Effective management of weeds in retained stubble systems is important because it means that growers can take advantage of the many benefits associated with stubble and residue cover, without suffering losses associated with poor weed control.
Take home messages:
- Herbicides known to bind to stubble include (from greatest to least): trifluralin (Treflan®) > prosulfocarb (Boxer Gold) > metolachlor (Boxer Gold)* > pyroxasulfone (Sakura)*
*residue can be washed off with rainfall.
- There appears to be no set stubble loads that can inhibit adequate penetration of herbicides applied at sowing time; product choice, water rates and rotations are all major factors.
- Killing weeds in-season prior to seed set is paramount – right time, right product, right conditions, right rate.
- Spray fencelines separately to increase chemical rotation options.
- Match seeder headlands to sprayed headlands to ensure the direction of travel is running with the stubble row.
- Match product type (pre-emergent) to ground cover per cent.
Over the last ten years farming systems have continued to evolve and this has changed the way we manage weeds, particularly in stubble retained systems. During the Millennium Drought stubble retention was widely adopted. When the drought broke in 2010, with a wetter-than-average season, stubble loads increased, challenging growers in terms of weed control in stubble residues.
With fewer paddocks being routinely cultivated, the weed spectrum has changed as certain species are longer being controlled by tillage. With the adoption of no-till farming, paddocks can be burdened with these weeds. Subsequently, more growers have adopted a chemical approach to weed control.
Effective management of weeds in these systems is important because it means that growers can take advantage of the many benefits associated with stubble and residue cover, without suffering losses associated with poor weed control.
Problem weeds in retained stubble systems
Weeds which are both prevalent and problematic in Victorian stubble retained systems include ryegrass, brome grass, wild radish and mustard. Herbicides commonly used to control those weeds can lose efficacy in stubble retained systems as they can bind to the stubble and residues can act as a barrier between the herbicide and the target.
Understanding the weed’s growth habit and pattern is essential to managing the weed and to delay the development of herbicide resistance a combination of chemical and non-chemical weed control strategies are recommended. Reliance on herbicides alone will invariably lead to the development of herbicide resistance. A combination of both chemical and non-chemical management strategies that are tailored to maximise control of the weed based on its behaviour is required to manage these weeds effectively and delay the onset of resistance.
Certain weeds have become more problematic in stubble retained systems such as fumitory, tares and marshmallow. Bind weed is also becoming a problem. While these weeds are not widespread, and do not have herbicide resistance to date, use of ‘effective’ herbicides at the most appropriate time is critical.
Stubble creates a physical barrier between pre-emergent sprays and target weeds. Spray efficacy can be further reduced if the stubble has been flattened by machinery or grazing livestock. Pre-emergent sprays are particularly sensitive to stubble tie-up (where the chemical becomes bound to stubble and then unavailable). Under high stubble loads (more than 40-50% soil cover) pre-emergent herbicides become less effective.
The key things to remember are:
- Where higher stubble loads are present growers need a good stubble management plan. Even if you choose to burn stubble off, the ash residue can also create a physical barrier to spray.
- Weed residue from previous spray applications also shield the soil and can prevent spray droplets from reaching the soil surface evenly. Pre-emergent herbicides may bind to weed residues and should be viewed with the same importance as stubble.
Herbicides that suit your system
With a heavy reliance on pre-emergent herbicides in cropping rotations which, particularly in low and medium rainfall zones are increasingly cereal dominated, it is important that we understand how various products work in our farming systems.
Adsorption is defined as adhesion of chemical onto the surface of particles; this may be stubble residue and or weeds. Sorption, on the other hand, is the physical and chemical process by which one substance becomes attached to another i.e. in the case of chemical being attached to stubble residue. Pyroxasulfone (for example) has a relatively low rating in terms of binding to stubble (Table 1). The less soluable the herbicide the more binding that will occur i.e. Trifluralin. Keep in mind when using products with greater solubility there is a requirement for rainfall to activate them and wash them into the root zone.
Improving herbicide application in retained stubble
Weed burdens that result from maintaining stubbles can be managed by understanding how to adjust pre-emergent spray applications to better suit stubble loads. Consider:
- Water rates – for pre-emergent spraying 70L/ha is recommended in high stubble loads. There is no specific stubble load under which you will then have to change your water rate, it is all dependent upon your individual system, i.e. row spacing, nozzle spacing, height of boom from stubble.
- Nozzle type – twin fan nozzles are now more commonly used as they create a slight angle which increases the potential of the spray to hit the side of the leaf. To increase coverage, these nozzles can be placed at a 30 degree angle facing forwards with every second fan placed on a 30 degree angle facing backwards. Bill Gordon – spraying in stubble videos.
- According to spray consultant Craig Day (Spray Safe & Save), “Using a nozzle with less than a 110 degree spray pattern and raising the boom height (from 50cm to above the target to 80cm above the target) will, in theory, see more droplets pass the stubble and reach the soil.
Selecting a sprayer
Are different sprayers more appropriate? At the 2014 BCG Expo a panel discussed the pros and cons of tow-behind and self-propelled spray machinery. You can listen to the full discussion here.
To summarise, travelling faster when spraying will result in finer droplets, while a slower speed will produce coarser droplets.
Some growers have suggested that self-propelled booms do not perform as well as a trailing boom. This can sometimes be attributed to incorrect set up or selecting the wrong nozzle for the speed they wish to travel. Adjusting water rates and bar pressure in accordance with the speed you would like to travel can overcome some spraying issues.
Herbicide application in retained stubble systems is one of 17 key areas being examined through this project.