditions only, this does not apply at night.

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Does ambient temperature affect herbicide performance?

with Chris Preston, Associate Professor, Weed Management
 at 
The University of Adelaide
Temperature affects the absorption, translocation and metabolic degradation of herbicides applied to plants. Herbicides applied under the wrong conditions can appear to fail, however the reason may not be herbicide resistance.
Dr Chris Preston, Associate Professor, Weed Management
 at The University of Adelaide says most herbicides have a temperature range at which they are most effective in controlling target weeds.
“Applying herbicides outside the optimal temperature range is likely to contribute to a spray failure, even in susceptible populations,” he says. “Alternatively, applying herbicides within the correct temperature range can improve the control in populations known to have a level of resistance to that herbicide.”
Dr Chris Preston suggests testing whole plants rather than seed for responses to a range of post-emergent herbicides. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions.
Dr Preston says the effect of frost on the efficacy of clethodim is a striking example. Spraying clethodim in non-frosty conditions achieves vastly better results than spraying after three days of frost, even on populations that are resistant to this chemical mode of action.
“Combining the optimal temperature with optimal weed size will give the best results possible,” he says. “The current common practice of applying clethodim to tillered ryegrass in the coldest months is not making the best use of this herbicide.”
As a general rule of thumb, Group A (fops), paraquat (Group L) and glyphosate (Group M) are more effective at lower temperatures while Group A (dims), atrazine (Group C) and glufosinate (Group N) are more effective at higher temperatures. However, weeds that are resistant to paraquat become less resistant in warmer temperatures.
“The other implication of this research is the effect of ambient temperature on herbicide test results,” says Dr Preston. “Seed collected in winter and grown out in the glasshouse in summer will be tested for resistance in conditions that are not representative of field conditions when growers are next treating that weed species. The Quick-Test using whole plants overcomes this problem and improves the reliability of herbicide susceptibility testing.”
How can I get the best performance out of clethodim?
Short answer: Avoid applying clethodim during frosty periods.
Longer answer: Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim.
Planning to apply clethodim for grass control outside the coldest months of June and July, and avoiding night spraying in winter, will see better results in both resistant and susceptible populations, particularly in tillered plants. Clethodim is most active when temperatures are over 20 degrees C.
Weed seed that is tested during summer may return false negative results, which could translate into spray failure in the field the next season.
Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim.
When it is it too hot for glyphosate?
Short answer: Efficacy is much better at 20 degrees C than at 30 degrees C.
Longer answer: Spraying glyphosate resistant barnyard grass at lower temperatures is more effective than under hotter conditions. If barnyard grass is tested for herbicide resistance during the cooler parts of the year it may appear susceptible to the field rate of glyphosate but then when this rate is applied to the population in summer there may be many survivors.
When glyphosate is taken up rapidly it tends to limit its own translocation, which can mean that although symptoms may appear more rapidly in warmer temperatures, plant kill is less reliable.
Which herbicide resistance test should I use?
Short answer: The weed resistance Quick-Test for post-emergent herbicides.
Longer answer: The Quick-Test involves testing whole plants rather than seed for responses to a range of herbicides and rates. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. The results of the Quick-Test are available within the same season, potentially giving growers an opportunity to apply an effective weed control tactic before the end of the season. The Quick-Test is not available for many pre-emergent herbicides.
The Quick-Test is available through Plant Science Consulting and results are normally available after four weeks.
Relevant links

Maximising clethodim performance and the impact of frost fact sheet
Keeping clethodim working in broafleaf crops
Plant Science Consulting herbicide resistance testing – Quick-Test
GRDC Update Paper – New developments and understanding in resistance mechanisms and management

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