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Diquat Alternative Trials and Seasonal Overview

Publication Date: 
3 October 2018
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AHDB’s final Strategic Potato Farm walk of the season took place at Somerby Top Farm, North Lincolnshire on the 18th September. This event included a demonstration by Graham Tomalin of VCS Agronomy on alternatives to Diquat in face of a potential ban. Dr Mark Stalham of NIAB CUF also provided an overview of this challenging season.

Potential Diquat ban challenges future harvest

Diquat is commonly used agrochemical for pre-harvest desiccation “burn off” however, the future of this desiccant is uncertain with the European Commission potentially giving a non-renewal of approval. This ban will have significant ramifications for multiple grower’s desiccation strategies, due to its common usage, and because there are limited alternatives available that are as effective and economical.

In response to this potential ban, AHDBs Spot Farm programme has been conducting demonstration trials to investigate alternative desiccants and combinations of these that growers can use in face of a Diquat ban.

The trial includes 13 treatments (including two flail treatments) each treatment has been sprayed on 26 varieties of potato. There are two timings of application with the first being 5-7 days before the 18th of September. All treatments and varieties will be assessed for defoliation, stem bleaching and re-growth 7-14 days following application 2 (0-10 scale). Following final assessments, a 10 tuber sample from all treatments of 2 commercially representative varieties will be harvested and assessed for skin set (0-10 scale).

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


Graham discussed how the 26 varieties react to different desiccants and the effectiveness of using novel combinations.

Regardless of the desiccants used Graham stressed that clear application quality is vital to achieving the best results from any desiccant application. Coverage is very important and a combination of slower forward speed, water volume and angled nozzles may all improve this significantly.

Graham stated that when it comes to finding a Diquat replacement, there is no universally perfect desiccation strategy and that growers will need to consider a range of strategies, dependant on crop senescence to provide adequate canopy desiccation.

He said if Diquat is banned flailing will become more important with immature crops and indeterminate varieties.

“If canopies are senescing current alternatives will desiccate most varieties but this may take longer. Currently approved alternatives are better targeted to stems and are less effective on leaves than Diquat”

Graham reassured that there are possible alternatives to Diquat in the pipeline, however, these still need approval and the costs will need to be taken into consideration.

Amber Cottingham, AHDB potatoes Knowledge Exchange Manager for the East Midlands said:

“AHDB views the potential loss of Diquat as a priority and is investigating the alternatives. Subsequently, we’ve been able to host this demonstration in response”

Amber explained that a good crop burn off is vital to ensure adequate skin set for harvest and that this can be difficult without Diquat. This poses a risk of tubers being damaged during harvest, in turn increasing exposure to infection during storage.

Amber made clear that planning ahead, routine maintenance and adapting machinery settings to the conditions are the key areas to reduce damage during harvest.

“There are many points throughout the harvesting and grading process where tuber damage could occur. Some components, when badly set or operated, will only result in small changes in damage, while others could show a massive reduction in damage if carefully set and operated”

AHDB has a guide on minimising harvest damage, with more information on these key areas.

Dr Mark Stalham seasonal overview

After the burn off demonstrations, Dr Mark Stalham of NIAB CUF provided an overview of this challenging season and the drought associated issues growers may encounter during harvest.

In one of the driest seasons on record, Mark reiterated the statements he made on the open day in July, that the main resource missing or lacking was water, and that irrigated crops stood a very good chance of being high-yielding.

Mark explained how the NIAB CUF Yield Model was showing that the variation in yield between crops was extremely large, with overall a very slight decrease in yield compared to average.

However, Mark’s main take-home message was that the key to high yields, if such hot seasons become more commonplace, is the irrigation frequency.

“Irrigating at 3-4-day intervals with 15-20 mm and maintaining a smaller soil moisture deficit has produced higher yields than 25-30 mm doses every 6-7 days”

Mark then provided insight on how lack of water may impact nutrient availability  and stated that despite such a dry season, where crops have been well-irrigated this was not an issue.

He detailed how the SPot Farm trials have shown that optimal nitrogen fertilizer rates have generally been at least 20-30 kg/ha lower than expected in terms of canopy development and maintenance.  Yield digs are about to take place to confirm these observations.

“People were frequently asking in July about foliar top-ups as crop growth appeared to be slowing down.  My universal answer was that crops were short of water and little else nutritionally.

Growth rates, determined by the warm, bright conditions post-emergence, were rapid and nitrogen uptake followed suit, so crops mostly had adequate nitrogen uptake to generate their canopies and maintain them until required.”

Mark went on to demonstrate the principle of determinacy and how varieties respond to applied nitrogen through different forms of branching.

He acknowledged a major talking point throughout the season has been the issue of secondary growth, with another generation of tubers in some crops, and the difficulty of setting skins in immature tubers.

Mark outlined the issues that may arise during harvest, as there are a large number of crops, particularly where unirrigated, that have significant quality issues moving forward towards harvest.

He stressed there is a very significant risk of bruising as a combination of two factors: rapid canopy senescence and the lack of irrigation capacity during August as many growers ran out of water.

“Tubers are likely to be in a susceptible status with respect to both hydration and biochemical maturity for bruise development”

Mark, however, declined to fully answer the question of whether he expected the harvesting season to be dry!

Full demonstration and trials data will be presented at the Strategic Farm North results day on 23 January 2019. Plese contact Chloe Lane to register for this event. 

AHDBs videos and guide on minimising damage during harvest can be found at the following link

Further reading



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