Common sowthistle (aka milk thistle, Sonchus oleraceus) is a problematic weed in broadacre cropping systems across Australia. Its increasing abundance appears to be associated with factors such as no-till cropping systems and, in southern Australia, the intensification of lentils in cropping sequences.
Sowthistle produces a large quantity of seed (25,000 to 80,000 per plant) which is sufficient to offset relatively low seed survival. Seed dormancy appears to be low, with most seed germinating within a year if not buried. Although seed is dispersed by wind, distribution is generally localised to within a few metres of the parent plant. Walker and Widderick (2009) reported that emergence is primarily responsive to soil moisture and not significantly limited by temperature.
In northern Australia, control in fallow can be problematic due to the increase in the abundance of glyphosate resistant populations. This is compounded by the decline in control as weed size increases. Applications to ‘small weeds in good conditions’ on the scale required in commercial operations are often difficult to achieve, mostly due to operational logistics, as well as environmental conditions.
The Northern Growers Alliance conducted studies investigating the use of double knock tactics and residual herbicides. It was found that: glyphosate or glyphosate followed by paraquat did not provide commercially acceptable control on a widespread basis; residual chemistry clearly had a benefit with encouraging levels of activity from a range of herbicide mode of action (MOA) groups; double knocks of Group I herbicides followed by paraquat appeared promising and may be suitable where grass weeds are of less concern.
In southern Australia, there is widespread sowthistle resistance to Group B herbicides, on the back of widespread usage over the last 30 years. Resistance to glyphosate (Group M) is increasing, but other MOAs, including Group I’s, remain generally effective (there are some localised populations with some resistance to Group I herbicides but cross resistance with other MOAs has not been reported).
Group B (ALS inhibitors) > 10,000
Group I (synthetic auxins) 5
Group M (glyphosate) > 50
Source: CropLife Australia www.croplife.com.au
Resistance to synthetic auxin herbicides (Group I) is relatively infrequent around the world compared to some other MOA’s. The level of resistance to Group I’s varies within the chemical class of the Group I (Preston et al 2013) and cross-resistance within chemical classes in this group is also highly variable. Recent work (Goggin et al 2018) on radish highlighted the ‘mystery and complexity’ of the MOA of auxinic herbicides and the mechanisms of resistance.
The importance of this MOA for global food security is perhaps hard to overstate, given their importance in enabling continued cultivation of glyphosate tolerant crops. However, there is no room for complacency as each particular case demonstrates. Greater understanding of the MOA of synthetic auxin herbicides (Busi et al 2017) has only highlighted how little we really understand and the potential complexity involved.
Arylex™ is a new synthetic auxin herbicide. It is from a new group of chemistry, the Arylpicolinates, within the Group I MOA family. Arylex has some distinctly different properties to the other sub-groups within this mode of action classification, and it has a unique spectrum of activity. Launched in 2015 in Paradigm™ herbicide, Arylex is also the key active ingredient in ForageMax™, Rexade™ and Pixxaro™ herbicides. Each of these herbicides contain two active ingredients.
Arylex is active at low use rates per hectare, does not accumulate in the stubble crop plants (in contrast to some other Group I herbicides such as clopyralid and aminopyralid) and it dissipates relatively quickly in soil, allowing rotational flexibility in most situations. Finally, Arylex products have been found to provide consistent results across a range of conditions, excellent crop safety and favourable toxicological and environmental profiles.
“We have developed the Arylex family of herbicides to give growers maximum flexibility – in the range of weeds controlled, the timing of applications, tank mix partners and rotational possibilities – whilst also ensuring robust control in the paddock. Reducing complexity during one of the busiest periods on farm is something we are really conscious of” advised Dan Dixon, Corteva Agriscience, Agriculture Division of DowDuPont, Market Manager.