If a grower experiences a poor spray outcome on annual ryegrass with glyphosate, or seemingly needs to continually increase rates to achieve the same level of control, it might seem obvious that herbicide resistance is the most likely problem.
On the contrary, Maurie Street, CEO at Grain Orana Alliance said that this is not always the case, having investigated ways to regain control of problematic ryegrass populations.
"Annual ryegrass has always been present on most farms in the Central NSW cropping region and many populations are resistant to Group A and B herbicides," Mr Street said.
"So, when growers started to have trouble controlling this weed with glyphosate it seemed natural to assume that populations were becoming resistant to our most relied on knockdown herbicide."
In an effort to finesse the available control options, GOA established trials over three years with GRDC investment on seven sites where poor control of annual ryegrass over previous years had resulted in increased weed seed banks.
"The first thing we did was collect samples from each site and have them tested using the Quick Test method for glyphosate resistance," Mr Street said,
"We were surprised to discover that five of the populations were in fact susceptible, even at lower label rates of glyphosate, and the other two populations were only moderately resistant to the lower rate and 100 per cent susceptible to higher label rates."
What this suggests is that there can be something other than resistance contributing to herbicide failures.
Testing will reveal if herbicide resistance is at play and identify herbicide products and rates that can be expected to provide acceptable control.
Next, critically assess the spray operation and identify factors that could have affected the efficacy of the spray job.
Finally, look for ways to implement the WeedSmart Big 6 tactics in your weed control program to keep weed numbers low.
So, if the ryegrass was susceptible to glyphosate, why was control poor?
Instances of poor weed control after a herbicide application could be associated with one or more factors such as poor spray water quality, incorrect spray timing, inappropriate sprayer set up delivering less than optimal spray droplet size and/or water rates, products with sub-optimal surfactant loadings, or environmental stress affecting the plant and or the herbicide activity.
In the event of an apparent herbicide failure, review and investigate all aspects of the application - including Stress, Timing, Application and Rate (STAR). Don't just assume that herbicide resistance is to blame.