Make the most of peri/post-em applications to counter current grass weed threats - Agrii - Connecting Agri-science with farming

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November 5, 2013

Make the most of peri/post-em applications to counter current grass weed threats

Excellent late growing conditions following plentiful October rain across the country this autumn mean cereal growers need to take full advantage of good soil moisture levels to build-up the residual component of their herbicide programmes if they are to counter serious black-grass, ryegrass and brome threats, advises Agrii head of agronomy, Colin Lloyd. All the more so where pre-planting or early pre-em control was restricted by dry seedbeds or ALS resistance is a particular concern.

“Enhanced metabolic resistance means the best black-grass control we’re getting from
Atlantis at our long-standing benchmark Stow Longa trials site these days is 25%,” he pointed out. “And once weeds reach any reasonable size control post-em is virtually negligible without good residual support.

“As well as encouraging rapid autumn weed growth, universally good soil moisture levels in most areas are providing the ideal conditions for peri and early post-em residual activity. So I would strongly advise anyone with grass weed problems to seize the opportunity for extra control now while it lasts. Active growth is far more likely this side of Christmas and our studies invariably show control at this stage is markedly better than in the spring.”

In planning their applications, Colin Lloyd reminds growers that not all pre-emergence options are supported at the peri-em timing, and warns that where cereals are not yet hardened-off some transient yellowing may occur – especially with multiple tank mixes and where crops are under nutrient stress.

He sees flufenacet as the key building block for managing resistant black-grass, stacked alongside other residuals such as the pendimethalin/picolinafen combinations which Agrii trials have shown to be valuable where ALS resistance is strong and weeds are still small.

For post-em applications when grass weeds are larger and more ‘oomph’ is required he suggests using mesosulfuron/iodosulfuron alongside the residual partners. Alternatively, prosulfocarb + clodinafop-propargyl or DFF/CTU combinations (while still available) will be best for those wanting to save the sulfonyl urea ‘bullet’ for subsequent use.

“Our Stow Longa work shows meso/iodo applications need to be at the 1-2 leaf stage of black-grass once resistance has started to bite,” he noted.

“The latest research also indicates autumn post-em applications – preferably after a robust pre or peri-em – are the best bet for controlling ryegrass and sterile and great brome.”

Where spraying windows are especially limited, Colin Lloyd recommends prioritising winter barley, followed by first wheat, oats and then second wheat. The major differences in weed competitiveness between wheat varieties recorded at Stow Longa also make variety prioritisation by the Agrii competitive index established his annual studies valuable.