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Balancing safety and weed control post-linuron

With linuron gone, the potato sector has been on a quest to identify alternative herbicide strategies. Teresa Rush reports.

 


Teresa   Rush

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Teresa   Rush
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Residual herbicides provide the mainstay of a programmed approach to weed control in potatoes.

 

For more than 25 years, active ingredient linuron was the most widely used residual, particularly in potatoes grown on sand and sandy loam soils. However, on June 3, last year, linuron’s approval for use lapsed and the active ingredient became yet another casualty of the EU’s hazard-based approach to pesticide regulation.

 

A number of alternatives to linuron are available, including metribuzin, clomazone and pendimethalin, but may have restricted use on sands due to possible crop damage following application.

 

Alternative residual options may also offer only limited control of some sand/sandy loam-spectrum weeds, including groundsel, fat hen, annual meadow-grass and black bindweed.

 

Identifying alternatives to linuron has been the focus of herbicide demonstration trials at AHDB’s SPot Farm East on sandy soils on the Elveden Estate in Suffolk since 2016. In the first year the trials focused on one variety, Maris Piper, and only on residual herbicides.

 

Programme extended

 

The programme was extended in 2017 and 2018 to include more varieties, 26 in total, and post-emergence contact herbicides in addition to pre-emergence residuals.

 

In choosing varieties for inclusion in the demonstration, the aim was to cover all market segments – pre-pack, crisping, salad and ware – and there was a focus on varieties grown on the Elveden Estate, explains project lead Graham Tomalin of VCS Potatoes.

 

Some of the varieties selected were newer ones, for which there was less crop safety information, particularly with regard to metribuzin. These included Sensation, Red Fantasy, Georgina (prepack) and Gwenne (salad).

 

The herbicides tested were applied in a single treatment and in 2017 and 2018 the new active ingredient aclonifen was also included.

 

Aclonifen, from Bayer, is a purely residual compound; it has no contact activity and so has to be applied pre-emergence of weeds in mixtures with other compounds. Aclonifen has Annex 1 approval and Bayer’s Ed Hagues says it may be approved in time for this season in the UK.

 

 

SPot Farm East herbicide treatments

Year

Herbicide application

Cost £/ha

Single treatments

 

Untreated

£0

2017 & 2018

VCS 1717 (aclonifen)

n/a

2016, 2017, 2018

Stomp Aqua (pendimethalin 455g/l) 2.8l/ha

£22

2016, 2017, 2018

Praxim (metobromuron 500g/l) 4l/ha

£72

2017

Gamit 36SC (clomazone 360g/l) 200ml/ha

£15

2017

(No product) (flufenacet 500g/l) 600g

n/a

2016, 2018

Defy (prosulfocarb 800g/l) 4l/ha

£26

2016, 2017, 2018

Shotput (metribuzin 70%) 600g/ha

£14

Combination treatments

2018

Praxim (metobromuron 500g/l) 2.5l/ha + Stomp Aqua (pendimethalin 455 g/l) 1.8l/ha + Shotput (metribuzin 70%) 200g/ha

£66

2018

Praxim (metobromuron 500g/l) 2.5l/ha + Defy (prosulfocarb 800g/l) 3l/ha + Shotput (metribuzin 70%) 200g/ha

Industry standard

£71

2018

Aclonifen 600g/l 2l/ha + Stomp Aqua (pendimethalin 455 g/l) 1.8l/ha + Shotput (metribuzin 70%) 200g/ha

n/a

2018

Aclonifen 600g/l 2l/ha + Defy (prosulfocarb 800g/l) 3l/ha + Shotput (metribuzin 70%) 200g/ha

n/a

2018

Aclonifen 600g/l 2l/ha + Shotput (metribuzin 70%) 400g/ha

n/a

2018

Stomp Aqua (pendimethalin 455 g/l) 1.8l/ha + Shotput (metribuzin 70%) 400g/ha

£26

2018

Artist (metribuzin 17.5% + flufenacet 24%) 1.2kg/ha + Stomp Aqua (pendimethalin 455 g/l) 1.8l/ha

£45

2018

VCS 1818 (new active) + clomazone

n/a

2018

VCS 1919 (3 ais)

n/a

Source: AHDB/VCS Potatoes

 

 

Weed control challenge

 

Elveden’s sandy soils provided a considerable weed control challenge, notes Mr Tomalin.

 

“Nearly all residual options have a get-out clause of ‘do not apply on sandy, stony soils’, so if we were to follow that to the letter many of these we could not use on the blowing sands at Elveden. But we have to, we just have to tailor rates. We’ve got approval to use these products, it is just at the agronomist’s and farmer’s risks.”

 

But while the SPot Farm East trials have demonstrated there are certainly a number of options for use in place of linuron, what has also become evident is weed control costs are set to increase.

 

“The costs of the programmes we are looking at now are probably that of when we had linuron.

 

“It is my belief that you will need good knowledge – better knowledge – of the weed spectrum present to better focus your herbicide applications on the weed spectrum you’ve got. If you haven’t got cleavers you don’t need an active to chase cleavers,” says Mr Tomalin.

 

Cost effective

 

Metribuzin is presently the most cost-effective herbicide available to potato growers, he says.

 

“Using metribuzin, taking into account soil type and varietal susceptibility and tailoring rates to as high as you can, will give you the most effective weed control.

 

“If you want a broader range of weed control, I still generally use a combination of three actives and metribuzin would be one of them, with the rate of metribuzin tailored to soil type and variety.”

 

However, metribuzin’s value may be short-lived as it is one of a number of herbicide active ingredients, including also pendimethalin and flufenacet, identified as ‘candidates for substitution’. Pendimethalin is approved for use through to 2024, but flufenacet’s approval expires this year, as does that of metribuzin.

 

“Metribuzin is probably the biggest risk at the moment,” suggests Mr Tomalin.

 

Aclonifen has Annex 1 approval and at the time of writing (January 2019) is awaiting approval in the UK, which might come in time for this season, suggests Bayer’s Ed Hagues.


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Target weed results

Cleavers

 

Three actives gave very good control of cleavers: prosulfocarb (Defy), clomazone (Gamit 36 EC) and flufenacet (Artist).

 

“If you know you’ve got a cleaver issue it is quite clear cut. Control is very good with these three actives and everything else pretty well gives no activity at all,” says Mr Tomalin.

Groundsel

 

Present in all three years of the trial, groundsel is seen as a serious issue on sands growing vegetable crops, where it is extremely difficult to control.

Mr Tomalin says: “It is not competitive in potatoes but we do not want to build the seed bank up for other crops.”

 

The take-home message from the trials at Elveden is groundsel control improves with increasing rates of metribuzin, and this was seen over the three years.

 

“At 300-400g/ha of metribuzin you are getting pretty good control of groundsel,” says Mr Tomalin.

 

Worth noting, however, are reports of resistance to metribuzin in groundsel from Worcestershire, although no resistance was observed on the Elveden site, he adds.

 

Gamit 36 EC (clomazone) in mixtures was also effective on groundsel at Elveden.

 

Small nettle

 

In 2017 populations of small nettle at Elveden were nearly 100/sq.m, a ‘massive’ population, according to Mr Tomalin.

 

“The result to pull out is generally the three-way mixes give good control. Linuron gave good control of small nettle. We are now seeing good activity with metobromuron, metribuzin, pendimethalin and the other interesting one is aclonifen. At 2.5 litres, aclonifen gave very good activity on its own and, if we get approval, it will add quite a bit on small nettle control.”

 

Fat hen

 

Aclonifen also showed good activity against fat hen.

 

Mr Tomalin says: “Other compounds which give good activity tend to be in the combinations; linuron, which we’ve lost, metobromuron, metribuzin and pendimethalin all add to the control of fat hen.”

 

Runch

 

Metribuzin gave good control of runch in the Elveden trials, with control improving with increasing rates. Combinations gave much better control than single actives, apart from the aclonifen.

 

“The brassica weeds are not normally an issue as we’ve always got rimsulfuron (Titus), which gives very good control, as a back up if brassica weeds get through the residual,” says Mr Tomalin.

Herbicide trials objectives at SPot Farm East

  • To assess the efficacy and weed spectrum controlled from a range of residual herbicides/residual herbicide combinations applied at standard crop timings
  • To assess any crop damage/phytotoxicity from residual herbicide applications with particular reference to a variety screen
  • To assess varietal effects of post-emergence applications of metribuzin, bentazone and rimsulfuron to a range of varieties

Source: AHDB/VCS Potatoes

 

SPot Farm East herbicide trials conclusions

  • Alternatives to linuron exist but herbicide programme costs are likely to increase
  • Knowledge of the weed spectrum present will aid cost-effective control – target applications to the expected weed spectrum
  • Metribuzin is the most cost-effective option for many weeds but care is needed with soil type and varietal susceptibility
  • Three residual active ingredient combinations provide control of a broader range of weed species
  • Potential new active ingredient aclonifen looks a useful addition when used in combination with other actives, if approval is granted

Source: AHDB/VCS Potatoes

 

Post-emergence treatments in potatoes

The post-emergence herbicide trials at Elveden focused on metribuzin at two different rates – 200g/ha and 500g/ha – in combination with Titus (rimsulfuron) at low rates (30g/ha) and straight Basagran (bentazone) at 1.1kg/ha, which is the new rate.

 

There was phytotoxicity from the Basagran but only on certain varieties, says Mr Tomalin.

 

Russet Burbank was the worst affected screen and a similar degree of phytotoxicity was seen on salad variety Leontine.

 

Results from the trials have been used to produce a guide (see Varietal sensitivity to post-emergence herbicides panel) to varieties’ sensitivity to herbicides, although growers are advised this is only a guide, not a recommendation.

 

Mr Tomalin says: “Rating varieties across four different levels is as good as we need.

 

“I would suggest you wouldn’t be going anywhere near the varieties in the moderate sensitivity and high sensitivity groups with a post-emergence herbicide. Those in the low sensitivity group you may do, with caution.”

 

Varietal sensitivity to post-emergence herbicides

 

Basagran 1.1kg/ha

Tolerant to very low sensitivity

Moderate sensitivity

High sensitivity

 

Gwenne

Angelique

Bambino

Gemson

Iodea

Georgina

Brooke

Challenger

Royal

Performer

Rooster

Lanorma

Nectar

Maris Piper

Maris Peer

Innovator

Forza

Soraya

Sensation

Red Fantasy

Shepody

VR808

Melody

Russet Burbank

Leontine

Jelly

 

 

Shotput 500g/ha

Tolerant

Low sensitivity

Moderate sensitivity

High sensitivity

Vales Sovereign

Brooke

Marfona

Saxon

Challenger

Markies

Royal

Daisy

Lanorma

Russet Burbank

Jelly

Angelique

Bambino

Gemson

Rooster

Soraya

Maris Piper

Leontine

Eurostar

Melody

Nectar

Performer

Gwenne

Shepody

Georgina

Iodea

Forza

Innovator

Maris Peer

VR808

Source: AHDB, VCS Agronomy

 

Weed control on fen soils

Controlling weeds in potatoes post-linuron has also been explored over the last two seasons at agronomy firm Hutchinsons’ fenland potato trials demonstration site on black soil near Mildenhall, Suffolk.

 

Hutchinsons root crop technical manager Darryl Shailes says: “One of the big changes in herbicide treatments this coming season is the fact we no longer have linuron. Although it has not been the mainstay for a lot of Fen growers, for growers on more mineral soil types a big adjustment will need to be made.”

 

The aim of the trials is to inform situations where a crop safety risk must be weighed up against a weed threat.

 

At the fenland site last year, Shotput had the greatest effect on Eurostar and Innovator; Daisy, Markies and Maris Piper were also impacted but damage was reduced at lower doses.

 

Damage from Basagran was worse on Markies, Royal and Sagitta, but again damage was at the higher dose.

 

Mixes had an effect on nearly all the varieties but Agria appeared to have the greatest percentage vigour reduction with Basagran and higher rate Shotput treatments compared to untreated, even though initially it did not look as though it was hurt too much, says Mr Shailes.

 

In 2017, Innovator and Manitou showed the most severe chlorosis and necrosis when treated with Shotput. Maris Peer and Eurostar also showed high levels of damage.

 

Basagran affected nearly every variety, particularly in the first 14 days after application. All varieties showed chlorosis after Titus but minimal necrosis. Maris Piper, Sagitta and Nectar were among the worst affected.

 

“A lot of similarities are coming through and we are starting to develop a bit of a picture based on two years of work in some cases, but these results cannot be seen as a recommendation,” says Mr Shailes.

 

Fenland potato site herbicide treatment

 

Untreated

Shotput 0.5kg/ha

Shotput 0.2kg/ha

Basagran 0.55kg/ha

Basagran 1.1kg/ha

Titus + Shotput 50g + 0.5kg

Titus + Basagran 50g + 0.55kg

Basagran + Shotput 0.55kg + 0.2kg

 

Fenland potato site varieties

 

Agria, Arsenal

Brooke

Challenger

Daisy, Diva

Eurostar

Fontane

Innovator

Maris Peer, Maris Piper, Markies

Royal

Sagitta

 

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