Timing the incorporation of pre-emergent chemicals - Elders Rural Services

Timing the incorporation of pre-emergent chemicals

Without incorporation, many active constituents have a greater disposition for breakdown and loss due to issues such as volatilisation and photodegradation.

Incorporation of chemicals is commonly performed through four methods: full cut mechanical, incorporation by sowing (IBS), irrigation and rainfall.

Volatilisation

Without proper incorporation, volatilisation can occur, especially of chemicals more disposed to this, such as trifluralin. Volatility refers to the likelihood of an applied chemical being converted into a gaseous state, and moving from the original application site, the result being a reduced weed control if left unincorporated in the soil.2

Photodegradation

The flip side of volatilisation is photodegradation. This occurs when an applied chemical reacts to sunlight, causing a breakdown of the chemical and reducing the efficacy of the chemical.3  Whilst normally not a major talking point, as standard incorporation practices are sufficient, application of chemical onto a dry soil surface with insufficient rainfall has the largest effect, especially for group C, B and K herbicides.

Incorporation

Several factors influence how quickly a chemical is lost without incorporation. A chemical’s active ingredients, formulation type, ambient temperature, and humidity all play a large role in influencing volatility.2

Pre-emergent chemicals are an important part of Australian agriculture, as we are heavily reliant on chemical application due to operating under minimum or no-till systems.

Pre-emergent chemicals are intended to be applied to the soil and most require some form of incorporation from rainfall, tillage or seeding operation. The most commonly used pre-emergent chemicals for weed control are prosulfocarb, pyroxasulfone and trifluralin.4

Rainfall and irrigation incorporation

Many pre-emergent chemicals can be applied pre or post-sowing without the need for physical incorporation. These chemicals require a certain amount of moisture present in the soil and more rainfall after application to be most effective.5 These chemicals typically are more soluble and have a lower volatility compared to the less soluble counterparts. In almost every case, with water usually being the limiting factor in Australian cropping systems, incorporation through rainfall is not going to be a viable method, with mechanical incorporation being the most viable option.

Mechanical incorporation

Physical incorporation is the most widely-used method when incorporating chemical into the targeted soil location. The act of mechanically incorporating minimises volatilisation and photodegradation from sunlight5. In the case of mechanical incorporation, it usually takes the form of knifepoint seeding mechanisms.

Depending on the volatility of the chemical in question, timing of incorporation varies between products. Pyroxasulfone’s timing between application and seeding is longer due to its low volatility rate, whilst trifluralin and tri-allate need almost immediate incorporation due to their high volatility.

Case study: Trifluralin volatility and incorporation

Trifluralin has long since been a staple in weed management practices in Australian cropping systems. Trifluralin’s chemical composition causes the product to have a strong affinity for soil and is extremely susceptible to rapid volatilisation, to the extent that it requires good incorporation to the soil to avoid such losses.6

An interesting study produced by Eureka! AgResearch and presented by the GRDC shows how quickly trifluralin is lost without incorporation. It was tested with four different times of incorporation once applied against the mean emergence of annual ryegrass. Rates of trifluralin are relatively low compared to current rates used across Western Australia, but we can extrapolate several important points from the data.

graph showing effective time of trifluralin

 

Figure.1 – Effect of time until incorporation and trifluralin application rate on mean emergence of annual ryegrass (of 25 seeds sown), in an alkaline, sandy soil (Eureka! AgResearch, 2015)

The findings showed that delaying incorporation of trifluralin into the soil significantly increased the required amount of trifluralin needed to produce the same level of control when incorporated immediately.

Current trifluralin rates have increased significantly but the data still shows the benefits of early incorporation of trifluralin to maximise its potential for weed control, and to minimise loss due to volatility.

The following tables shows the ED50 and trifluralin loss with time to incorporation on an alkaline, sandy soil.

 0 hours2 hours24 hours48 hours
ED505775136150
Trifluralin lost0 %24 %58 %62 %

The study also showed that, in 48 hours, as much as 62 per cent of trifluralin was lost due to volatilisation. A gap between application and incorporation as little as two hours meant up to 24pc was lost. When higher rates are applied, the percentage of trifluralin lost without incorporation would be lower than shown in the table above. The information can still be extrapolated to show the effects of a volatile pre-emergent chemical and the importance of timing of incorporation.

Final thoughts

Pre-emergent chemicals remain a staple in weed management for Australian agricultural practices. Managing zero and no-tillage systems comes with its own sets of benefits and problems, and actively understanding the volatility of chemicals can drastically increase the performance of weed control in pre-emergent herbicides.

Because of this, the timing of pre-emergent chemicals should not be overlooked, as correct timing will both produce better weed management and resulting in greater crop yields, and overall help achieve a more profitable farming margin.

 

Elders offers a range of pre-emergent and crop protection products


References

[1]  GRDC – Soil Behaviour of Pre-Emergent Herbicides in Australian Farming Systems. Accessed 2021
[2] A New Approach to Quantify Herbicide Volatility. 2018
[3] GRDC – Soil Behaviour of Pre-Emergent Herbicides in Australian Farming Systems. Accessed 2021
[4] Flower Et. Al. 2019. Rainfall affects leaching of pre-emergent herbicide from wheat residue into the soil
[5] NSW Department of Primary Resources. Accessed 2021
[6] D., R., Wallace. 2014. Triluralin. Environmental Fate and Behavior

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