WSU herbicide resistance testing program eliminates the guessing

By Ian Burke and Rachel Lindell

A wheat farmer’s most expensive herbicide is the one that doesn’t work. In this respect, herbicide resistance costs real money.

Even when retailers and manufacturers stand behind their products, the time it takes to realize the herbicide application has failed can be the difference between a profit and a loss. And it’s more than a one-year event. Seed produced from herbicide resistant plants can continue to infest a farm for years or decades to come.

Ian BurkeBut what does a farmer do in the absence of information about which weeds exhibit resistance? Our experience is that they and their fieldmen simply choose the next shiny new herbicide product. If it works, they keep using it until it doesn’t work. Then they find the next product. Weed scientists call that the “herbicide treadmill.”

Farmers seldom have the space, time and the information necessary to make sound decisions on the herbicide program that will be most effective for them on a field-by-field basis. Why? The key piece of information they need–is the weed sensitive to the herbicide–is missing.

The goal of the Washington State University (WSU) herbicide resistance testing program is to eliminate the guessing game that occurs in the spring of every season. Although we may ultimately be able to test before the field is sprayed, for now we are focusing on developing our capability to rapidly assess large numbers of seed and tissue samples in the fall and winter.

GotResistance-2Weed seed can be quite difficult to test, as weed species have varying amounts of seed dormancy. Grass seed needs to ‘after-ripen’ following harvest for a few weeks to a few months before they will germinate. Broadleaf weeds like wild mustard and common lambsquarters have dormancy mechanisms that enable weed seed to last years in the soil profile, and often requires special treatment like sanding the seed coat or an acid bath to germinate.

As a consequence, it takes much longer to process samples submitted to our program for resistance testing than you would expect. When a sample arrives at our offices, it is assigned a unique identification number and a receipt is sent to the submitter. The seed sample is then cleaned up and tested for germination. If the seed germinates, fifty or so are planted in a single pot to test for resistance to the herbicide in question. If the weed doesn’t die when it’s supposed to, additional herbicides are tested on it to see what works. Reports are sent by email as each step is complete. It can often take more than four months before a submitter has a clear answer.

In an effort to cut down the time down to get farmers or fieldmen an answer, we have also been working on standardized genetic tests for specific types of common herbicide resistances. Target site resistance, where there is a change in the gene coding for the enzyme target of the herbicide, can be detected using genotyping facilities at WSU. And the test is very rapid with an answer available in days sometimes.

Using green leaves of the weed in question, we can test for target site resistance to the Group 1/A ACCase inhibitors (herbicides like Axial XL and Select), the Group 2/B ALS inhibitors (herbicides like Powerflex HL and Affinity BroadSpec); or metribuzin, a photosystem II inhibitor. Resistance in Italian ryegrass, wild oat, prickly lettuce, mayweed chamomile, redroot pigweed, and common lambsquarters can all be verified using this method.

The disadvantage of the approach is that we can only target certain weeds using a genetic test, and the test will miss resistance where the weed metabolizes the herbicide, that is when it breaks the herbicide down. Herbicide resistance due to metabolism is less common, but it could set up a situation where the sample tests negative for resistance, but the weed is in fact resistant. We are working on identifying a solution and a test for metabolism-based herbicide resistance so we can avoid such situations.

Additional genetic tests are coming, so if a farmer is in doubt about our capabilities, please ask! We’ll take tissue samples by arrangement, as they have to be frozen immediately after collection, or delivered to us live.

The seed/tissue testing program is in place and functioning (see side bar for directions on submitting samples). In the last 8 months, more than a hundred samples have been submitted. Of those, we have identified four new herbicide resistances that were not previously known in the state: dicamba resistant kochia, florasulam (a component of Orion and Starane Flex) resistant shepherd’s purse, Group 1/A ACCase and Group 2/B ALS resistant Italian ryegrass (in the same plant), and Olympus resistant downy brome.

We’re certain there are more cases of resistance in farmers’ fields. The difficulty is we really have very little understanding of the magnitude of the problem in the state. One of the positive outcomes of running a testing service, however, is that we will have a very good idea of the severity of herbicide resistance as well as where the problem is occurring. Not only will the testing service help farmers, it will also help researchers by identifying significant weed problems earlier.

The program will be organized as a fee-for-service, similar to a soil testing lab, starting this fall. But until then, farmers can submit samples for free thanks to underwriting by the Washington Grain Commission.

Don’t hesitate. If you have a questionable weed that doesn’t appear to be impacted by your current herbicide regimen, now’s the time to send in your samples!

How to submit a sample:

Submitting a sample to the WSU Resistance Testing Program is easy. The best approach is to put a few coin envelopes (size A4 is best) in your combine or scouting rig. The amount of seed required for testing varies for each species – we need several hundred seeds if possible. Put this envelope in another envelope or a small box for mailing and do not use plastic bags.

Make sure the sampling occurs across the area of interest, not just at one spot. If you are interested in the entire field, you need to sample the entire field! If you are looking at a small patch, sample throughout that area.

Print out a submission form (it’s easy to find at the WSU Small Grains website at and include it with the sample. If you include an email, you’ll receive updates including confirmation of receipt.

Send in those samples!