There are several reasons why weeds might survive a herbicide treatment but it is increasingly common for herbicide resistance to be the culprit.
Testing weeds for herbicide susceptibility and resistance can save growers thousands of dollars, making the investment of a few hundred in testing very worthwhile.
There are two main tests – The Quick Test and The Seed Test
- The Quick Test is done using weed plant samples collected in-crop and provides the results within a few weeks.
- The Seed Test is done on weed seed samples, usually collected around harvest time and the results take a few months.
The Quick Test uses plant samples collected on-farm and sent to the laboratory. The plants are revived and planted into pots, then tested against the required herbicides. The Seed Test requires the collection of ripe seed, which is planted out at the laboratory. After dormancy has been broken and the seedlings have started to grow they are tested for their response to herbicides. Both tests are equally accurate. The Quick Test can not test for resistance to some pre-emergent herbicides, such as trifluralin.
The value of the Quick Test is that you can find out what herbicides still work on the weeds collected and this gives you the option to use a different herbicide to treat the weed patch within the same season and before the plants set seed.
Gathering samples for the Quick Test
- collect from the middle of the patch of weeds that are suspected to be resistant
- if the weeds are large, collect 20 plants
- if the weeds are small, collect 50 plants
- shake off the loose dirt and place the sampled weeds in a zip-lock plastic bag
- do not add water to the bag
- keep the sample cool
- if possible, collect and send samples on a Monday or Tuesday
- sample from different patches in the paddock, note the location/s and keep samples from different patches separate
- send by express post to Plant Science Consulting Check the website for details about the services offered, costs and specific instructions before submitting samples.
Gathering samples for the Seed Test
Collecting weed seed before or at harvest is the most common method used. The collected seed must be mature, from green to when the seed changes colour. Before harvest, collect 30 to 40 ryegrass seedheads or several handfuls of wild oats seed. After harvest, it is common to find seedheads still in the paddock or samples of contaminated grain can be sent for analysis.
Keep samples from different locations separate and details noted on the bag. Only use paper bags (double layer) to collect and send seed samples. Ensure bags are sealed so that the samples don’t mix during transit.
There are two weed seed testing services in Australia:
Visit the website/s for details about the services offered, costs and specific instructions before submitting samples.
How to collect samples for the Quick Test
How the Seed Test works
- Testing for herbicide resistance with Dr Peter Boutsalis
- Testing for herbicide susceptibility pays off
- What herbicides still work