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Get the best results using pre-emergent herbicides

with Chris Preston, University of Adelaide and Mark Congreve, ICAN

Resistance to glyphosate and to a range of post-emergent herbicides is driving the increased use of pre-emergent herbicides in Australian cropping systems, but getting a good result with these herbicides is not always straightforward.   

To assist growers and agronomists, Dr Chris Preston, Professor, Weed Management at The University of Adelaide and Mark Congreve, Senior consultant with Independent Consultants Network Australia, have shared their expertise in the online Diversity Era course ‘Pre-emergent Herbicides 101’.

Presenters Mark Congreve (left) and Dr Chris Preston (centre) with WeedSmart extension agronomist Peter Newman deliver the Pre-emergent herbicides 101 course.

“In the southern farming systems, resistance to in-crop herbicides has been the main driver for the increased use of pre-emergent herbicides,” says Chris. “When pre-emergent herbicides are used to provide early weed control in a competitive crop, the amount of weed seed set in-crop can be vastly reduced.”

In northern cropping systems the main driving force for increased use of pre-emergent herbicides is found in the fallow periods where a rapid increase in glyphosate resistant and glyphosate tolerant weeds in the last five years leaves no-till farmers with few weed control options.

“Northern growers are looking to add pre-emergent herbicides to their fallow management program and cotton and sorghum growers are using lay-by applications of pre-emergent herbicides to control weeds in the inter-row,” says Mark. “The use of pre-emergent herbicides in these situations requires extra care and planning to avoid crop damage and to keep future cropping options open.”

WeedSmart’s ‘Diversity Era Pre-Emergent Herbicides 101’ course can be completed in less than 10 hours, giving you a solid grounding in the basics of how pre-emergent herbicides work and how to get the best results in winter and summer cropping systems.

Follow the link to the free, online Diversity Era Pre-emergent Herbicides 101 course.

Are there different pre-emergent herbicides suited to different situations?

Short answer: Yes. A key difference between pre-emergent herbicide products is their mobility in the soil, which is largely driven by soil type, chemical solubility and level of absorption to soil and organic material.

Longer answer: All pre-emergent herbicides need to reach the soil and create a band of treated soil around the weed seeds to reduce germination. In high stubble situations, and especially where the stubble is laying flat on the soil surface, the first challenge is to get the herbicide through the stubble layer and into the soil. Some herbicides wash off stubble better than others. Products like trifluralin are generally not an effective option in high stubble situations.

In very high residue years growers may decide to burn paddocks prior to sowing and applying pre-emergent herbicide. The herbicide will not bind to the ash, but sometimes there is a layer of chaff left after a stubble burn, and this can still be a barrier to the herbicide reaching the soil.

Once in the soil, some pre-emergent herbicides move more quickly down the profile than others. For example, S-metolachlor (one component of Boxer Gold) and Sakura both have a ‘medium’ rating for binding to soil and organic matter, but quite different solubility ratings. S-metolachlor is reasonably soluble and reaches the weed seeds in the topsoil easily, while Sakura has low solubility, which can mean that in dry years the herbicide might not reach some deeper weed seeds before they germinate.

However, in high rainfall zones or in wet years s-metolachlor is likely to move further horizontally and vertically in the soil profile, increasing the likelihood of coming into contact with the crop seed.

What else do I need to have in place to support pre-emergent herbicides in-crop?

Short answer: Crop competition, harvest weed seed control and double breaks all help make the most of pre-emergent herbicide applications.

Longer answer: Relying on pre-emergent herbicides only is not a good idea in weedy situations. If weed numbers are high it is essential to reduce the weed seed bank using other tactics first. Pre-emergent herbicides are most effective when used to provide early weed control in competitive crops where the crop itself can further suppress weed growth and seed set later in the season.

The extensive and long-term use of in-crop herbicides in southern grains regions has selected for longer dormancy in weeds like annual ryegrass. As a result, growers can make significant gains through sowing early into warmer soils to promote vigorous early crop growth and using a pre-emergent herbicide either incorporated by sowing or, for certain herbicides, applied soon after planting. If soil conditions are warm and moist at sowing it is very important to incorporate the herbicide quickly and realise that microbial degradation can be more rapid in this situation, making strong crop competition even more important.

How does chemical degradation impact on pre-emergent herbicides when used in a summer fallow?

Short answer: Pre-emergent herbicides primarily degrade, or ‘run out’, due to microbial breakdown in the soil. Photodegradation (UV light exposure), volatilisation, leaching and hydrolysis may also be significant loss pathways for certain herbicides.

Longer answer: Soil temperature and moisture are the key drivers for microbial degradation of pre-emergent herbicides. These factors need to be considered as a risk when using pre-emergent herbicides with long plant back periods and the possible impact on rotational crop options if there is insufficient rainfall to allow breakdown of the herbicide.

Sorghum seedling exhibiting symptoms of pre-emergent herbicide damage (S-metolachlor, in association with short term waterlogging). Photo: M Congreve

On the other hand, some pre-emergent herbicides applied soon after harvest may not provide full-season fallow weed control in high rainfall years.

The frequent use of a single pre-emergent herbicide may favour the build-up of the suite of microbes that degrade that herbicide. Over time, pre-emergent herbicide efficacy can ‘run out’ quicker than it did when it was first used.

When applied in a no-till fallow, the pre-emergent herbicide may sit on the soil surface for a period of time awaiting rainfall for incorporation. To minimise losses due to UV light and volatilisation it is important to follow the label instructions for the timing of incorporation by rainfall, irrigation or tillage.

Follow the link to the free, online Diversity Era Pre-emergent Herbicides 101 course.

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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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