Every decision in winter weed control involves a compromise. Joseph Watts, Technical Specialist in Agronomy at PGG Wrightson, says farmers are always weighing weed control against clover safety, and every choice has a consequence either way.

Phenoxy herbicides are the main tool for controlling thistles, with MCPA and 2,4-D the products most commonly used, he says. They work by mimicking a natural plant growth hormone, causing uncontrolled cell elongation and division. The plant does not so much die as grow itself apart.

“It can be three weeks plus before these are actually dead, because they need to be actively growing for that hormone to express itself.”

That same requirement for active growth is what creates the risk to clover, Watts says. When conditions are warm enough for thistles to be growing well, clover is likely growing well too.

“The biggest complaints we get normally boil down to a disconnect between expectation and the timing that they used.

“You’re not spraying air temperature, you’re spraying plant activity.

“Think about what the temperature has been that week or the weeks prior, and think, has this plant been growing?”

In colder, slower periods, 2,4-D ester formulations get into the plant faster than amine formulations and cause more damage at low temperatures.Those formulations are harder on clover, but it is usually inactive by then, and grazing the paddock beforehand to strip clover leaf reduces the risk further. In warmer conditions the kill is stronger, but clover is more exposed.

He says the choice between actives comes down to potency on weeds versus clover safety. For scotch or variegated thistles, 2,4-D amine can be a good option. Nodding or wing thistles call for stepping up to MCPA or 2,4-D ester, and for large wing thistles, clopyralid is probably needed, Watts says.

Farmers in areas with known or suspected resistance should follow up after spraying and look closely at what they find, he says.

“If you’re noticing that this thistle’s not dying, then definitely consider adding some of those other products into the system to help control them.”

This is where a PGG Wrightson technical field representative can help — working through active selection, timing, and resistance risk for a specific situation. Brand matters less than getting those decisions right, Watts says.

“Get the right active, get the right time for what you’re wanting to achieve out of it, and do it in the right conditions.”

Make sure to contact PGG Wrightson to plan your crop and pasture requirements. You can find your local Technical Field Representative or store team at https://store.pggwrightson.co.nz/find-your-store 

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