Retaining our pre emergent herbicide options

Author: Peter Boutsalis (School of Agriculture, Food & Wine, University of Adelaide, Plant Science Consulting P/L), Christopher Preston and Gurjeet Gill (School of Agriculture, Food & Wine, University of Adelaide). | Date: 07 Feb 2017

Take home messages

  • Trifluralin resistance in annual ryegrass has increased in Victoria (VIC) and South Australia (SA).
  • Resistance to Group J herbicides has been detected.
  • Rotating between different mode of action herbicides combined with non-herbicide strategies is important to reduce the risk of resistance. 
  • Herbicide resistance testing takes the guesswork out of herbicide selection and can identify effective post-emergent options.

Resistance to pre-emergent herbicides

Random weed surveys of cropping regions across SA and VIC funded by the GRDC have been conducted annually since 2005 by the University of Adelaide to monitor for resistance levels in key weed species. The methodology involves collecting weed seeds from paddocks chosen randomly at pre-determined distances at harvest. Weed seeds are tested in pot trials the following winter. These surveys have shown that the greatest level of herbicide resistance is in annual ryegrass. The incidence of resistance to trifluralin in SA and VIC as identified by these surveys is presented in Table 1. In most regions, resistance to trifluralin has increased significantly between 5-year intervals.

Table 1. Percentage of paddocks identified to contain trifluralin resistance. Paddocks were scored as resistant if the seeds collected exhibited >20% survival in a pot test conducted the following autumn. Samples that exhibited less than 20% survival were not classed as resistant.

  Survey 1 (2005-2009)  Survey 2 (2010-2014)  Survey 3 (2015-2019) 
 Western  5  25  31
 Northern  2  0  
 Southern  0  2  
 South Australia      
 Mallee  19  40  
 South East  39  78  
 Mid North and Yorke  40  66  
 Eyre Peninsula  5  34  

Since 2014, screening with other pre-emergent herbicides was incorporated into the testing. To date, resistance to triallate and one case of resistance to Boxer Gold® has been detected.

Table 2. Percentage of paddocks identified to contain resistance to alternative pre-emergent herbicides. Paddocks were scored as resistant if the seeds collected exhibited >20% survival in a pot test conducted the following autumn. Samples that exhibited less than 20% survival were not classed as resistant.


Trillate   Boxer Gold® Sakura®  Propyzamide 
 Victoria  Western -2015  3  0 0  0
   Southern - 2014  11  0  0
 South Australia  Central and Yorke P. 2013  6  0  0  0
   Eyre Peninsula - 2014  1  1  0  0

Group C herbicides

Resistance to Group C herbicides (atrazine, simazine, diuron, terbuthylazine, etc.) remains low in weed species occurring in broadacre crops. A few cases of resistance in ryegrass, wild radish, mustard and silvergrass have been confirmed. However, there are no indications that resistance will increase rapidly even with the use of triazine tolerant (TT) canola. One reason is that the main mechanism of resistance in weeds confers a fitness penalty, therefore in the absence of Group C herbicides, resistant individuals are disadvantaged. 

Group D herbicides

Substantial increases in resistance to trifluralin in ryegrass have been detected in most regions (Table 1.). Although propyzamide is also classed as a Group D herbicide, it is not a dinitroaniline herbicide. No resistance to propyzamide has been detected and all studies to date have not shown cross-resistance between trifluralin and propyzamide. 

Group J herbicides

Triallate and Boxer Gold® (prosulfocarb + S-metalochlor) are the only currently registered Group J herbicides for pre-emergence use against annual ryegrass, with prosulfocarb only products such as Arcade® likely to be available in 2017. To date there is limited information on the cross-resistance between triallate and Boxer Gold®. One ryegrass sample (198-15) from central NSW in 2014 was not controlled with Boxer Gold® as reported by an agronomist (Table 3). Herbicide resistance testing confirmed resistance to field rates of Boxer Gold® and triallate. Another ryegrass sample from the Yorke Peninsula (375-14) in SA was not controlled with triallate in 2014. Testing confirmed resistance to triallate but not to Boxer Gold®. This indicates that triallate resistant ryegrass will not always be controlled with Boxer Gold®. A third sample (EP162) identified in the 2014 random weed survey in the southern Eyre Peninsula was confirmed resistant to Boxer Gold® and triallate, but not Sakura® or propyzamide. 

Cross-resistance between triallate and prosulfocarb 

Both are Group J herbicides belonging to the thiocarbamate chemical class. Cross-resistance between these herbicides has been documented in a ryegrass biotype that was selected from recurrent selection of a susceptible population (Roseworthy-R) treated with high rates of triallate (Table 3). As more Group J ryegrass populations are confirmed, cross-resistance between both herbicides will be tested. Rotating between prosulfocarb and triallate is unlikely to be a good strategy for ryegrass control. 

Table 3. Survival (%) of four ryegrass biotypes to field rates of pre-emergent herbicides in a winter pot trial. 

 Herbicides EP162  198-15  375-14  SLR31  Roseworthy-R   S
 Sakura®  0  0  0  0  0  0
 Boxer Gold®  32  29  0  0  0  0
 Avadex®  54  43  100  0  60  0
 Trifluralin  7  20  -  79  0  0

EP162: was identified from the 2014 random weed survey of southern Eyre Peninsula.
198-15: identified from central NSW after poor control with Boxer Gold®.
375-14: identified from the Yorke Peninsula, SA, after poor efficacy with triallate.
SLR31: trifluralin resistant biotype.
Roseworthy R: selected with high rates of triallate in a field trial from a susceptible population and resistance enriched after recurrent selection.
S: a susceptible field population.

Group K herbicides

Sakura® is the only isoxazoline Group K herbicide currently registered in Australia. It is registered for control of ryegrass in wheat (and other weeds in other crops — check label). Prior to Sakura®, the only other Group K herbicides (chloracetamides) used were metolachlor and S-metolachlor, although not for ryegrass control (registered only for suppression in barley and oats). This suggests that there has been less selection with chloroacetamide herbicides in ryegrass than with the Group J herbicide triallate over the past three decades. In 2017, another chloroacetamide herbicide Butisan® S (metazachlor) is due to be registered in canola for control of ryegrass making available another Group K herbicide for broadacre cropping. FMC is developing a Group K (napropamide) + Q (clomazone) pre-emergence herbicide (Altiplano®) product to be registered in 2018 for controlling ryegrass in canola. The Group K herbicide napropamide belongs to a unique chemical class (acetamides) different to other Group K herbicides. Furthermore, the introduction of a Group Q herbicide into broadacre cropping is positive from a resistance perspective. 

To date, no resistance to Group K herbicides has been detected. Although Group K herbicides are inhibitors of very-long-chain fatty acids (VLCFAs), they are chemically dissimilar, therefore resistance to one herbicide does not guarantee resistance to the other. In addition, Group K herbicides have multiple target sites, therefore if resistance develops in one target site, other target sites will still be inhibited. 

Key resistance management considerations

Selection pressure

Selection pressure is one of the factors that determines the rate of resistance development. Mechanisms of resistance that have a high frequency in unselected populations and only a slight fitness penalty are more dominant. The frequency of resistance in weed populations never exposed to a Group B herbicide is higher than to any other mode of action herbicide. This was identified in selection studies in the 1990s where 1:20,000 plants were found to possess a Group B target site mutation. This is a contributing factor to why Group B resistance is most prevalent. In similar selection studies, target site resistance to Group A herbicides in ryegrass was documented at a frequency of 1: 500,000 plants. In addition, there is little evidence of major fitness penalties in Group A and B target site resistant ryegrass suggesting that it is difficult for a population to resort back to susceptibility even if these herbicides are no longer used. For this reason, Group A and B resistance is widespread across numerous cropping regions. 

The selection pressure imposed by Group C, D, J, K and Q herbicides is likely to be much lower than to Group A and B herbicides. One reason is chance — for some reason, Group A and B resistance is more prevalent in natural populations, whereas the frequency of resistance to pre-emergent herbicides is likely to be much lower, especially for herbicides that have multiple sites of action. Generally, a greater proportion of a weed population is exposed to post-emergent than pre-emergent herbicides. In knifepoint-press-wheel tillage systems, seeds occurring in the crop row often germinate in the absence of a pre-emergent herbicide — this has the effect of reducing selection pressure in a weed population. 

Cross-resistance patterns 

It is likely that in the next few years, more pre-emergent herbicides will become available. Altiplano® has already been discussed.  A second herbicide coded as F9600SC from FMC is planned for registration in 2020. Initial studies show selectivity in cereals, pulses and canola with ryegrass a key target. The mode of action has not been released, but indications suggest it is a rare mode of action group. Adama Agricultural Solutions Ltd. are planning to register AG-C4-900 in 2019, a canola/pulse selective pre-emergent herbicide targeting ryegrass and brome. As to whether there is cross-resistance between herbicides still in development and existing herbicides is unknown. Generally, the more closely related a herbicide is, for example, if it has the same mode of action and is chemically related, then the chances of cross-resistance are greater than between herbicides that are not related. 

Rotating herbicides 

Rotating herbicides aids in reducing the risk of resistance to any one herbicide. With numerous pre-emergent herbicides, available rotation can be an effective strategy if combined with other management techniques. It is important to consider the seedbank life of weed species. Not all weeds will germinate the following year. About 20% of ryegrass seed will germinate in the second year after seed-set. Studies by the Department of Agriculture and Food, WA (DAFWA) have shown that only about 50% of wild radish seed germinates the following year. Thus, if the same herbicide is used every two years, a significant proportion of plants could be exposed to the same herbicide. 

Herbicide rate

Some herbicides have a rate range on the label. Using higher label rates can often improve weed control and counteract poor conditions at spraying. Weeds with weak resistance mechanisms can often be killed with higher rates. 

Ensuring optimum coverage and sowing speed is essential to maximise placement of herbicide in the close proximity to weed seeds. To maximise coverage and herbicide performance, it is important to spray during ideal weather conditions with the correct nozzles, speed, water volume and water quality.

Herbicide resistance testing 

The GRDC sponsored random weed surveys have shown that in some regions, there are significant opportunities for Group A and D herbicides to control ryegrass in cereal crops. A reliable way to determine which herbicides remain effective is to have the sample tested for herbicide resistance. Studies by the University of Adelaide have shown that in neighbouring paddocks, herbicide resistance profiles can differ from no resistance to significant resistance. Some growers find it convenient to assume they have herbicide resistance to all Group A and B herbicides without ever having tested and rely on products such as Sakura® or Boxer Gold®, because of their low resistance risk. However, not only is this costly because there may be cheaper alternative herbicides that are still effective, but unnecessary selection pressure for resistance is imposed. If only a small proportion of the population is resistant (and therefore not controlled), reducing the population size (susceptible plants are killed) reduces the seed-set for next season and can make seed-sterilisation techniques more effective if there are fewer target weeds. 


The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the author would like to thank them for their continued support.

Contact details 

Peter Boutsalis 
University of Adelaide, Waite Campus, Glen Osmond SA 5064

Herbicide resistance testing

Peter Boutsalis 
Plant Science Consulting
0400 66 44 60