Understanding alphabet resistant annual ryegrass
Take home messages
- Alphabet herbicide resistance is more common in high rainfall zones.
- Resistance to Group J and Group K herbicides in ryegrass has been confirmed in SA, Vic and NSW.
- The use of new mode of action (MoA) herbicides should be included to delay resistance to any one herbicide.
- Use herbicide resistance testing to identify opportunities to use groups still effective.
Weed populations resistant to many different modes of action (MoA) herbicides are sometimes referred to as possessing ‘alphabet resistance’. Resistance in these cases is usually conferred by multiple-resistance types, that is, the accumulation of two or more resistance mechanisms. This usually occurs by sequential selection, i.e. repeated use of one herbicide (or herbicides from the same MoA e.g. Group Bs) selects for resistance and is followed by overuse of a second MoA which develops resistance to the second herbicide. The population is now resistant to two MoA herbicides. Alternatively, resistance to more than one MoA herbicide can be conferred by cross-resistance. In this circumstance, one mechanism of resistance confers resistance to more than one MoA. The most common mechanism conferring cross-resistance is metabolic resistance, whereby herbicides are more rapidly detoxified by metabolising enzymes. Some forms of metabolic resistance can confer cross-resistance to Hoegrass®/Decision® (Group A) and Glean®/Logran® (Group B), such as ryegrass biotype SLR31 which has been extensively investigated by Preston and Powles.
Resistance can also be conferred by multiple-resistance comprising of Group A target site resistance (mechanism 1) plus Group B target site resistance (mechanism 2). One ryegrass biotype identified in the 2016 north-eastern Victorian survey (Sample 76) collected south-west of Rochester was confirmed resistant to triallate (J), sulfonylureas (B), Axial® (A) and glyphosate (M). It is most likely that there are four individual mechanisms of resistance (multiple resistance) in this population. Each mechanism can confer exclusive resistance to one MoA (multiple resistance) or more than one MoA (cross-resistance). While resistance to four MoAs is not common, resistance to three MoAs, e.g. A, B and to one other MoA herbicide (e.g. trifluralin (D), triallate (J), glyphosate (M)) is becoming increasingly common in ryegrass.
Weed resistance surveys supported by GRDC indicate that in the more intensively cropped areas of southern Australia where herbicide selection pressures have been high, alphabet resistant ryegrass is becoming more prevalent (Figure 1). The highest proportion of alphabet resistant ryegrass has been detected in the SA South East (SE) with the majority of ryegrass samples resistant to three MoA, and a significant number resistant to four and five MoA. In lower rainfall areas such as the SA Mallee, almost 30% of ryegrass samples were not resistant, with the majority (45%) resistant to one MoA, and 15% to two MoAs. In the higher rainfall zones of south western and north-eastern Victoria, the majority of ryegrass samples were resistant to two MoA. Needless to say, in every region there are a proportion of paddocks with no resistance or resistance to only one MoA. Through the use of resistance testing, these benefits can be exploited by determining those MoAs that are still effective.
Figure 1. Percentage of samples from random weed surveys resistant to different MoA herbicides.
Herbicide resistance in Victoria
Resistance to post-emergent herbicides
The GRDC has supported the University of Adelaide’s weeds group since 2005 to conduct annual weed surveys across three regions of Victoria. The methodology involves randomly selecting 150–200 paddocks and collecting weed seeds prior to harvest. Reasons for the presence of plants at the end of the season could include: (1) Resistant plants that have survived herbicides, (2) Germination occurred post-herbicide application, (3) Sub-lethal effects due to shading or poor herbicide/adjuvant choice/poor application equipment, and (4) Reduced herbicide uptake due to plant stress or (5) a combination of points 1 to 4. Testing using pot trials under optimum spraying conditions (accurate herbicide application on non-stressed weeds at correct timings) is conducted the following season under outdoor winter conditions. These studies are aimed at identifying the incidence and changes in resistance mechanisms at 5-year intervals in addition to monitoring for resistance to recently registered herbicides. The results are expressed as the percentage of paddocks containing resistant ryegrass as determined by a pot test and where survival in that pot test was ³20% (Tables 1 and 2).
Table 1. Percentage of paddocks containing herbicide resistant ryegrass in Victoria to post-emergent herbicides. Paddocks were scored as resistant if the seeds collected exhibited ³20% survival in a pot test conducted the following winter. Samples that exhibited <20% survival were not classed as resistant. Letters below the herbicide name represent the MoA labelling.
Region of Victoria
For each herbicide, recommended field rates plus recommended adjuvants (if required) were used.
The surveys have identified that resistance to the cereal selective Group A and Group B herbicides ranges between 33% and 96% across Victoria. The effectiveness of Group A and Group B herbicides varies between regions. For example, ryegrass from 33% of paddocks surveyed in Western Victoria in 2015 exhibited resistance to Axial®, whereas resistance is higher in the other two regions.
Clethodim (Select®) is arguably the most important herbicide in break crops. The incidence of resistance to 500ml/ha clethodim in the pot trials under winter conditions on non-stressed ryegrass was below 5% in the Victorian samples. To put this into perspective, the incidence of glyphosate resistance was similar to clethodim. However, frequently during the growing season, there are reports of poor control with clethodim. This discrepancy of reduced performance with clethodim is, therefore, likely to be due to other factors such as environmental stress reducing herbicide performance.
Across 450 paddocks randomly surveyed in Victoria between 2014 and 2016, glyphosate resistance in ryegrass was detected in 4% of paddocks. This is a real concern because glyphosate is the most commonly used herbicide pre-sowing and for pre-harvest crop-topping. Studies to investigate the viability of glyphosate resistant ryegrass seed after crop-topping with glyphosate have revealed the strategy was ineffective in sterilising seed. This suggests that in a mixed population, including glyphosate resistant and susceptible individuals, crop-topping at the correct timing with glyphosate would only sterilise susceptible individuals, but not resistant individuals. Resistance testing for glyphosate resistance would aid in product choice in pulse crops where paraquat is an alternative.
Table 2. Percentage of paddocks containing herbicide resistant ryegrass in Victoria to pre-emergent herbicides. Paddocks were scored as resistant if the seeds collected exhibited ³20% survival in a pot test conducted the following autumn-winter. Samples that exhibited <20% survival were not classed as resistant. Letters below the herbicide name represent the MoA labelling.
Region of Victoria
For each herbicide, recommended field rates were used.
Resistance to pre-emergent herbicides
With the exception of resistance to trifluralin in western Victoria (31% of paddocks resistant) and triallate resistance in southern and north-eastern Victoria (11-12% of paddocks resistant), resistance to pre-emergent herbicides remains low. In north-eastern Victoria, the high incidence of mixed farming has likely contributed to the reduced selection pressure on pre-emergent herbicides. In situations, where high stubble loads and large ryegrass seedbanks are common such as in southern Victoria, the use of trifluralin alone is not likely to be effective due to poor herbicide-seed contact. Here, full rates of diverse pre-emergent herbicide mixtures are common. Heavier reliance on trifluralin in western Victoria with fewer herbicide/weed control strategies compared to mixed farming in northern and southern Victoria has most likely contributed to the higher incidence of trifluralin resistance. Since no resistance to propyzamide has been detected from the random weed surveys or from commercial resistance testing indicates that it is rare.
Resistance to Group J and Group K herbicides is generally low in Victoria except triallate, which has been available for more than 30 years. It is likely that the long-term exposure of ryegrass to triallate has eliminated susceptible individuals, increasing triallate resistance. Such resistant individuals are more likely to develop cross-resistance to other herbicides in Group J. Group J and Group K herbicides have become widely adopted as the main herbicide defence against alphabet resistant ryegrass in both cereal and broadleaf crops, and resistance is likely to increase if they continue to be heavily relied upon. Recent University of Adelaide studies have shown that cross-resistance between the Group J and Group K herbicides is highly unpredictable, making it difficult to determine if resistance to one herbicide will elicit cross-resistance to another Group J/K herbicide. Resistance testing can aid growers identify which pre-emergent herbicides are still effective.
New MoA herbicides
Several herbicides, most which have MoAs that differ to currently registered herbicides (Groups E, Q, R), are in development (Table 3). The Group K herbicide Devrinol-C® (napropamide) belongs to a unique chemical class (acetamides) different to other Group K herbicides. Devrinol-C® has been registered for the 2019 growing season, whereas the registration for the other three herbicides (bixlozone, cinmethylin and carbetamide) is pending. The herbicides in Table 3 are all pre-emergent herbicides and selective in a variety of crops. Trials have shown that in addition to other weed species, these herbicides control some alphabet resistant ryegrass. If used as part of an integrated weed control strategy, these alternative MoA herbicides are likely to reduce selection pressure on any one MoA.
Table 3. Summary of new herbicides expected for registration by 2020.
Mode of Action
Grasses and broadleaf weeds
Ryegrass, brome, barley grass
*registration in November 2018
? Under investigation
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.
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