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Life after Diquat: desiccation trials across AHDB’s SPot network

Publication Date: 
26 July 2019

Last season’s desiccation demonstration at Strategic Potato (SPot) Farm North came as somewhat of late addition after it became apparent that the EU Commission would be making the decision not to renew the approval of Diquat.

A report from last year’s work can be found here, the results providing useful insight as AHDB planned replicated multi-site trials across the country for 2019.

This ban has had significant ramifications for multiple grower’s desiccation strategies, due to its common usage, and the limited alternatives available that are as effective and economical.

In response to this potential ban, AHDBs Strategic Farm programme has been conducting national demonstration trials to investigate alternative desiccants and combinations of these that growers can use in face of the popular herbicides absence.

This season’s trials work is currently underway across four of our current strategic farms, including: SPot North, South, East and West – with additional work being carried out in Pembrokeshire.

AHDB potatoes desiccation trials 2019

Post-diquat, some crops will be relatively easy to desiccate. Where it is possible to select determinate varieties and to grow them without excess nitrogen on light land; few problems are expected.

AHDB’s two projects this year are designed to help prepare growers for the difficult situations:

  • Indeterminate varieties
  • Heavier land
  • Salad crops with precise size requirements
  • Seed crops with precise size requirements
  • Foliar or tuber disease in the crop
  • Wet weather

Most of the trial sites have four replicates and results will be analysed statistically. Which will be presented at SPot farm Results Days which will take place in January 2020.

Haulm destruction trials

The chemical desiccants available next year are pyraflufen-ethyl (Gozai) and carfentrazone-ethyl (Spotlight Plus). These are much weaker foliar desiccants than diquat. Where a crop must be finished while it is still vigorously growing these chemicals will normally be used following mechanical destruction. The flail is the usual implement, but there are other possibilities. Use of a root cutter can immediately cause foliage to wilt.

This project compares efficacy of a haulm topper and root cutter, with and without chemical desiccants. The four sites cover a wide range of situations:

  • Gemson salad crop at West Orielton, Pembrokeshire
  • Georgina packing crop at Dillington, Somerset
  • Royal processing crop at Elveden, Suffolk
  • Daisy seed crop at St Cyrus Aberdeenshire

Haulm destruction trial treatments

T1 T2 (2 days) T3 (9 days)

T4 (16 days)

1 control natural senescence

2   diquat 1l diquat 3l  
3   diquat 1l diquat 2l  
4 haulm topper      
5 root cutter      
6   Gozai 0.8l Gozai 0.8l Spotlight 1.0l

haulm topper +

root cutter


8 haulm topper Gozai 0.8l Gozai 0.8l Spotlight 1.0l

root cutter

Gozai 0.8l Gozai 0.8l Spotlight 1.0l

haulm topper +

root cutter

Gozai 0.8l Gozai 0.8l Spotlight 1.0l

Fungicide will be applied separately from the desiccant treatments.

Chemical desiccation trial

Chemical desiccation alone, with a weakened armoury, will still sometimes be required. For example; when a flail is not available, wet ground would cause machinery to damage the soil, or there is a risk of flailing spreading disease. The second project compares flailing followed by desiccant with chemical desiccants alone. Treatments include those most likely to be employed in certain situations next year. Products based on pelargonic acid are not yet approved for desiccation, but may be in future. Salt water readily scorches potato plants when accidentally applied. A salt water treatment is included to see if it has the power to finish a crop deliberately.


Treatment T1 T2 T3
1 Reglone 1.5l Reglone 2.5 l  
2 Flail + Spotlight 1.0 l    
3 Flail + Gozai 0.8 l+ Toil 1.5l + Spotlight 1.0l    
4 Gozai 0.8l + Toil 1.5l Spotlight 1.0l  
5 Spotlight 1.0l Gozai 0.8l + Toil 1.5l  
6 Spotlight 1.0l Gozai 0.8l + Toil 1.5l  
7 Spotlight 1.0l + Gozai 0.8l + Toil 1.5l Spotlight  0.6l + Gozai 0.8l + Toil 1.5l  
8 Control, No desiccant.    
9 Pelargonic acid Spotlight 1.0l  
10 Spotlight 1.0l Gozai 0.8l + Toil 1.5 Spotlight 0.6l
11 Saltex Plus 2 x 562 Spotlight 1.0l  

In the chemical desiccation trial fungicides will be applied together with each desiccant application.

Funding has been set aside to continue the desiccation work in 2020. This may include alternative machinery such as a haulm puller. It is likely to repeat some of the chemical combinations used in 2019, with refinements based on results this year. Views of Board members, agronomists, chemical and machinery companies have all contributed to the design of experiments in 2019.

If readers have views on next year’s research, please visit one or more of the trials when dates are announced, and contact your regional knowledge exchange manager or Anne Stone; mob 07813 527527, email

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