Published on 21/07/2022
Since mid-winter is a key time to start keeping this particular crop clean and profitable, a quick review of latest sow thistle control options in white clover is timely for growers.
FAR herbage research manager Dr Richard Chynoweth says that when the herbicide Jaguar, (a mix of diflufenican and bromoxynil), was removed from the New Zealand market in 2019 it left quite a gap for sow thistle control.
For 10 years prior, Jaguar was part of the most commonly used strategy to control sow thistle in white clover seed crops, in combination with 2,4-D Ester, applied during late winter when clover is dormant but about to start growing.
There are three sub-species of sow thistle – perennial, spiny leaved and annual – and these can all be present in one crop at the same time, which is one of the reasons it can be hard to control. Spiny and annual sow thistles are typically the most prevalent in Canterbury. In white clover, they compete for light, and can shade plants and stolons, limiting flowering and reducing seed set.
FAR began researching alternative herbicide control options before Jaguar was withdrawn, testing a wide range of different treatments for three years. It was important for crop profitability to find a replacement for late winter herbicide application, particularly for poorly-established, weaker or dryland crops, Richard says.
“You can use herbicides in late autumn to ‘hold’ the sow thistle and slow growth before those late winter applications. But bare ground and aggressive growth regulation of clover can then allow spring germination of sow thistles which will suppress seed yield if the crop established slowly, or where growth is reduced by pest pressure.”
Year 1 of the alternative treatment trials showed paraquat had good sow thistle control, as did mixing two other herbicides (Quantum and Bromotril) to create the same active ingredient com-bination as Jaguar originally provided.
It also showed applying Tropotox Ultra in June would ‘hold’ thistle growth and development for later follow up applications, but applying it in July or August reduced seed yield.
Year 2 and 3 confirmed these results, and further showed that adding 2,4-D Ester (Relay Super S) to the diflufenican and bromoxynil combinations provided high seed yield (790-860 kg/ha) and low sow thistle density when applied in late July.
The Year 2 trial included a new diflufenican and bromoxynil formulation (Argosy) and this was shown to have similar efficacy to Jaguar.
Additionally, Year 2 and 3 also showed that applying paraquat or Tropotox Ultra in early June was useful when thistles were large, allowing better efficacy during follow up applications.
All herbicide treatments reduced sow thistle numbers per plot compared with the untreated control when assessed in early January, and all diflufenican and bromoxynil mixtures/formulations were similar in weed control and seed yield.
Using MCPA (Agritone) at two different rates in July, however, reduced clover growth and resulted in sow thistles germinating between drill rows in spring due to a lack of ground cover.
Margins over cost relative to the un-treated control varied; the July application of Quantum, Bromotril and Relay Super S had the highest of $670/ha. MCPA was very detrimental to seed yield, reducing it by 22% compared to the control, and leading to a loss of $940/ha when considering costs of application and resulting seed yield.
Unless weed pressure is high, Richard says, herbicide application might not increase crop yield, but will reduce sow thistle seed returned to the soil seedbank.
For more detail contact Richard Chynoweth at FAR.