By Clark Brenzil, PAg., Provincial Specialist, Weed Control, Crops and Irrigation Branch, Regina
Computer programmers have a term—garbage in, garbage out or GIGO, meaning that bad input will produce lousy output. The same rule applies to herbicide resistance testing at the Saskatchewan Agriculture Crop Protection Laboratory; collection and preparation of the sample is critical to getting a result you can trust.
Most herbicide resistance testing is conducted on weed seeds. These seeds are grown out as part of the testing process, whether in agar plates treated with the test herbicide or for spraying young plants grown from that seed. The only testing that can be done on living plant samples is a polymerase chain reaction (PCR) test that looks for the presence of the resistance gene. Although they are the only option, PCR tests have some drawbacks. Firstly, they can be expensive compared to conventional testing. Secondly, they are specific to known herbicide resistance mutations. Because of this, they can miss resistance that is the result of a new mutation or is metabolism (multi-gene) based. PCR testing for herbicide resistance is not done at the Crop Protection Laboratory.
To get a consistent result from your test, it is important that the sample be harvested when the weed seeds are mature and dry. A good tool for collecting wild oat seed is an insect sweep net. Allow the wild oats to mature before collecting. Make sure to turn the net inside out so the seams are to the outside and won’t catch the weed seeds as you dump them out.
If the weed is prone to shattering (stinkweed) or tumbling (kochia), it may be necessary to collect whole plants when they are near maturity and allow the seed to shatter naturally. Store collected plants in tightly woven cloth bags, such as a pillowcase and hang in a protected but well-ventilated area. Thresh by crushing the bag and allow seeds to fall to the bottom of the bag for collection.
For windblown seed, such as annual sow-thistle or narrow-leaved hawk’s-beard, a cordless handheld vacuum is a handy tool.
It is important to make sure that seed is collected before the field is treated with a preharvest herbicide treatment since this treatment can affect the seed viability. Seed handling and storage is also important to maintain viability. Avoid storing the sample sealed in locations that get very hot, such as vehicle dashboards, as temperatures there can reach 70 C, which is lethal to seeds, even with the windows open a crack. Storage in a dry location in moderate temperatures is best to ensure maximum germination during testing.
For more information on herbicide resistance testing, check out our YouTube channel, visit our website or contact Clark Brenzil, Provincial Specialist, Weed Control at 306-787-4673 or firstname.lastname@example.org.