Plan your attack on the weed seed bank

Here we summarise the key components of the WeedSmart Big 6 strategies and the principles that underpin them. You can also test your plan using AHRI’s Weed Integrated Management (WIM) decision support software for annual ryegrass, brome grass, barley grass, wild oats and barnyard grass.

1. Increase crop competition

A competitive crop will suffer less yield loss at the hands of the weeds, and will also reduce seed set of the weeds compared to an un-competitive crop. In other words more crop, fewer weeds.

Principle #1 Set your crop up to stay ahead of the weeds.

• Sow competitive crop types and varieties.
• Improve soil health (fertility and structure) and crop nutrition, e.g., soil amelioration (if necessary), no-till, stubble retention, nutrient budgeting.
• Plant clean seed and consider grading retained planting seed. Test for germination, vigour and 1000 seed weight.

Principle #2 Adopt at least one competitive strategy (but two is better).

• Target higher plant populations using increased seeding rates.
• Aim for even seed distribution and establishment.
• Utilise early sowing and adopt east/west sowing if practical.
• Reduce row spacing where possible.

2. Rotate crops and pastures

Short rotations speed up herbicide resistance evolution! This is because of the inherent lack of diversity.
Weed populations quickly respond to routine management practices – those that survive will set seed and their progeny have an advantage when the same control tactic is used again soon after.

Principle #1 Diversity in herbicide choices, diversity in cultural practices.

• Target grass weeds in broadleaf crops and broadleaf weeds in cereal crops.
• Add greater diversity to weed management strategies by adopting herbicide tolerance traits.

Principle #2 Create a low weed farming system.

• Use break crops and double break crops, fallow and pasture phases to drive the weed seedbank down over consecutive years.
• In summer cropping systems, use diverse rotations of crops including cereals, pulses, cotton, oilseed crops, millets and fallows.

3. Mix and rotate herbicides

Before herbicide selection has taken place it is very rare for an individual weed to be resistant to two herbicides. Mixing herbicides at full label rates in a single application takes advantage of this fact.

Within an integrated weed control program, try to make sure there is rotation and mixing going on in each phase – in the fallow, pre-seeding, in-crop and for desiccation. When done in conjunction with a determination to stop seed set and remove survivors, it is possible to keep weed numbers low.

Principle #1 Rotating buys you time, mixing buys you shots.

• Rotate between herbicide modes of action groups.
• Mix different modes of action within the same herbicide mix or in consecutive applications.

Principle #2 Follow resistance-prone herbicides with another tactic to control survivors in a fallow or pre-sowing situation and prevent weed seed set.

• Incorporate multiple modes of action in a double knock. For example, glyphosate/Group 1/Group 2 knockdown followed by paraquat and Group 14 and pre-emergent herbicide.
• Consider using a non-herbicide tool after resistance-prone herbicides to prevent seed set of survivors. For example, the ‘Canola Combo’ – crop top followed by harvest weed seed control (HWSC), or herbicide followed by tillage.
• In cotton systems, aim to target both grasses and broadleaf weeds using two non-glyphosate tactics in crop and two non-glyphosate tactics during the summer fallow, and always remove any survivors (2 + 2 & no survivors).

Principle #3 Low dose applications result in herbicide resistance.

• Always use full label rates.

Principle #4 Know what herbicides will and won’t work for you.

• Test weeds for resistance and susceptibility to individual modes of action and mixes.

4. Optimise spray efficacy

Spray drift is of great concern for sensitive crops and environments, along with the fact that if the spray doesn’t hit the intended target, you do your dough, and your weeds live. There are many factors to consider so every spray job is as effective as possible. Make every drop count.

Principle #1 Set up correctly to maximise efficacy and reduce spray drift.

• Follow spray application guidelines and ensure the correct speed, nozzles, water volume, boom height, and adjuvants are used.
• Avoid antagonistic tank mixes.

Principle #2 Hit the target.

• Always use the largest spray droplet feasible that gives the highest efficacy.
• Use the best quality water available. Test water quality and use adjuvants if required.
• Avoid spraying during hazardous inversions (particularly from evening through to early morning), in high temperatures, frost and dew conditions, and when the wind speed is below 5 km/h or above 20 km/h.

4. Stop weed seed set

Reducing the weed seed bank is the ultimate goal of any weed management program and it becomes super-critical once herbicide resistance has gained a foot hold.

Principle #1 Take no prisoners and act early on weedy patches.

• Aim for 100% control of weeds and diligently monitor for survivors in all post weed control inspections.
• Consider hay or silage production, brown manure or long fallow in high-pressure situations.
• Manage crop borders, fence lines and other non-crop areas carefully – avoid evolving resistance in these areas.

Principle #2 Use tactics that work to drive down the weed seed bank after (or to prevent) a blow-out.

• Crop top or pre-harvest spray in crops to manage weedy paddocks.
• Use all appropriate strategies in the pasture phase to reduce the weed seed bank prior to cropping phase.
• Consider shielded spraying, optical spot spraying technology, targeted tillage, inter-row cultivation or chipping.
• Windrow (swath) to collect early shedding weed seed.
• Use two or more different weed control tactics (herbicide or non-herbicide) to control survivors.
• In cotton farming systems, consider late season strategic tillage operations for better overall weed and Helicoverpa pupae control.

6. Implement harvest weed seed control

Collecting and destroying any weed seeds as part of the harvest operation is rapidly gaining traction on Australian grain farms.

Principle #1 Avoid spreading weed seeds at harvest.

• Capture weed seed at harvest using weed seed impact mills, chaff lining, chaff tramlining/decking, chaff carts, narrow windrow burning or bale direct.
• All the six HWSC options are effective – choose the one that suits your system best.
• All HWSC tools (except the seed impact mills and chaff lining) involve some action after harvest to remove or destroy the weed seed collected at harvest.

Principle #2 Ensure optimal harvester set-up.

• Get maximum weed seed into the header. Consider harvest timing and harvest height.
• Adjust the harvester to ensure all the weed seed and the grain lands on the sieves and that the weed seeds stay in the chaff fraction so they are delivered into the harvest weed seed control tool.

WeedSmart Wisdom

• Never cut the rate – always follow label instructions
• Spray well – choose correct nozzles, adjuvants and water rates
• Clean seed – don’t seed resistant weeds
• Clean borders – avoid evolving resistance on the fence line
• Test – know your resistance levels

Key resources

Integrated weed management in Australian cropping systems manual

Integrated weed management (workshop handbook)

AHRI’s WIM decision support software

How to make the most of spring planning sessions

Case study – Curry family, Junee

WeedSmart Big 6

WeedSmart Wisdom

Updated July 2023.