Herbicide resistance trials confirm early season weed control change

With the arrival last season of a new cereal pre-emergence and early post-emergence (EPE) herbicide offering growers a potential step-change in their early season weed control, high profile researchers in WA and SA last year conducted extensive trials featuring multiple herbicide-resistant weeds to confirm the value of the herbicide.

The news was good, and hence growers can now be confident of having an effective new tool at their disposal.

Roberto Busi, Weed Scientist with the Australian Herbicide Resistance Initiative (AHRI) at the University of WA, said there were few grass selective post-emergent herbicide options for use in wheat crops and most already had weed resistance levels of up to 50 per cent.

This had resulted in a reliance on pre-emergent herbicides, which are particularly important with no-till seeding systems, for effective weed control, and where weed resistance has developed over time.

Roberto said AHRI testing of annual ryegrass resistant samples last year showed resistance to trifluralin, one of the most popular pre-emergent herbicides, was now consistently at 15pc, which equates to one in every six paddocks treated with trifluralin.

Last year, Bayer released the pre-emergent and EPE herbicide, Mateno Complete^®, for grass and broadleaf weed control in wheat and barley, and Roberto assessed the value of the new herbicide in controlling multiple herbicide-resistant annual ryegrass populations.

Mateno Complete contains aclonifen (Group 32), a herbicide mode of action available last season for the first time to the Australian industry, in a unique, complementary co-formulation with pyroxasulfone (Group 15), which also is the active ingredient used in Sakura^®, and diflufenican (Group 12) herbicides.

Roberto said he wanted to study the long-term efficacy of the herbicide in worst-case scenarios, evaluating it against three annual ryegrass populations with some resistance to prosulfocarb and pyroxasulfone, including one from Western Australia and two from Victoria, and three populations with some resistance to trifluralin, all of which were from South Australia and included one that also featured resistance to prosulfocarb and pyroxasulfone.

The extensive pot trial included three replicates and compared a range of standard pre-emergent or incorporated by sowing (IBS) herbicides; Mateno Complete applied pre-emergence, post-emergence following the IBS treatments, post-emergence stand-alone, and also in a tank mix with Buctril^® broadleaf herbicide; as well as the application of an experimental post-emergent herbicide mixture, Sakura Flow and diflufenican.

The trial assessed weed control and biomass suppression through to the reduction of weed seedset, which Roberto said was important to evaluate longevity of control.

He said the trial showed the EPE application of Mateno Complete was very convincing, giving growers confidence it would control annual ryegrass post-emergence across the full soil profile, including in the furrow, while it also provided far superior weed control when compared with the Sakura Flow and diflufenican mixture.

“It was impressive to observe the efficacy of Mateno Complete as a true post-emergent herbicide capable of fully controlling the annual ryegrass in the pots,’’ Roberto said.

“Mateno Complete controls it no worries. If you have a herbicide-resistant annual ryegrass population, Mateno Complete can control it, as long as the population is not heavily herbicide Group 15 (previously Group K) resistant.’’

He said the trial outcome in the worst-case scenarios was clear that in those tough situations, it was best to use Mateno Complete following an effective pre-emergent grass herbicide with an alternate mode of action.

“Even with the better performing IBS treatments, the weed control runs out; with the EPE application of Mateno Complete giving an improved period of control.’’

The trial generally showed a 10-15pc improvement in annual ryegrass control and more than 20pc in some cases when Mateno Complete was applied following the pre-sowing application of an effective pre-emergent herbicide.

Roberto said the post-emergent application of Sakura and diflufenican, undertaken to understand the benefit of the aclonifen component that features in Mateno Complete, was the least effective in the trial and, in turn, clearly demonstrated the strength of the three active ingredient combination, including aclonifen, for annual ryegrass control.

Meanwhile in SA last year, Plant Science Consulting conducted pot trials to evaluate the effectiveness of IBS and post-emergent applications of Mateno Complete on herbicide Groups 3 and 15 resistant annual ryegrass from the South East and Eyre Peninsula regions of the State respectively. Further pot trials assessed post-emergent applications of the new herbicide on Groups 4 and 12 resistant wild radish from WA and Group 4 resistant sowthistle from NSW.

The trials featured Mateno Complete applications compared to standard pre-emergent grass herbicides including Sakura, prosulfocarb and trifluralin, as well as broadleaf weed herbicides including diflufenican plus MCPA, halauxifen and florasulum, and Precept^®.

Both biomass reduction and the overall weed control were measured in the trials and the findings were similar to those from the WA research.

The 750 mL/ha and 1 L/ha application rates of Mateno Complete were highly effective on the susceptible and resistant annual ryegrass, and EPE applications following the application of an effective pre-emergent herbicide treatment showed further improved control.

Against resistant wild radish and sowthistle, EPE applications of Mateno Complete proved the most effective, especially when used at the higher application rate on wild radish.

The strength of the aclonifen component in Mateno Complete was well highlighted, with diflufenican and its mixture with MCPA proving the least effective in the trial and weed control levels from applications of Sakura plus diflufenican also being significantly lower.

Plant Science Consulting Director Dr Peter Boutsalis agreed that the effectiveness of applying Mateno Complete EPE could add another dimension to early weed control in addition to effective pre-emergent herbicide treatments, especially when contending with resistant annual ryegrass.

“There is more resistance to prosulfocarb than herbicides like pyroxasulfone (Sakura) and, as a result, where weed control from products like Boxer Gold^® and Arcade^® may be suffering, the trial showed Mateno Complete controls those resistant ryegrass populations well,’’ Peter said.

He said for grass weed control, Mateno Complete was predominantly taken up through weed roots rather than leaves and, hence, a good rainfall event following application was required, while targeting annual ryegrass prior to tillering would achieve the best results and mixtures with cost-effective broadleaf herbicides would further enhance broadleaf weed control.

“If growers can knock out those grasses with weak resistance mechanisms and then achieve good crop competition, they will be putting a handbrake on resistance development.’’

“Mateno Complete was a standout on two to three-leaf wild radish, and either alone or adding MCPA LVE at 440 mL/ha to the 1 L/ha rate took the control to 100pc, whereas the control from Tigrex^® was about 25pc and 50pc from Paradigm^® 25 g/ha and MCPA LVE at 440 mL/ha,’’ Peter said.

With the resistant sowthistle at two to three-leaf, the application of Mateno Complete achieved 100pc control whether applied with or without bromoxynil plus MCPA LVE.

“The EPE application over the entire soil surface can provide control in the interrow and the crop row for both wild radish and annual ryegrass,’’ Peter said.

Growers and advisers seeking further information on the application of Mateno Complete can contact their local sales agent or Bayer Crop Science Territory Business Manager.

Buctril^®, Precept^®, Tigrex^® and Mateno^® are Registered Trademarks of the Bayer Group.

Sakura^® is a Registered Trademark of Kumiai Chemical Industries Co. Ltd.