Rotating herbicide groups explained

GroundCover Live and online, stay up to date with daily grains industry news online, click here to read more

Herbicide resistance testing is helping to take the guesswork out of herbicide selection

Random weed surveys of cropping regions across South Australia and Victoria funded by the GRDC have been conducted annually since 2005 by the University of Adelaide to monitor resistance levels in key weed species. The method involves collecting weed seeds from paddocks chosen randomly at harvest, with the seed tested in pot trials the following winter.

At the GRDC Update held in Bendigo, Victoria, the university’s Dr Peter Boutsalis reported that the greatest level of herbicide resistance is in annual ryegrass and that the incidence of trifluralin resistance in SA and western Victoria had increased significantly between five-year intervals.

Dr Boutsalis said that, since 2014, screening with other pre-emergent herbicides had been incorporated into the annual surveys (see Table 1). This reflects the increasing reliance on pre-emergent herbicides as weeds become more resistant to post-emergent herbicides.

This screening has detected the first cases of resistance to pre-emergent herbicides. “Resistance to triallate and one case of resistance to Boxer Gold® has been detected,” Dr Boutsalis said. “However, the selection pressure imposed by Group C, D, J and K pre-emergent herbicides is proving to be much lower than Group A and B herbicides because of the greater background prevalence of resistance genes to the Group A and B post-emergent herbicides.”

He said that rotating herbicides remained the primary strategy in reducing the risk of resistance to any one herbicide, especially given the numerous pre-emergent herbicides available, with more likely to become available in the next few years, including Altiplano®.

“It is important to consider the seedbank life of weed species when selecting herbicides,” Dr Boutsalis said. “Not all weeds will germinate the following year. About 20 per cent of ryegrass seed will germinate in the second year after seed-set. Studies by the Department of Agriculture and Food, WA, have shown that only about 50 per cent of wild radish seed germinates the following year.

“That means if the same mode of action herbicide is used every couple of years, a significant proportion of plants could be exposed to the same herbicide.”

Table 1 State of resistance to pre-emergent herbicides.
GroupHerbicide resistance
C
(Atrazine, simazine, diuron, terbuthylazine)
Resistance remains low in weed species occurring in broadacre crops and there is no indication resistance will increase rapidly due to a fitness penalty incurred in the few cases of resistance in ryegrass, wild radish, mustard and silvergrass.
D
(Trifluralin, propyzamide)
Substantial increases in resistance to trifluralin in ryegrass detected in most regions.
No resistance to propyzamide (or cross-resistance between trifluralin and propyzamide) has been detected.
J
(Triallate, prosulfocarb, S-metolachlor)
To date there is limited information on the cross-resistance between triallate and Boxer Gold®, but instances have occurred of triallate-resistant ryegrass that were not controlled by Boxer Gold®.
In 2017, prosulfocarb-only products such as Arcade® will be available.
K
Pyroxasulfone (Sakura®)
Sakura® is the only isoxazoline Group K herbicide currently registered in Australia. Its main use is for control of ryegrass in wheat.
In 2017, the chloroacetamide herbicide Butisan® was registered in canola for the control of ryegrass.
There has been less selection with chloroacetamide herbicides in ryegrass than with Group J triallate over the past three decades.
FMC is developing a Group K (napropamide) + Group Q (clomazone) pre-emergence herbicide product, Altiplano®, to be registered in 2018 for controlling ryegrass in canola. Napropamide belongs to a unique chemical class (acetamides) different to other Group K herbicides.
No resistance to Group K herbicides has been detected. As they are chemically dissimilar, resistance to one herbicide does not guarantee resistance to the other.

GRDC Research Codes UCS00020, UA00158

More information:

Peter Boutsalis,
peter.boutsalis@adelaide.edu.au