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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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What can I do to control large FTR grass in fallow?

Feathertop Rhodes grass (FTR) is a major weed in chemical fallows in Australia, and is notoriously hard to kill with glyphosate.
Bhagirath Chauhan, professor at the University of Queensland’s Centre for Crop Science, says some other herbicide control measures have potential to manage large FTR plants (40 to 50 leaf stage) that have escaped earlier treatment.
Professor Bhagirath Chauhan says there are some tank mixes and herbicide sequences that growers could deploy to help manage FTR and stop seed set.
“Feathertop Rhodes grass is an aggressive weed that can establish in bare fallow situations and produce a large quantitiy of seed if left uncontrolled,” he says. “Several biotypes of this species are resistant to glyphosate and can also survive a double knock of glyphosate followed by paraquat, particularly once the weed is larger than 4 to 5 leaf stage.”
To give growers more options, a study was conducted to assess the potential of other herbicides and use patterns that can control large feathertop Rhodes plants or stop seed set.   
Alternative herbicide options are available to help manage large FTR and reduce seed set in fallow.
“An integrated approach is essential to controlling feathertop Rhodes grass,” says Bhagirath. “In applying the WeedSmart Big 6 to FTR in a bare fallow situation we have identified some tank mix and herbicide sequences that growers could deploy to help manage this difficult weed and stop seed set.”
Can anything be done to improve the efficacy of glyphosate or the double knock against large FTR plants?
In brief: Adjuvants did not improve glyphosate efficacy on mature (40 to 50 leaf) FTR plants. In glyphosate resistant populations, the second knock product is doing the heavy lifting when applied to large (8 to 10 leaf) FTR plants.
The details: None of the commercially available adjuvants improved the efficacy of glyphosate (740 g a.e. per ha) as a single product application on FTR at the 40 to 50 leaf stage. All the plants survived and produced seed after being treated with glyphosate, indicating that the population used in the study was resistant to glyphosate at this rate and weed growth stage.
Glyphosate and the double knock tactic can often provide good control of resistant FTR plants if the herbicide is applied when the plants are small and actively growing.
The traditional double knock of glyphosate (Group 9 [M]) or glyphosate + 2,4-D, followed by paraquat (Group 22 [L]) or glufosinate (Group 10 [N]), applied to older FTR plants (8-10 leaf) achieved increased phytotoxicity through improved mortality, reduced biomass or fewer seed panicles.
However, the double knock was no better than using paraquat or glufosinate alone when applied to 8 to 10 leaf FTR plants. FTR is not listed on glufosinate labels in Australia but is used to control other weeds in fallow situations at the rate (750 g a.i. per ha) tested in this study. For best results, glufosinate needs to be applied in warm, humid conditions, which is not a common scenario for summer fallow situations.
Rate response (0, 187.5, 375 and 750 g a.i. per ha) to glufosinate applied to large FTR plants.
Are clethodim or haloxyfop suitable alternative herbicides to treat large, glyphosate resistant FTR plants?
In brief: Possibly. Excellent results were achieved in pot trials conducted in an open environment, but will be more difficult to achieve in the field.
The details: Clethodim and haloxyfop were tested on FTR plants at the 24 to 28 leaf stage. Clethodim is registered for use against FTR in a number of summer crops, but without any crop competition many FTR plants survived the registered rate (90 g a.i. per ha), although weed biomass and seed production was severely curtailed.
Haloxyfop efficacy against FTR at this growth stage was 100 per cent at the registered rate of 80 g a.i. per ha.
A combination of these two treatments also resulted in 100 per cent control. The effective use of these two herbicides (both Group 1 [A]) relies on excellent coverage and application when the plants are actively growing. This is difficult to achieve in field conditions, which is why the label recommendations are typically for younger weeds.
A combination of clethodim and haloxyfop can provide good control of large feathertop Rhodes plant and curtail seed production.
These herbicides are known to readily select for resistant biotypes so when applied in a chemical fallow situation (with no competition), it is necessary to target small weeds with robust application rates and to apply a second knock with a contact herbicide, such as paraquat. 
Did you find any new and exciting prospects for controlling mature FTR plants?
In brief: Yes, it seems that there is a truly synergistic effect when isoxaflutole (Group 27 [H]; e.g. Balance) is mixed with paraquat.
The details: Neither of these herbicides provided useful control of FTR at the 40 to 50 leaf stage when applied individually. When mixed together, these herbicides achieved a higher level of weed mortality and prevented panicle production. For example, a tank mixture of isoxaflutole 75 g a.i. per ha, with paraquat 600 g a.i. per ha, resulted in 92 per cent FTR mortality and no panicle production.
Even at a paraquat rate of 300 g a.i. per ha mixed with isoxaflutole 75 g a.i. per ha, only 17 per cent of the large FTR plants survived when the mixture was applied to both the plant and the nearby soil – allowing uptake through both the leaves and the roots.
The benefit of this mixture may be reduced if the weed patch is dense, potentially reducing the amount of the isoxaflutole that reaches the soil. Even the prevention of seed set in large FTR plants is of significant value in managing the seed bank of this invasive weed, as FTR seed remains viable for less than 12 months.
Such a use pattern is not currently specified on product labels, although both products are registered for weed control in fallow situations.  
Web resources
Read the research paper.

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Can pulse cover crops tackle multi-resistant ryegrass in irrigated systems?

The best weed control comes from tactics that also bring other benefits to a farming system.
Greg Sefton, principal agronomist with Sefton Agronomics in the Riverina, says multi-resistant annual ryegrass is becoming a major problem in irrigated systems.
Greg Sefton, principal agronomist with Sefton Agronomics in the Riverina, says legume cover cropping is providing effective control of multi-resistant annual ryegrass in irrigated systems.
“Herbicide resistance can move easily through irrigation areas, particularly when the control methods used on the supply channels are limited to just a few herbicides,” he says. “The ryegrass here is generally accepted to have resistance to glyphosate (Group 9 [M]), Group 1 [A] such as clethodim, Group 2 [B] and Group 3 [D], such as trifluralin. Growers are now relying heavily on Group 15 [K] products such as Sakura, and doing their best to rotate out of the problem.”
To regain control, Greg is working with growers to incorporate a multi-purpose fallow crop such as field pea into the system as a winter fallow clean with the added benefit of contributing biological nitrogen into the soil ahead of planting rice or wheat. 
Earlier maturing varieties of field pea provide better weed control options than Kaspa field pea, chickpea and lupin, all of which generally mature later, sometimes after the target weeds have set seed.
“A competitive pulse crop terminated at maximum biomass is an excellent way to reduce weed seed set,” says Greg. “It is a cultural control that also enables the use of some herbicides that are rarely used in our system. Combining the herbicide and cultural methods in the WeedSmart Big 6 is an effective way to keep our cropping options open and to maximise the value of applied water.”

What is the best fit for the legume crop as a winter clean?
In brief: In the Riverina, the optimal place in the rotation is ahead of rice.
The details: Fields selected for rice production are usually bare fallowed for the preceding winter. The aim of the fallow is to control weeds and conserve soil moisture.
Some growers are having success with field pea sown in May as a winter cover crop then terminated for silage or as a brown manure in early September. This fits well with preventing seed set in annual ryegrass, including late germinating plants.
Field pea is a competitive legume and can suppress weed germination and growth when planted in the most competitive configuration possible with minimal soil disturbance and no gaps.
A knockdown treatment of glyphosate (Group 9 [M]), clopyralid (Group 4 [I]) and carfentrazone (Group G [14]) is applied at planting then a mix of pendimethalin (Group 3 [D]), clomazone (Group 13 [Q]) and paraquat (Group 22 [L]) is applied after an irrigation flush to initiate rice germination and prior to rice germination to knockdown both newly emerged barnyard grass (BYG) and persisting ryegrass. This provides a double knock on ryegrass whilst applying a pre-emergent herbicide for barnyard grass in the rice phase.
When implemented once every 4 or 5 years, with a diverse rotation of winter and summer crops in-between, growers can keep a lid on herbicide resistant annual ryegrass populations. 
Field pea is a competitive legume crop that can reduce annual ryegrass germination in the paddock and halt encroachment from the crop borders.
How do you manage weeds on the non-crop areas?
In brief: The same herbicide mix is applied to the whole paddock, including the weeds growing in the check banks.
The details: Weed seed, often carrying herbicide resistance genes, travels easily through irrigation systems and can colonise non-crop areas. Seed from these plants readily infests the cropping areas if not controlled effectively. The control measures used on non-crop zones are often limited to herbicide tactics, so it is important to make sure the herbicide is applied to maximum effect to prevent seed set.
Farm hygiene and physical removal of isolated weeds will also have a positive impact on weed seed production. 
What farming system benefits come with growing a legume cover crop?
In brief: A legume crop grown for biomass rather than grain can improved soil tilth and reduce crusting on some sodic soils. This practice also allows better soil nutrition management and keeps the grower’s options open if the water allocation situation changes.
The details: The field pea crop will fix atmospheric nitrogen and this allows the grower to use 100 to 150 kg/ha less urea to grow the following rice crop without any yield penalty. If there is insufficient irrigation water available for a rice crop, then the fixed nitrogen is still available for a winter crop of canola or wheat.
The phosphorus fertiliser required for rice can be applied when the field pea crop is planted, giving the phosphorus time to become more available in the soil and ready for uptake when the rice is planted.
Field pea is quite drought tolerant, so if irrigation water is not available for rice, the field pea can be grown through to harvest the grain and will usually yield 0.7 t/ha, which can be more profitable than, say, a 1 t/ha drought-affected wheat crop.
Building an integrated farming system based on methods that have multiple benefits is fundamental to staying ahead of weed pressure.
Practical tips for growing field peas as a brown manure crop
Pulses to attack weeds on many fronts

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Can multi-species planting provide effective weed control?

Crop competition is one of the most effective weed control tools available to growers, but some crops simply don’t have a competitive edge.
Dr Andrew Fletcher, a farming systems scientist with CSIRO, says companion planting and intercropping is an option that growers can consider to bolster the competitiveness of an otherwise uncompetitive but valuable crop in the rotation. International research suggests that it can!
Andrew Fletcher, CSIRO farming systems researcher sees potential for multi-species plantings to compete with weeds. Photo: GRDC
“When two or more species are grown together they can occupy ecological niches that might otherwise be taken up by weeds,” he says. “Multi-species plantings have several potential benefits including increased crop yield and improved soil health, but the right combination can also reduce weed biomass by over 50 per cent.”
Multi-species plantings can be quite challenging to integrate into a grain cropping rotation but are more easily used in mixed grain and livestock operations and in intensive pastures for dairy cattle. International research suggests there is a significant untapped opportunity to increase the use of these systems in Australian grain production systems. However, relevant Australian data is scarce and more research is required to understand this untapped potential in Australian systems.    
A mixed-species cover crop can provide multiple soil health benefits, grazing and fodder for livestock and weed control through crop competition and stopping weed seed set.
“Crop competition is a non-herbicide pillar in the WeedSmart Big 6, with the potential to do some serious heavy lifting in terms of weed control,” says Andrew. “Intercropping and companion planting offers a means to bolster the competitiveness of some crops and to keep them in the rotation without risking a weed blow-out.”
What is intercropping, companion planting and mixed-species planting?
In brief: These systems all involve planting two or more crop species together. The combinations are almost limitless.
The details: Intercropping involves planting two or more species together and harvesting the grain of multiple crops. This generally relies on the grain species having different size seed and compatible harvest times.
Companion planting involves two or more species planted together with the intention to harvest grain from one species only after grazing or terminating the other species before seed set.
Sowing a low-growing species like clover between the rows of cereal can compete with weeds in the inter-row area, fix nitrogen and provide the basis of a pasture after the cereal grain is harvested. This is one example of companion planting.
Mixed-species planting is used to describe plantings of several species grown together primarily for the soil health benefits, and that may have potential for grazing and or forage conservation.
How do these systems suppress weed growth?
In brief: These multi-species systems are designed to take up the ecological space that might otherwise present and opportunity for weeds to fill.
The details: Intercropping and companion planting provide additional weed control in situations where one of the species is a relatively poor competitor as a sole crop. By maximising competition, weed growth is suppressed by up to 58 per cent compared to the least competitive species grown on its own. If a competitive crop such as barley is sown in the most competitive configuration possible, there is little additional benefit from adding a second species.
The downside of using this multi-species strategy for weed control is that in-crop herbicide options the choice of herbicides is limited. This is mainly due to the common combinations being a grass crop with a legume or brassica, meaning grass and broadleaf herbicide options can’t be used, except for when one species is terminated. This needs to be factored into decisions around intercropping and companion cropping.  
What are the best-bet combinations for enhanced weed control?
In brief: It depends on the farming system and the other reasons for considering a multi-species planting.
The details: If the aim is to produce grain, the species selected should have easily separated seed. A well-known example is peaola (field pea plus canola). A recent review of historical trials showed that the median yield increase was 31 per cent compared to sole crops of peas and canola, but the weed control effects of peaola in Australia are unquantified.
An effective companion planting combination is wheat undersown with tillage radish and a legume. The broadleaf companions are sprayed out at stem elongation, leaving the cereal to mature through to harvest.
If there is livestock in the farming system, dual purpose combinations such as grazing canola plus vetch and oats can provide excellent weed suppression. This mix could be grazed and then terminated as hay or silage at stem elongation.
Multi-species plantings add a layer of complexity to the farming system, but many growers have taken on the challenge and are reaping the rewards in crop yield, soil health and weed suppression.

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