• Standing stubble and processed stubble trash can inhibit efficacy of herbicides by 
preventing herbicide contact with target weeds & soil
• Herbicides are one part of integrated weed management systems and cannot be the sole control measure. To avoid herbicide resistance and successfully manage weeds an IWM approach is vital
• Harvest and post-harvest operations are important HRZ considerations to increase herbicide efficacy in summer and pre-sowing herbicide applications in the next season
KEY WORDS:
Herbicide, efficacy, retained stubble, integrated weed management,
Background
Successful herbicide applications in a retained stubble systems require careful planning and management. This is even more important in the high rainfall zones (HRZ) of southern Victoria and Tasmania. Heavy stubble loads, upwards of 10t/ha, regularly impact on herbicide efficacy. Reliance on herbicides alone will not successfully control weeds in retained stubble situations and an integrated weed management plan that uses cultural and mechanical control measures is vital to ongoing success.
There has been a large body of work done in this area across many regions which extrapolate well to the HRZ. Barry Haskins NSW DPI conducted extensive research into efficacy of pre-emergent herbicides in conservation farming systems. (see reference area) Areas covered include stubble architecture, harvest setup, crop safety, soil throw, seed depth, furrow closure, herbicide rate, solubility and volatility.
SFS conducted one specific herbicide efficacy trial comparing retained stubble with incorporated stubble and burnt stubble. Each treatment was then applied with 118gm/ha of Sakura pre-emergent herbicide. They were then sown using both a tyne with press wheels seeder and a Vaderstaad Rapid disc seeding system. The Vaderstaad Rapid has offset coulters in front of the seeding disc’s which gave an excessive degree of soil incorporation.
Aim
To measure the impacts of stubble management techniques, (burn, incorporate, retain) on Sakura (pyroxasulfone) efficacy comparing disc & tyne seeding equipment
Trial details:
- Site – Streatham
- 2014 rainfall: 388mm (Avg annual 621mm)
- Soil – sandy loamy over clay
- Variety – Beaufort wheat
- Sown – 5 June 2014
- Treatments – Burn v Incorporated v Retained stubble 3.8t/ha
- Equipment – Vaderstaad Rapid disc v tyne & press wheel
- Assessments – herbicide efficacy, crop safety, NDVI, yield
RESULTS AND INTERPRETATION
The SFS trial work did not provide reliable information on which growers could base management decisions. The Rapid disc coulters cultivated the soil in front of the seeding discs and provided a high degree of soil incorporation. Establishment was also vastly different and appears to have had a negative influence on grain yields & weight because of low rainfall during grain fill. This system is vastly different to common single disc openers that have a very low level of soil disturbance.
Work ongoing in 2015 is examining herbicide efficacy in stubble comparing a JD single disc opener with a Morris contour drill with tyne & press wheel setup. The disc seeder has sown into a range of stubble management practices. Weed control scores will be collected and reported at a later date.
Training was provided for growers on spray application management by Bill Gordon a leading expert in this field. The training examined boom spray setups in relation to nozzle selection, recommended pressures, spray drift reduction and water rates. Craig Day, Spray Safe, has been engaged to conduct further training in spring 2015. Craig will focus specifically on herbicide application techniques that improve herbicide efficacy in retained stubble situations.
Key factors for successful herbicide applications in retained stubble:
Stubble & trash provides a physical barrier that prevents herbicide reaching the soil or target weeds. This can be managed in several ways during the production cycle
Harvest
To minimise the amount of ground cover, it is vital for trash to be spread evenly across the header swathe. If header trails become too thick to achieve good herbicide efficacy, consider using stripper fronts or windrow burning to ensure good herbicide and soil/weed contact. The stubble takes up a greater surface area when lodged when compared to standing stubble, so the use of controlled traffic farming is ideal for this scenario because the limited paddock traffic will keep the stubble upright and avoid wide spread compaction. Using pre and post-harvest weed seed control such as spray topping, a Harrington seed destructor (HSD) or chaff carts can reduce the weed seed bank for the subsequent year.
Autumn
Grazing should be limited throughout the Autumn period so that the stubble can remain upright in high stubble load situations, especially when the succeeding crop is canola, which can suffer from shading. If grazing is necessary, graze paddocks that will have good post emergence herbicide options. Controlling volunteer and summer weeds will prevent an increase in dry matter that will consequently intercept pre-emergence sprays.
Sowing
Farmers using zero-till practices will have fewer weed seed germinations due to minimal soil disturbance at seeding.
Wider row spacing’s allow for greater penetration of spray applications, this combined with short statured varieties will minimise stubble trash during sowing and consistent seeding depth and sufficient furrow closure will avoid crop safety issues. Rotating crops and utilising a broad sowing window (also beneficial for risk management) during sowing allows for a varied use of herbicide groups and knockdown sprays. If a particular weed cannot be controlled without the use of herbicides and managing the rotation of mode of action, then burning stubble should be considered.
Spraying
High water rates combined with coarse droplet size will improve the penetration of chemicals through stubble which will improve the efficacy of herbicide to soil contact. The selection of highly soluble herbicides that can be washed off the stubble onto the soil by rainfall is good practice. Trifluralin and triallate are the least soluble pre-emergent herbicides. If using trifluralin the label supports the usage of a higher rate especially in high stubble load situations. Using the correct boom spray nozzle spacing’s and spray patterns setup up for a specific row spacing, the efficacy can be improved in high stubble load situations. It should be noted to be cautious when applying the above techniques to avoid crop safety issues.
Burning
When stubble covers >50% of the ground, herbicide efficacy will be reduced due to stubble interception of spray droplets. Additionally, ash from burnt stubble residue will bind to herbicides, again reducing the efficacy, therefore ash should be concentrated into seed furrows by wind and rain or removed entirely from the paddock surface. To reduce ash, aim for hotter burns weeks before sowing to reduce herbicide tie up in the ash. When herbicide is applied to ash there is an increased chance that the herbicide tied up within the ash can be transported during windy conditions.
Further information
References:
By – Adelaide University, Dr Sam Kleeman
Title – Seeding systems & pre-emergent herbicides
By – FarmLink Research – Paul Breust, Grassroots Agronomy – Greg Condon
Title – Disc v’s Tyne Illabo 2013
By Paul Breust
Research and trials manager
Southern Farming Systems
E: pbreust@sfs.org.au
This rese
arch is being conducted by SFS as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 ‘Maintaining profitable farming systems with retained stubble in Victoria and Tasmania’).
