Herbicide trial for controlling Californian thistle
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The prickly and persistent problem of Cali thistle is costing the agricultural sector millions. A new partnership may have answers for New Zealand farmers.
AgResearch estimates Cali thistle is responsible for the loss of some $700 million in pastoral farm revenue every year.¹ Farmers report they are disillusioned and losing confidence in their ability to stay on top of the continual threat.
Corteva AgriScience and PGG Wrightson have joined forces this season to find a permanent solution to the invasion.
Together, they have set up control sites across the country to demonstrate best practice methods in the management and elimination of Cali thistles. Ten dairy and red meat farms will show what is achievable through an effective, multi-spray programmatic approach.
Cali thistle's aggressive rhizomatous root system is capable of extending more than two metres deep. Its adventitious buds produce new above ground shoots, so getting chemical to this
part of the plant is vital. This, and the plant’s natural growth cycle, has informed Corteva and PGG Wrightson’s procedure.
The first phase will occur this summer, during the hard-ball stage, when the sugars within the plant are actively travelling to the root system. Spraying after flowering occurs is too late as the sugars remain in the upper part of the plant to enable flower and seed development.
An application of Corteva’s Tordon™ PastureBoss™ herbicide containing the necessary active ingredient – aminopyralid – uses these same pathways to the root zone to disrupt the metabolic
process. Tordon PastureBoss is the premium herbicide to apply at the front end of a control programme, especially for dense infestations.
In autumn, an application of Grassmaster™ MCPA herbicide from PGG Wrightson will deliver the final blow. It is readily absorbed by the leaves and carried to the roots of the plant causing abnormal growth, then death.
Come summer 2025, only spot spraying of Tordon PastureBoss will be necessary to target any new, emerging thistle shoots.
On these selected sites, observers will record and document the process. Through a programme of multiple herbicide applications, and defoliation through mowing or hard grazing, this partnership will show how farmers across the country can find success.
The joint project will run for a minimum of 18 months.
¹ Economic cost of Californian thistle and other pasture weeds much larger than
previously estimated, AgResearch, 27 January 2016
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