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The Diquat Dilemma

Publication Date: 
4 December 2018

Since the EU Commission has decided not to renew the approval of diquat, AHDB is actively seeking new ‘burn-off’ strategies that growers can adopt after the ‘use-up’ period ends.

Diquat* dibromide is a commonly used agrochemical for pre-harvest desiccation or ‘burn-off’. However, the European Commission has not renewed its approval for use, meaning this desiccant will no longer be available for grower’s when the ‘use-up period ends in February 2020.

The non-renewal of diquat was based on the concerns surrounding exposure of bystanders, residents and birds. Diquat is an active ingredient in popular desiccants such as Reglone, and following the ban of sulphuric acid in 2009, diquat has become the quickest and most reliable method for ‘burn-off’. Alongside Linuron, this ban is another example of the continually shrinking agrochemical tool-set available to grower’s disposal.

The EU has proposed a market withdrawal date of 4 May 2019, with a use-up period until 4 February 2020. The UK Chemicals Regulation Division of the Health and Safety Executive has still yet to confirm the use up periods for UK growers, but it is only in isolated cases that they deviate from EU timings.

Due to its common widespread usage, and limited time still available for use, many growers will have to adapt their ‘burn-off’ strategies. In preparation of the ban, AHDBs Strategic Potato (Spot) Farm programme, which champions on-farm implementation of research findings, has been conducting demonstration trials to investigate alternative ‘burn-off’ strategies that growers can use in face of the ban.

One such trial is at Somerby Top farm, SPot Farm North, which is a mixed arable farm in North Lincolnshire, it is the newest addition to the SPot Farm programme.  The farm spans the Lincolnshire/Yorkshire border and grows around 440 ha of main-crop potatoes for the packing market each year. Here ongoing demonstrations have been conducted investigating alternative desiccants to diquat and combinations of these that growers can use.

The demonstration trials include 13 treatments, including two flail treatments. Each treatment was sprayed on 26 varieties of potato. There were two timings of application with the first being the 12 September 2018 and the second being 7 days later. After 14 days of the final spray, all treatments and varieties were assessed for defoliation, stem bleaching and re-growth. Following assessments, a 10 tuber sample from all treatments of 2 commercially representative varieties was harvested and assessed for skin set.

Agronomist Graham Tomlin from VCS (UK) Ltd has been conducting the SPot Farm demonstrations and has assessed how the 26 varieties react to different desiccants, and the effectiveness of using combinations.

From the results of the demonstration, Graham believes that mixes of desiccants can offer growers approximately 60% as effective ‘burn-off’ performance as diquat.  The currently approved alternatives were shown to be better targeted to stems and less effective on leaves than diquat. From the desiccants investigated a combination of desiccants Spotlight plus and Gozai were the most effective.

Table 1: Outline of the treatments used during the desiccation trial

This combination was shown to be effective at desiccating stems and preventing regrowth from green stems, which is important as regrowth can act as a conduit for blight infections and also reduces skin set, exposing tubers to more potential damage during harvest.

However, Graham believes that present application methods of the alternatives will not provide as sufficient desiccation as diquat in large canopies of indeterminate varieties, unless improvements in desiccant application are made, and that this will be the key to achieve the best results from any burn off strategy, and to make the most of the currently available alternatives.

He stressed that a combination of slower forward speed, water volume and angled nozzles may all improve application and coverage significantly. Graham stated that for any application of desiccant the most important strategy is to “get low and go slow” as any initiatives that increase the amount of desiccant reaching the stem will significantly improve desiccation.

One such strategy to increase desiccant to stem contact is flailing, and the desiccants under study were all shown to be even more effective when used in combination with flailing. Using an all-in-one flailing/spraying machine will allow this, but does incur an increased cost.

Graham states that flails should leave about 20cm of the stem to allow uptake of desiccant and that any flailed foliage from the stems should be removed to allow spray to hit the stems more precisely. Flailing and spraying is an effective ‘burn-off’ strategy in dry seasons like 2018 and on lighter soil types, however, it may be problematic in wetter conditions.

Graham aims to extend the Spot Farm demonstrations to full canopy experiments that has much greener foliage and continue to test desiccants on different potato varieties. Additionally, Graham wants to conduct new trials that are concerned with improving desiccant application and investigate different nozzles for the application that will improve efficiency and coverage.

AHDB will soon be announcing Strategic Farm demonstration programmes for 2019, and growers can expect to see diquat replacement work high on the agenda.

Further reading

*What is diquat?

Diquat is a contact herbicide and desiccant, it quickly acts on the parts of the plant to which it is applied, and causes desiccation and defoliation by inhibiting the plant's photosynthetic components. It has been used by growers in the UK for more than 50 years and is applied to a broad spectrum of different crops. For potatoes, it is used for burn off due to its desiccation properties, and application, which is usually 2-3 weeks prior to harvest, results in crop canopy foliage removal.

For growers an effective ‘burn-off’ results in the tubers skin setting, which significantly reduces the risk of foliar tuber diseases during storage and end use. ‘Burn-off’ also stops the tuber bulking, in an aim to maintain the tuber at the desired marketable size before it enters storage. The removal of diquat will have significant impacts on UK potato production, due to its popularity and because it is currently much more effective than the approved available alternatives. Diquat is also of the only two non-selective herbicides available, in which weed resistance hasn’t significantly evolved.

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