- Winter grain retained for crop seed needs to be effectively cleaned to avoid weed contamination.
- Significant numbers of weed seeds often remain in cleaned grain, especially from annual ryegrass (Lolium rigidum) and wild radish (Raphanus raphanistrum L.).
- A high proportion of these seeds come from weeds that are resistant to a range of commonly used post-emergent herbicides.
- Testing weed seeds to assess herbicide resistance status can ensure effective herbicides are applied in-crop.
- This can be part of an integrated weed control strategy that uses multiple non-herbicide and herbicide tactics in each cropping season to prevent weed and weed seed blow-outs.
Levels of weed seed contamination in retained grain, even after cleaning operations, are often under-estimated. But some WA growers are able to achieve clean seed and this is a worthy goal.
Research shows many weed seeds that contaminate grain at harvest are from populations resistant to a range of commonly used post-emergent herbicides.
This risks further spread of herbicide resistant weeds in cropping paddocks during the next season and highlights the importance of effective seed cleaning and weed seed herbicide resistance testing.
Cleaning and contamination
A survey of 74 WA farms in 2008 by the GRDC-supported Australian Herbicide Resistance Initiative (AHRI) found most growers undertook seed cleaning.
Of the cleaned crop samples in the survey, 27% were weed-free and 73% had some degree of weed seed contamination.
The average weed seed contamination from cleaned grain was 62 weed seeds (from 11 foreign weed and volunteer crop species) per 10 kilogram grain sample.
In the paddock, this equated to 465 foreign seed contaminants per hectare (based on an average seeding rate of 75 kilograms per hectare) – many of which were likely to be herbicide resistant.
Crop samples collected in the survey that were not cleaned had 25 times higher levels of weed seed contamination, at an average of 1545 weed seeds per 10kg grain sample.
The most common weed contaminants of grain samples in the survey were annual ryegrass (Lolium rigidum) at 45%; wild radish (Raphanus raphanistrum L.) at 30%; brome grass (Bromus spp.) at 29%; and wild oats (Avena fatua L.) at 24%.
Sowing crop seed that is contaminated with weed seeds – even at low levels – will have long term impacts on weed control and overall farm business productivity.
Seed cleaning methods
The AHRI survey found using a gravity table led to the lowest levels of weed seed contamination in grain samples across all crop types.
A rotary screen and a combination of more than one cleaning method were the next most effective, followed by sieves. Cleaning by external contractors produced better results than self-cleaning by growers.
Herbicide resistance risks
The bulk of annual ryegrass populations found in crop grain samples studied by AHRI were resistant to Group A (89%) and Group B (90%) herbicides.
Most wild radish sample populations from the AHRI survey were resistant to Group B (78%) herbicides.
During the past six years, herbicide resistance testing by Charles Sturt University (CSU) in NSW, Dr Peter Boutsalis’ Plant Science Consulting (PSC) and The University of Adelaide has also found high levels of herbicide resistance in annual ryegrass samples submitted for testing.
Testing typically shows strong resistance to Group A & B herbicides, but lesser rates of glyphosate resistance in samples for which glyphosate testing is requested. However, the incidence of glyphosate resistance is increasing.
Weed seed herbicide resistance testing services
Testing weed seeds at or after harvest will provide a baseline status of herbicide resistance for each paddock and indicate which herbicides will and won’t work next year.
This is recommended as part of an integrated weed management (IWM) and monitoring strategy, as promoted by the national grains industry’s WeedSmart campaign. This is aimed at combating herbicide resistance and more information can be found at: www.weedsmart.org.au
Resistance testing to a wide range of herbicides is offered by PSC and CSU.
A single herbicide test typically starts at about $125 and it costs about $350-400 for cross resistance tests (usually four to five herbicides and/or the addition of glyphosate).
Tips for testing weed seed:
- Collect at least one cup full of clean weed seed – or an envelope full of seed heads (more for wild radish and wild oats).
- If sampling a patch, take samples from plants by hand.
- To measure the average resistance status of the whole paddock, samples can be taken from the header grain tank.
- Sample each paddock separately.
- Put the sample(s) in a sealed bag or paper envelope for dry seeds.
- Include some identification, including herbicides to be tested and paddock details - such as herbicide history and crop rotations.
- Put the sample or samples in an envelope and send as soon as possible to CSU or PSC to avoid seed deterioration.
- Tests take 4-12 weeks, so look out for results from the end of March to mid-April and discuss these with an agronomist or advisor if necessary.
Weed hygiene and biosecurity
It may be impossible to completely remove all weed seeds from grain at harvest, but aiming for 100 per cent clean seed is a worthy goal to strive for.
Strategies to achieve weed-free retained seed include:
- Correct seed cleaning at harvest
- Obtaining seed from weed-free paddocks
- Using good farm hygiene and biosecurity measures, including clean harvesting and tillage equipment.
Coupled with good IWM during the cropping phase and rotation planning, these steps can help to reduce weed populations and extend the life of herbicides.
Peter Newman, AHRI, 08 9964 1170, email@example.com
John Broster, CSU, 02 6933 4001, firstname.lastname@example.org
Peter Boutsalis, PSC, 040 066 4460, email@example.com
National grains industry WeedSmart hub: www.weedsmart.org.au
Australian Herbicide Resistance Initiative (AHRI): www.ahri.uwa.edu.au
AHRI crop seed contamination survey: www.ahri.uwa.edu.au/Research/Surveys
Audio: Hear John Broster talking about herbicide resistance testing: www.grdc.com.au/Podcast-HerbicideResistanceTesting2013
PSC YouTube video: Seed Resistance Testing: http://youtu.be/aSNj-5p9tkk
Sending weed seed samples for testing to CSU: Herbicide Resistance Testing, School of Agricultural and Wine Sciences, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW, 2678. Website: www.csu.edu.au
Sending weed seed samples for testing to Plant Science Consulting: Log in via the website: www.plantscienceconsulting.com.au and send to PSC, 22 Linley Avenue, Prospect, SA, 5082.
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