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How can I avoid getting stuck in an imi herbicide cycle?

with Sarah Wilson, Senior Technical Services Specialist, BASF

Group B herbicides are very handy tools in the weed control toolkit, but weeds can evolve resistance relatively quickly to their mode of action. To keep these herbicides as an option, and to maximise the benefits of imi-tolerant crops, it is essential that they are used correctly within farming systems.

Senior Technical Services Specialist with BASF, Sarah Wilson says it is easy for growers to get caught up in the imi-cycle of using imi-tolerant crops to avoid plant-back issues with imi residuals in the soil.

Senior Technical Services Specialist with BASF, Sarah Wilson says it is easy for growers to get caught up in the imi-cycle of using imi-tolerant crops to avoid plant-back issues with imi residuals in the soil.

“The problem arises when an imi-tolerant crop is sown to avoid imi-residues from the previous crop or fallow, but then the grower also wants to use the imi chemistry in the crop,” she says. “This leads very quickly to over-use of the imi herbicides, and research has shown that as few as four applications of group B herbicides (to which imi herbicides belong) to the same population of weeds can result in the selection of resistant individuals, so resistance can evolve within a very short period of time.”

In Australia there are four imidazolinone or ‘imi-type’ active ingredients registered to control a variety of grass and broadleaf weeds in crops and fallow. These actives are imazamox (e.g. Intervix®*, Raptor®), imazapic (e.g. Bobcat I-Maxx®*, Flame®, Midas®*, OnDuty®*), imazapyr (Arsenal Xpress®*, Intervix®*, Lightning®*, Midas®*, OnDuty®*) and imazethapyr (Lightning®*, Spinnaker®). The other types of herbicides in Group B are the pyrimidinylthiobenzoates, sulfonylureas (SUs) and triazolopyrimidines herbicides. They all inhibit the plant’s production of specific essential proteins.

Use the WeedSmart Big 6 to develop an integrated weed control program that keeps Group B herbicides as a viable option well into the future.

*products that contain more than one active.

How do Group B herbicides work?

Short answer: The Group B herbicides, including the imis, interfere with the activity of the ALS enzyme that is used in the production of certain essential plant proteins.

Longer answer: The Group B mode of action is to inhibit the production of the acetolactate synthase (ALS) enzyme in the plant cells. This enzyme is needed to produce essential plant proteins. By inhibiting ALS production, a foliar herbicide application causes the plant to deplete its supplies of the essential proteins and the plant will slowly die, often taking about three weeks. Group B herbicides with residual activity inhibit the production of amino acids so the plant uses up the reserves in the seed as it germinates and is exhausted before it breaks through the surface of the soil.

Whether using Group B herbicides as a pre-emergent, or post-emergent application; consider the use of registered tank mixes with herbicides from other modes of action.

What conditions do Group B herbicides need to work best?

Short answer: Small weed size is critical for effective foliar application. Imis will not kill older weeds, so applying these herbicides to large weeds is a waste of money.

Longer answer: ALS concentration is highest in young plant tissue and so foliar Group B herbicides are most effective when plants are small and actively growing. When plants are moisture-stressed there will be reduced uptake and translocation of foliar-applied imis.

Foliar uptake of imis is very sensitive to high temperatures. In summer, temperatures in the 30s will require much more active ingredient for the same level of control achieved at lower temperatures. Follow the label instructions.

There is a wide range of soil characteristics and environmental conditions that affect the efficacy of soil-applied Group B herbicides.

What are recommended Group B use patterns?

Short answer: Apply no more than two (2) Group B herbicides in any four (4) year period on the same paddock, and choose the right product for the situation.

Longer answer: A Group B herbicide application in either a summer crop or summer fallow is equivalent to a winter crop pre-emergent application, so no further Group B applications should be made in that paddock, that year.

Use Group B herbicides strategically, if you use imazapic (Flame) in the summer fallow and Ally, Logran, Atlantis or Intervix (for imi-tolerant cereals) over the top of your cereal crop in the winter, you then need to wait three years before using any other Group B chemistry in that paddock.

If you are planting imi-tolerant varieties to get around an imi residue problem, do not use imi chemistry over the top – it’s not good practice for resistance management and you will get stuck in the imi-cycle!

ALWAYS READ AND FOLLOW LABEL INSTRUCTIONS.

What are my options if there’s sufficient planting rain but the plant-back requirements for the Group B herbicide haven’t been met?

Short answer: Consider planting a Clearfield or imi-tolerant crop, but try to avoid using imis or other Group Bs in-crop. Imazapic (e.g. Flame) applied in a summer fallow is cheap and effective, but it will have implications for crop rotation flexibility.

Longer answer: Imis have a broad range of soil binding characteristics and the period of residual decay varies markedly. Microbial activity is the primary mechanism for breakdown of soil-applied imis. Consequently, soil moisture and temperature play an extremely important role in how long the herbicide remains effective in the soil and when it is safe to plant a sensitive crop.

Source: Soil behaviour of pre-emergent herbicides in Australian farming systems (GRDC)

Even if the residual has not broken down sufficiently to safely plant sensitive crops, there may be poor weed control due to sub-lethal amounts of herbicide remaining in the soil. This scenario represents a serious risk of partially-resistant weeds setting seed. Other weed control options must be set in place to control weed escapes.

While Clearfield and imi-tolerant crops are the most tolerant crops available, there are several non-Clearfield crops, such as chickpea, field pea, mungbean, peanut and soybean that have a degree of natural tolerance to imi herbicides. Look for a safe option that also enables the use of non-Group B herbicides and or cultural methods to manage weeds in-crop. If you need to use a pre-emergent, be sure to choose from an alternative herbicide MOA group.

Other resources:

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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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