RESISTANCE to the Group I herbicide 2,4-D in sowthistle has been confirmed in SA.
The development is of great concern to the nation's cropping industry because sowthistle, also known as milkweed or milk thistle (Sonchus oleraceus), has become an increasingly difficult weed to manage in no-till crops and fallow.
It is a problematic weed because of its ability to germinate on the soil surface and having populations already resistant to Group B herbicides and glyphosate.
A major weed of cropping systems north of Dubbo in NSW for more than 15 years, sowthistle is now considered a significant emerging weed in southern and western farming systems and will be a focus of a GRDC project on the evolution mechanisms and inheritance of herbicide resistance, being led by the University of Adelaide's Chris Preston.
Dr Preston said recent testing of five sowthistle populations from SA had confirmed three populations resistant to solid rates of 2,4-D.
"These populations come from irrigation areas in SA's South East and are resistant to the equivalent of 2.2 L/ha 500 g/L 2,4-D," he said.
"This is another world-first and highlights the need for growers to be using diverse weed management tactics to prevent ongoing development of resistance to our most important herbicides."
Across Australia most sowthistle populations are resistant to Group B herbicides such as chlorsulfuron and metsulfuron.
Three cases of glyphosate resistance have been confirmed in northern NSW, with more suspected populations undergoing testing.
The three populations with resistance to 2,4-D showed typical 2,4-D symptoms within days of herbicide application and were severely damaged by the herbicide, but within two weeks of treatment grew new leaves that were normal in shape. More than half the plants survived treatment.
"The plants were treated at the four to six leaf stage and were quite small," Dr Preston said.
"As sowthistle grows larger it can be more difficult to control with 2,4-D, and 2,4-D would be even less effective on large plants from the resistant populations."
Dr Preston said mixtures with 2,4-D and other Group I herbicides were still likely to be effective at controlling these populations, so long as there was a robust rate of the mixing partner and the herbicide was applied to small seedlings.
"Otherwise, alternative herbicides will have to be used to control the resistant populations," he said.
"Apart from Group B and Group I herbicides, there are few herbicides that can be used to control established sowthistle in crops, so attacking it early will be key."
