Herbicides are an essential tool enabling cost-effective management of weeds as part of an integrated strategy. Herbicides represent a major input cost for grain growers.
Hence, understanding how herbicides work and the factors which impact their usefulness is critical to maximise the effectiveness of these valuable tools.
Although widely used, herbicides are complex and optimising their effectiveness can be challenging. Maximising herbicide performance becomes especially critical as herbicide resistance increases.
Understanding the science behind how herbicides work in different environments also helps enable forensic evaluation of the causes of spray failures when they occur.
To assist growers and advisers with their decision-making, the Grains Research and Development Corporation has developed, and will continue to produce, resources focused on optimising herbicide use through fostering a greater understanding of their behaviour.
Reference Manuals and Guides
Rotational Crop Constraints For Herbicides
Herbicides designed to be used for residual weed control need to provide weeks or months of persistence in the soil to achieve their desired weed control objectives. Additionally, some herbicides that typically are used for post-emergent weed control and which are not typically considered ‘residual’ herbicides may also persist in the soil for some time after application.
It is important to understand the soil persistence of a herbicide and its potential impact on following crops in the rotation prior to the herbicides’ use.
This guide has been developed to provide grain growers and advisers with relevant information to assist in planning the use of herbicides into crop sequences and in managing rotation constraints.
Soil Behaviour Of Pre-Emergent Herbicides
To understand how pre-emergent herbicides perform, it is important to know the properties of the herbicide, the soil type and how it is broken down in the environment. Availability of a pre-emergent herbicide is an interaction between the solubility of the herbicide; how tightly it is bound onto soil colloids and organic matter; soil factors such as structure, cation exchange capacity and pH; herbicide volatility; the environment and particularly soil water and the rate of herbicide applied.
Understanding Post-Emergent Herbicide Weed Control
Since the advent of herbicides for weed control, growers have readily adopted herbicide tactics, as they are generally a very robust and cost-effective way of managing weed populations in cropping situations.
Following many seasons of extensive herbicide use, Australian farming systems have selected for herbicide resistance in key weed species. This has resulted in many situations where a particular herbicide, or mode of action group, is no longer effective on a particular weed population, or other situations where resistance is developing in the paddock and the herbicide may now be only providing a partial level of control.
Summer Fallow Weed Management Manual
How farming country is managed in the months or years before sowing can be more important in lifting water use efﬁciency (WUE) than in-crop management. Of particularly high impact are strategies that increase soil capture and storage of fallow rainfall to improve crop reliability and yield.
While many factors inﬂuence how much plant available water is stored in a fallow period, good weed management consistently has the greatest impact.
In Australia, more than 400 branded products are registered for use as spray adjuvants (208), surfactants (72) or wetting agents (165) (APVMA Pubcris). These include about 30 different ‘active ingredients’, some of which are combined in individual products to provide different functions.
Spray adjuvants are used within formulations and in tank mixes to improve the efficiency and effectiveness of agricultural chemical application. The Grains Research and Development Corporation (GRDC) has developed this publication to equip consultants, agronomists and growers with a greater understanding of adjuvant attributes and their properties.
Mixing Knockdown Partners With Group G Herbicides
The use of Group G (PPO inhibitor) herbicides has increased in recent years as users are looking to put more diversity into their fallow weed control programs. Most Group G herbicides require the addition of a non-selective knockdown partner when used in fallow or pre-sowing. Most commonly the choice of partner herbicide is a decision between either glyphosate or a paraquat based herbicide.
Glyphosate, a systemic herbicide that takes time to enter and translocate throughout the weed, may not always be the ideal mixing partner for a fast-acting Group G contact herbicide that has limited ability to translocate within the weed.
Rotational Constraints For Pulse Crops
Herbicides from the Group I Mode of Action are effective knockdown herbicides against many broadleaf weeds. However, care must be taken as several have significant soil residual properties with implications for rotational crops.
The potential for residual effects causing crop injury to following pulse/legume crops in the crop sequence is particularly high with some herbicides from the pyridine sub class of Group I herbicides. Their capacity to persist and damage subsequent crops is complicated by how these herbicides persist in crop residues.
When applied in winter cereals, aminopyralid, clopyralid and picloram based herbicides can provide knockdown of a range of broadleaf weeds. Additionally, these winter applications may also provide useful residual control of spring germinations of weeds such as fleabane.
Optimising Group A Herbicides In Canola
Group A herbicides were first commercialised in Australia in the early to mid 1980s.
In the absence of herbicide resistance, these herbicides quickly and effectively shut down growth of small grass weeds. This removes competition within days of application, despite often taking a number of weeks for herbicide symptoms to become visible.
Several different Group A herbicides are used in Australia. Those that are registered for use in broadleaf crops such as canola are from the aryloxyphenoxypropionate (fop) or cyclohexanediones (dim) sub-classes. Other Group A herbicides used in certain cereal crops are not covered in this Fact Sheet.
Keeping sorghum safe when using metolachlor-based herbicides
Weed competition during the establishment phase of sorghum can result in poor establishment, uneven maturity and harvest delays. Where weeds compete with the crop for nutrients and moisture, a yield
penalty often occurs.
A limited range of post-emergent herbicide options are available for the control of broadleaf weeds in sorghum. However reliable and effective post-emergent herbicide options for grass weed control are limited to either inter-row cultivation, chipping or some suppression via high application rates of atrazine.
Application considerations for in-crop herbicide use
The mode of action (MOA) of the chemical being applied and the crop situation determine the water volume and droplet size and how a sprayer should be set up. It is also important to know how a product enters the plant and how it is translocated within the plant to determine where the spray coverage is required and what adjuvants may assist with the uptake of the product into the plant.
Crop growth stage, canopy size and stubble load will influence decisions about nozzle selection, application volume and sprayer operating parameters.
Effective Double Knock herbicide applications
A Double Knock (sometimes called sequential applications) is where growers apply two different weed control tactics to a single flush of weeds to stop any survivors from the first application setting seed. The tactics do not need to be herbicides. Cultivation, heavy grazing or fire could also be used as a second knock.
By using a Double Knock approach in fallow paddocks, growers have a reliable technique to control difficult weeds while also assisting in the management of herbicide resistance.
Double Knock herbicide strategies are critical to reducing weed seed banks.
Using Group A herbicides for control of grass weeds in fallow
With the increasing frequency of glyphosate resistance, growers require alternative solutions to control grass weeds in fallow. The use of Group A herbicides in fallow to control grass weeds such as the Chloris species – feathertop Rhodes grass (Chloris virgata) and windmill grass (Chloris truncata) – has shown to be a viable option for controlling young weeds.
However, Group A herbicides have a high frequency of resistant individuals in unselected populations, so will quickly develop resistance if survivors are not controlled.
Stewardship for pre-harvest application of herbicides in winter crops
The responsibility to avoid herbicide residues in delivered cereal, pulse and oilseed grains sits squarely with grain growers and their advisers.
The application of herbicides late in the season to prevent weeds setting seed or to desiccate crops must be carried out with caution and in line with herbicide label recommendations. It is essential to check if these practices are acceptable to buyers, as in some situations markets have extremely low or even zero tolerance to some pesticide and herbicide residues.
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