What herbicides still work?

A cheap herbicide quickly becomes very expensive if it is not effective.

Multiple resistance is beginning to show up in wild radish outside WA and testing is recommended if you plan to use certain herbicides again within the next three years.

Testing for herbicide resistance has played an important part in understanding how resistance occurs in field situations but now that researchers have a good understanding of the mechanisms involved it is becoming more important for growers to find out which herbicides are still effective in controlling target weeds.

John Broster, senior technical officer, Charles Sturt University (CSU) is recommending growers collect weed samples for ‘susceptibility testing’. Dr Broster heads up the herbicide resistance testing service at CSU and the data he has collected over the last 24 years has helped identify resistant populations of a wide variety of weeds across Australia.

“If a particular weed, say annual ryegrass, is present in reasonable numbers at harvest time, having survived in-crop treatment, say with a Group A (fop) or Group B herbicide, we can assume with some confidence that herbicide resistance is the reason for survival,” he says. “From a grower’s point of view then, it is not cost effective to test those plants to see if they are in fact resistant to Group A or Group B.”

“The big question is ‘what herbicides are those plants still susceptible to?’ and that is what we can help answer through susceptibility testing.”

Testing of suspected herbicide resistant ryegrass samples at CSU in 2014 suggest that if you think your ryegrass is resistant to Group A (fop) or Group B herbicides, it probably is and testing is not necessary. If you think it could be resistant to glyphosate, always have it tested. Annual ryegrass is almost always expected to be susceptible to atrazine, so only test if you think your weeds could be resistant. For all other herbicides it is worth testing if you are suspicious of herbicide resistant or cross-resistance.

Although annual ryegrass has a reputation for its resistance to important herbicide groups there are herbicides that are still effective options for annual ryegrass control. These include trifluralin, glyphosate, atrazine, propyzamide, Boxer Gold, paraquat, Sakura and Avadex.

Having several options still available is good news for growers and it is essential that these herbicide options are protected and incorporated into a diverse weed control plan that includes non-herbicide options.

Table 1. Low levels of herbicide resistance in annual ryegrass to a range of herbicides tested in 2014.

For wild radish the story is similar. There are high levels of resistance to Group B (e.g. Glean, Logran), especially in Western Australia, so testing for resistance is generally not warranted, although in other states it may still be worthwhile. Resistance to Group C (e.g. atrazine), Group F (e.g. diflufenican) and Group I (e.g. 2,4-D ester 680) is beginning to show up and testing is recommended if you plan to use these herbicides again within the next three years. Similarly with glyphosate, testing is recommended if you suspect resistance. To date there is no resistance to pyrasulfatole (e.g. Velocity, Precept) in Australia, so only test if you are suspicious. For all other herbicides it is worth testing for cross resistance, particularly for herbicides you plan to use over the next few years.

Table 2. High levels of herbicide resistance in wild radish to commonly used herbicides tested in 2014.

**This sample is being re-tested to confirm the result.

For wild oats, if you suspect Group A (fop) resistance you are probably right and testing is unnecessary. Most samples are susceptible to atrazine (Group C) and glyphosate, so only test if you suspect there is a problem. There is a 50:50 chance of samples being resistant to Axial or Achieve so testing for susceptibility is worthwhile if you plan to use these herbicides over the next few years. For all other herbicides you plan to use over the next few years it is worth checking for cross resistance to determine which products the wild oats is still susceptible to.

Table 3. High levels of herbicide resistance in wild oats to commonly used herbicides tested in 2014.

CSU researcher Dr John Broster is encouraging growers to collect samples for herbicide susceptibility testing next season.

Dr Broster urges growers to not be complacent about herbicide resistance in these weeds. “The situation is becoming increasingly serious for a wide spectrum of weed species,” he said. “These results suggest that growers have time to implement herbicide and non-herbicide strategies to avert a crisis situation, but immediate action is required.”

In preparation for next season Dr Broster suggests growers allow for susceptibility testing in the budget and be prepared to collect samples at harvest, even carrying sample bags in the header if possible.

To have samples tested simply collect seeds or seed heads from the weeds of concern, often those present at harvest. Place the sample into a paper bag inside an A4 envelope. Double bagging the seeds protects the sample. Don’t use plastic bags because the seeds can easily become mouldy if there is any moisture in the bag. Post the samples to either of the herbicide resistance testing services—Graham Centre for Agricultural Innovation at CSU, or Plant Science Consulting at University of Adelaide.

For ryegrass, collect about a coffee cup full of clean seed or an A4 envelope full of seed heads. For wild oats or wild radish, a comfortably full A4 envelope of seed heads or pods is ample. Record information on the sample bag about where the sample was collected and what herbicides you would like the sample tested against.

Make sure the sampling occurs across the area of interest, not just at one spot. If you are interested in the entire paddock you need to sample the entire paddock, if you are looking at the area around a blow-out then sample throughout that area.

For more information on herbicide testing services visit www.csu.edu.au/research/grahamcentre or www.plantscienceconsulting.com/seedtest.