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Growers should consider spring spray programmes carefully to avoid carry-over of any residual activity from sulphonyl urea (SU) herbicides, say agronomists. Think ahead when planning spring herbicide programmes

• Don’t hold back on hard weeds

• Avoid sulphonyl urea carry-over

• Watch out for persistency in soil

Growers should consider spring spray programmes carefully to avoid carry-over of any residual activity from sulphonyl urea (SU) herbicides, say agronomists.

“This is particularly the case for those growers thinking about increasing the areas of cover crops they will grow next summer or for those thinking of coming back into or extending their oilseed rape acreage, says Dick Neale, technical manager for Hutchinsons.

“The value of both crops is high so it’s best to avoid compromising their establishment if avoidable. This puts more significance on how weed control is built up in the spring to avoid residual herbicide activity carrying over.

“It doesn’t mean holding off controlling the difficult weeds, but look at how ‘softer’ sulphonyl ureas (SUs) could work while still controlling the weeds without the long tail of residuality. It’s the fine details that will matter.”

Grassweed contacts such as SUs Hatra (mesosulfuron-methyl and 2g/L iodosulfuron-methyl-sodium) and Pacifica Plus (30g/kg mesosulfuron-methyl, 10g/kg iodosulfuron-methyl-sodium and 50g/kg amidosulfuron) can have long lasting effects in the soil.

Growers should consider this if planning to incorporate them in spring herbicide programmes where cover crops or oilseed rape is following, says Mr Neale.

“We used to apply these products in the autumn and issues rarely occurred, but as their efficacy on blackgrass has declined they are now increasingly used to tackle wild oats, bromes and ryegrass with a spring application.”

“Wild oats can emerge anytime from February to April and it is when herbicides are applied later to these crops that issues can be seen in following covers and oilseed rape.“

Broadleaf weed SUs, such as those in Ally Max (metsulfuron methyl and tribenuron methyl), cover a whole range of weeds. It has an extended growth stage on its label so is a popular option – but it is also very persistent in the soil.

Mr Neale suggests an alternative to this would be to use tribenuron methyl and thifensulfuron as a mix with no metsulfuron.

“Although there is a reduced weed spectrum  there is nowhere near the same levels of persistence. Other options would be products with actives containing florasulam and fluroxpyr or florasulam and hauloxyfen-methyl.”

Mr Neale says it is important to avoid stacking herbicides with the same active as it is easy to reach damaging levels this way without realising it.

Avoiding build-up

“You want to avoid levels of DFF building up as it is very residual and can form a pencil thin line in the top of the soil,” he explains. “When a new crop meets this it can really knock the vigour out of it.

“It’s not difficult for this to happen without realising it. For example, if a pre-emergence application of Liberator (flufenacet and diflufenican) goes on at 0.6l/ha this contains 60gms of DFF.”

“If using a split label rate then it is possible to go back on post-emergence six weeks later, of which is perfectly legal and safe to do, with 0.3l/ha (30gms DFF) making that a total of 90gms DFF.”

But if Crystal (flufenacet and pendimethalin) is also being used in the programme, DFF is often added to this. That adds in another 50gms of DFF which if then followed up with 0.6l/ha Liberator takes the amount of DFF up to 110gms.

“As its been dry it’s possible that one of the applications hasn’t worked, so blackgrass might be starting to come through, which means another 0.3l/ha Liberator (30gms DFF) is added to the mix, which would bring the total amount of DFF up to a total of 140gms.

“This shows how important it is to scrutinise the partner contents of all products and use product sequences that reduce the overall loading of persistent actives. For example, flufenacet can be sourced with picolinafen or pendimethalin and as a straight active.”

How to avoid residual herbicide carry over

When establishing cover crops within an regenerative farming situation,  experience shows that everything has to be in your favour in the summer to get covers away well, says Ed Brown, head of agro-ecology at Hutchinsons.

Adequate moisture, timely harvest, effective establishment technique are all imporetant, says Mr Brown. We can’t guarantee that all of those come together every year, so we don’t need herbicide residues adding another potential impact on success.

“It’s important that all cultural controls should have been employed meaning that a winter crop is only considered when the grassweed burden is at a manageable level and doesn’t require huge stacks of residual, therefore avoiding the DFF issue.”