Pre-emergence strategy comes under scrutiny

Blackgrass control from pre-emergence herbicides averaged just 46% last year, compared with 67% in the previous season, according to DuPont and Bayer CropScience.

That discrepancy highlights the effect seasonal factors can have on herbicide performance and the need to get the best from available chemistry, say both companies.

Dry conditions were to blame for the dramatic drop in 2011, reports Chris Cooksley of Bayer CropScience, with the lack of soil moisture at drilling preventing chemical uptake by vulnerable weed plants in many situations.

“There’s no doubt the conditions were a challenge,” he says. “But it is important to not just look at one year in isolation. The pre-emergence materials still did a job last year.”

Leading the field for efficacy was flufenacet, says David King of DuPont. “Where it wasn’t used, blackgrass control levels were lower. It is the best active ingredient at the pre-emergence timing.”


Atlantis – iodosulfuron + mesosulfuron

Crystal – flufenacet + pendimethalin

Lexus – flupyrsulfuron

Liberator – diflufenican and flufenacet

Unite – flupyrsulfuron + pyroxsulam

But there was also a vital performance lift from stacking. “That’s where some of the older chemistry comes in, albeit with a new fit. It can be used to boost control.”

Flupyrsulfuron, or FPU as it is now being called, was relaunched last year with a new position as a pre-emergence material. “Previously found in the post-emergence herbicide Lexus, FPU can now be tank mixed with another residual active ingredient and used pre-emergence of the blackgrass.”

The Chemicals Regulation Directorate’s approval of a herbicide sequence based on two sulfonylureas means it can be added to a pre-emergence residual at a rate of 20g/ha, and then followed by a post-emergence application of Atlantis, says Mr King.

“It gives growers more choices. The pre-emergence position means the weeds don’t have the levels of enzymes in them which cause metabolic resistance, so the FPU remains effective.”

Adding FPU to one of the flufenacet-based herbicides (Liberator or Crystal) has two important results, he says. “It raises control by an average of 15-20% and it takes the pressure off the following contact materials.”

Even this year, when control levels from pre-emergence herbicides were lower, there was a benefit from adding FPU, he says.

However, the active ingredient is still affected by target-site resistance, he warns, so has little value where this mechanism predominates. “And obviously it also can’t be used where Unite is planned as the post-emergence spray, as that also contains flupyrsulfuron.”

FPU also has useful broad-leaved weed activity, so complements flufenacet as a pre-emergence mixer. “It brings in weeds such as volunteer oilseed rape, cranesbill, charlock and groundsel.”

Suffolk experience 

AICC chairman and independent agronomist Mike Warmer of Avocet Agronomy tried adding FPU to both Liberator and Crystal last autumn, in an effort to stop blackgrass making further inroads on the farms he oversees in Suffolk.

Where conditions were favourable, he hoped to boost control by up to 30%, helping to reduce the pressure on the post-emergence spray by getting the most from the earlier timing.

“In one particular situation I used 0.6 litres/ha of Liberator together with 20g/ha of FPU pre-emergence, followed by an autumn application of 0.4kg/ha of Atlantis + Biopower,” he says.

The resulting 30% extra control should give a yield benefit of 0.5t/ha, worth about £74/ha. “The cost of the FPU added almost £18/ha to the pre-emergence spray, so it should more than pay for itself.”

Mr Warner believes the current pre-emergence herbicides are often not giving the required 50-60% control of blackgrass, leaving him no option but to stack residuals and make best use of cultural control methods.

Good trial results with FPU persuaded him to give it a go last autumn. “As well as blackgrass control, it has activity on broad-leaved weeds such as cranesbill, fumitory, redshank and poppy.

“What’s more, it can be used in pre- or peri-emergence situations. So if there isn’t much moisture around, you can wait for the crop to emerge and add it to the insecticide spray.”

Where growers are opting for a “normal” September drilling date, he suggests using flufenacet + FPU at drilling, followed by November-applied Atlantis. “Again, if conditions are right, you should get up to 90% control from this approach.”

Where drilling will be delayed – probably because the blackgrass has enhanced metabolic resistance and stale seed-beds are being used – he recommends a flufenacet + FPU treatment to hold the blackgrass until a spring application of Atlantis can be made.

“It is important to note that soil moisture is critical for the pre-emergence herbicides to work effectively. Dry, cobbly seed-beds can really compromise the results.”

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