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Group I resistance
The Group I herbicide 2,4-D was commercially released after World War II and was the first of the "new" synthetic herbicides that have been the basis of weed control practices for the last 60 years.
Group I herbicides are synthetic plant hormones (auxins) and cause unregulated plant cell growth leading to distortion of growing parts and plant death. See AHRI Insight for a more detailed explanation.
In 2009 wild radish (Raphanus raphanistrum) was becoming a problem in the central and northern wheat belt of Western Australia. The image above shows a significant infestation that survived pre-plant and early post emergent herbicide applications.
The Group I herbicide mode of action contains five chemical classes (Table 1), which while targeting the same pathways in plants, have different characteristics such as differing efficacy on different weed species and behaviour in the soil.
Table 1. Herbicides included in the Group I mode of action (Source: http://www.croplife.org.au/resistance-strategy/herbicide-mode-of-action-groups/)
* Part of a pre-mix
While primarily herbicides for the control of broadleaf (dicotyledon) weeds and some monocotyledon weeds, one class - the quinoline carboxylic acids - also has activity on grasses through the accumulation of cyanide in the plant tissues.
Despite Group I herbicides being used for over 70 years and on an estimated 200 million hectares each year cases of resistance are relatively low. Internationally there are 32 weed species with populations resistant to this herbicide mode of action consisting of 24 broadleaf weeds, 5 grasses and three non-grass monocotyledon weeds (Heap 2016). The first two cases of Group I resistance occurred in 1957 in wild carrot (Daucus carota) found on roadsides and scurvy weed (Commelina diffusa ) found in sugar cane only 11 years of commercial use. Group I resistance remains relatively minor when compared to more recent modes of action such as Groups A and B despite the long and widespread use of this mode of action.
Australia has four weed species with populations resistant to 2,4-D. They are wild radish (Raphanus raphanistrum), Indian hedge mustard (Sisymbrium orientale), sowthistle (Sonchus oleraceus) and capeweed (Arctotheca calendula) (Table 2). All have evolved in winter cereal cropping where the weeds were treated with 2,4-D or MCPA over many years.
Table 2. Weed species with populations resistant to 2,4-D
Group I resistance cases can be located on a State by State basis by going to the Bayer herbicide resistance mapping page http://www.diversitycantwait.com.au/iwm/maps/mapdata.asp
Management of Group I resistant weeds
Generally the levels of resistance to Group I herbicides is modest and varies from 3 fold to 100 fold depending on weed species and chemical class (Preston et al 2013). Cross resistance between different Group I classes is also highly variable.
The modest strength of Group I resistance in wild radish in Western Australia has been demonstrated by Walsh et al (2008), Campbell (2013), Newman (2013) and Thompson (2014).
This moderate resistance means that resistant plants can be controlled with combinations of tactics such as robust rates of herbicide plus crop competition. However this should also be combined with a range of tactics such as weed seed harvest techniques including narrow windrow burning, chaff lining and the Harrington seed destructor.
The effectiveness of the in-crop techniques of spray-topping with a non selective herbicide or windrowing on wild radish seed set control is highly variable because the timing of these operations are too late if a satisfactory grain yield was to be achieved (Walsh and Powles 2009).
Using a limited number of weed management techniques will select for weed populations that have characteristics to avoid these tactics, such as becoming prostrate or flowering and forming viable seed earlier.
To manage Group I resistant weeds it is necessary to:
The image above shows wild radish seedlings(Raphanus raphanistrum) beginning to regrow from an application of 2,4-D.
Group I resistance management links
Ashworth MB, Walsh MJ, Flower KC, Powles SB (2016) Recurrent selection with reduced 2,4-D amine doses results in the rapid evolution of 2,4-D herbicide resistance in wild radish (Raphanus raphanistrum L.). Pest Management Science. wileyonlinelibrary.com
Campbell W (2013) Understanding and managing proposed different development stages of herbicide resistance in wild radish (Raphanus raphanistrum). In 'GRDC Crop Updates'. Perth WA. (GIWA).
Craig S, Burdett D, Taylor C (2014) Broadleaf herbicide resistance targeted survey of Victorian Wimmera-Mallee. Project BWD00023 Birchip Cropping Group (BCG inc), Birchip, Victoria.
Goggin DE, Cawthray GR, Powles SB (2016) 2,4-D resistance in wild radish: reduced herbicide translocation via inhibition of cellular transport. J. Exp. Bot. Online. http://jxb.oxfordjournals.org/content/67/11/3177.full
Newman P (2013) We can kill wild radish in lupins if we have enough spray capacity. In 'GRDC Crop Updates'. Perth, WA. http://www.giwa.org.au/2013-crop-updates.
Preston C, Belles DS, Westra PH, Nissen SJ, Ward SM (2009) Inheritance of Resistance to The Auxinic Herbicide Dicamba in Kochia (Kochia scoparia). Weed Science 57, 43-47.
Preston C, Dolman FC, Boutsalis P (2013) Multiple resistance to acetohydroxyacid synthase-inhibiting and auxinic herbicides in a population of oriental mustard (Sisymbrium orientale). Weed Science 61, 185-192.
Preston C, Malone JM (2015) Inheritance of resistance to 2,4-D and chlorsulfuron in a multiple-resistant population of Sisymbrium orientale. Pest Management Science 71:1523-1528
Owen MJ, Martinez NJ, Powles SB (2015) Multiple herbicide-resistant wild radish (Raphanus raphanistrum) populations dominate Western Australian cropping fields. Crop & Pasture Science 66, 1079-1085.
Thompson G (2014) Controlling stacked resistant radish with herbicides. In 'GRDC Crop Updates'. Perth, WA. (GIWA).
Walsh MJ, Powles SB, Beard BR, Parkin BT, Porter SA (2004) Multiple-herbicide resistance across four modes of action in wild radish (Raphanus raphanistrum). Weed Science 52 8-13.
Walsh MJ, Powles SB (2009) Impact of crop-topping and swathing on the viable seed production of wild radish (Raphanus raphanistrum). Crop & Pasture Science 60, 667-674.
Walsh MJ, Owen MJ, Powles SB (2007) Frequency and distribution of herbicide resistance in Raphanus raphanistrum populations randomly collected across the Western Australian wheatbelt. Weed Research 47, 542-550.
Walsh M, Maguire N, Powles S (2008) Combined effects of wheat competition and 2,4-D amine on phenoxy herbicide resistant Raphanus raphanistrum populations. Weed Research 49, 316-325.