HERBICIDE application failure can be complex. Last year, Greg Condon, WeedSmart's extension agronomist in southern New South Wales, saw widespread failure of clethodim applications for ryegrass control, particularly in canola crops.

A combination of factors contributed to this spike in weed numbers while driving the weed seedbank down will take time and require an immediate farming systems response.

Condon said clethodim has underpinned grass weed control in Clearfield (CL) and triazine-tolerant (TT) canola hybrids in southern NSW since the herbicide was released, but conditions in 2025 highlighted the need to have multiple effective control measures in place.

"Conditions at the start of the 2025 winter cropping season were stacked against effective early weed control across southern NSW," Condon said.

"Most canola crops were dry sown with no pre-seeding knockdown due to the late break and low soil moisture, which meant that many of these crops then emerged later than usual. The pre-emergent herbicides typically applied at sowing, trifluralin and propyzamide, also rely on moist soil and rainfall for incorporation and availability for uptake to control weeds.

"This combination of conditions resulted in unusually high pressure on in-crop ryegrass control with clethodim."

"To compound the situation, the previous run of very wet years had built up a significant carryover weed seedbank. High weed pressure plus less-effective early weed control resulted in a perfect storm with growers generally on the back foot the whole season."

COLD PRESSURE

The cold conditions in June across large parts of southern NSW, suppressed early crop vigour and reduced the efficacy of clethodim applications, which rely on warmer atmospheric temperatures for effective weed control.

While canola offers other weed control options, the pressure on clethodim makes it a weak link in the tight wheat-canola-wheat rotation that is common on farms in southern NSW.

"The upshot has been a spike in clethodim resistance, which had been building in previous years," Condon said.

"Growers have been mixing clethodim with butroxydim (Factor) and metazachlor (Tenet) in the legume phase in a bid to wind back increasing levels of resistance, but the result has been that some ryegrass populations now have strong resistance to all three actives."

DIVERSITY

He said the circuit breakers for herbicide-resistant ryegrass will always lie outside a farm's herbicide program and in cultural practices such as harvest weed seed control, hay production and brown manuring vetch or field peas.

The second step is to diversify the herbicide-tolerant canola hybrids used in the rotation. Long-term farming systems will include all these strategies, not simply as a reset, but a targeted program based on stacking cultural tools, diverse rotations and alternative canola herbicide traits to reduce reliance on clethodim.

Single trait glyphosate-tolerant (Roundup Ready, TruFlex and Optimum GLY), and glufosinate-tolerant (LibertyLink) canola hybrids can be rotated with imidazolinone-tolerant (Clearfield) and triazine-tolerant (TT) hybrids to broaden the range of in-crop herbicides used in canola to drive down ryegrass numbers. 

In addition, there are stacked traits available with TruFlex + Clearfield Combined Technology (glyphosate + imi tolerance), Clearfield + Triazine Tolerant Combined Tolerance (imi + triazine tolerance), LibertyLink + TruFlex (glufosinate + glyphosate tolerance) and LibertyLink + TT (glufosinate + triazine tolerance).

"While maintaining a tight wheat-canola rotation, growers can easily target ryegrass with a different herbicide in the canola phase with glyphosate in Optimum GLY or TruFlex hybrid in year 1, clethodim + glyphosate in Roundup Ready hybrid in year 3, glufosinate + glyphosate in stacked LibertyLink + TruFlex hybrid in year 5 then glufosinate + atrazine in stacked LibertyLink + TT hybrid," Condon said.

"By also mixing and rotating the pre-emergent herbicides Overwatch, trifluralin, propyzamide and atrazine, combined with early sowing, growers can expect to lower the ryegrass seedbank by 50 per cent, based on research led by Dr Chris Preston at Adelaide University."

For long-term weed control, Condon reiterated the value of implementing diverse and competitive rotations that keep the weed seedbank low and maintain profits using the WeedSmart Big 6 strategy.

"We have seven years of data from the southern NSW farming systems project that supports the shift into diverse cropping rotations," he said.

"Really robust and profitable farming systems routinely use double-breaks to broaden the herbicide program, build soil nitrogen and allow the implementation of croptopping with a Group 14 (for example, Sharpen or Sledge) plus paraquat to target late-emerging weeds and prevent seed set.

"Adding a legume opens up many weed control options for growers. For instance, faba bean is very competitive against ryegrass, and vetch and field peas are excellent options for brown manuring to maximise fixed-nitrogen while eliminating weed seed set with a double-knockdown tactic. Vetch is also a good option for capturing immature ryegrass seed in hay or silage."

Shifting to an integrated weed management system that includes the WeedSmart Big 6 pillars can turn the tables on clethodim-resistant ryegrass in southern NSW.

This article was provided by Cindy Benjamin, WeedSmart.

For more information about integrated weed management strategies, visit: www.weedsmart.org.au