• 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.
Herbicide, efficacy, retained stubble, integrated weed management,
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.
To measure the impacts of stubble management techniques, (burn, incorporate, retain) on Sakura (pyroxasulfone) efficacy comparing disc & tyne seeding equipment
- 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 – Spread trash evenly across header swathe to minimise ground cover
Harvest high to minimise amounts of trash to spread, LRZ growers are using stripper fronts for this purpose.
Limit paddock traffic to keep stubble upright and avoid wide spread compaction. Matching seeder, boom and header widths will reduce surface compaction to smaller areas. CTF is the ideal scenario.
Autumn – Limit grazing to keep stubble upright in heavy stubble situations
Control volunteers & summer weeds to prevent increases in dry matter that can intercept pre-emergent sprays
Sowing – Inter-row seeding will leave stubble standing and reduce herbicide capture post sowing
Low soil disturbance at seeding will minimise weed seed germinations, zero-till
Consistent seed depth and sufficient furrow closure will avoid crop safety issues
Wider row spaces allows greater penetration for spray applications
Select short stature varieties if available to minimise stubble cover
Spraying – High water rates in combination with coarse droplet size will improve herbicide to soil contact
Select herbicides with higher solubility to allow herbicides to be washed off stubble by rainfall. Trifluralin & Tri-allate are the least soluble the pre-emergents.
Use the upper level of herbicide rate recommendations. Important for Trifluralin as label supports higher rates in high stubble situations.
Boom spray nozzle spacings and spray patterns setup for specific row spaces can improve efficacy in high stubble loads
Caution should be used to avoid crop safety issues when adopting above techniques
Burning – Stubble above 50% ground cover can negatively influence herbicide efficacy. Be flexible in your approach to burning, baling or incorporating stubble in these situations
Ash from burnt stubbles will bind to herbicides reducing efficacy (Using pre-emergent herbicides in conservations framing systems – Haskins). It can be concentrated into seed furrows by wind & rain or removed entirely from the paddock surface. Aim for warmer burns weeks before sowing to reduce this impact.
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
This research 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’).