The cost of desiccation without diquat

Friday, 31 January 2020

Spotlight and/or Gozai straights or combinations, along with flailing, can give virtually as quick desiccation as diquat, as trials and demonstrations across our Strategic Potato (SPot) Farm network to evaluate potential replacements have shown. But at what cost?
Dr Mark Stalham, Head of NIAB CUF who led the trials, reveals the results, Mark Topliff from our Farm Economics team crunches the numbers and we speak to farm manager Will Gagg for his views

The withdrawal of diquat and the absence of like-for-like herbicide replacements available on the market poses a real challenge to growers because of its use as a desiccant as well as a herbicide. Potato haulm destruction, which is usually carried out by chemical desiccation, plays a crucial pre-harvest role as it stops the tubers bulking and allows their skins to set. It also reduces the risk of bacteria, fungus and viruses getting in to the crop.

Led by Dr Mark Stalham, new head of NIAB CUF, demonstration plots and experiments designed to help potato growers re-think their desiccation strategies have been taking place in different areas around the country. These include North Lincolnshire-based SPot Farm North, (RJ and AE Godfrey), Elveden Farms (Suffolk), the James Hutton Institute (Dundee, Scotland), SPot West (Heal Farms, Shropshire) and SPot South (Dillington, Somerset).  

Using different varieties, and targeting vigorous, complete canopies at close to commercial defoliation timings, the work evaluated speed of leaf and stem desiccation, skinset, and effects on yields and internal defects on both ware and seed crops.

Two applications at weekly intervals of different combinations of Gozai (pyraflufen-ethyl), Spotlight Plus (carfentrazone-ethyl), Finalsan (pelargonic acid) and Saltex (concentrated brine solution) were made, while one combination of Spotlight/Gozai was applied three times. Actives were also combined with flailing. A hand-simulated haulm puller was applied to the seed experiments.

Assessments were then made at weekly intervals from the first application or defoliation.

The initial results have been assessed from work on the popular ware variety Maris Piper, and the slightly later-maturing Jelly grown for seed.  The maincrop processing variety Royal was assessed at Elveden Farms, the packing variety Georgina at Dillington and the crisper Titan at Heal Farms.

Grow-on after application

While Reglone’s speed of desiccation was known as its major advantage, overall, applications of straights or combinations of Spotlight and/or Gozai, were only two to four days slower in dying off, says Mark, adding that pelargonic acid (Finalsan) was the slowest acting of the trialled actives.

The slightly slower kill-time achieved by Spotlight/Gozai, and in-turn, pelargonic acid allowed for a slightly greater yield increase after application than from Reglone. This is would be unlikely to have a major impact on main-crop, but is something worth monitoring for those growing to a strict specification for seed or salads.

Meanwhile, most of the plots received two treatments and no benefits were noted from applying a third treatment.

“We found that flail and haulm-pulling caused instant death, and even at four weeks after treatment there was no regrowth from any of the different methods used.”

He and the team observed Reglone and Saltex as the most effective means of removing leaves, although for Saltex, efficacy depended on atmospheric conditions. 

“With the later-maturing varieties such as Royal, it is more difficult to flail effectively to an even length, which should be between six and eight inches,” says Mark. “Once the haulm has lodged, it can get stuck in the wheelings, and some flailed stems were up to three feet long.”

The length and greenness of the stems also affected on the ease of cutting of the tops off the plants. “Most varieties were cut vertically, but the late-maturing stems were more difficult to do correctly.”

Using a skinning barrel, skinset was also assessed, since this is the factor used by most growers in determining readiness for harvest, and results show that three weeks after the initial treatment, Spotlight and Gozai combinations were not significantly different to those using Reglone, flailing, haulm pulling or Saltex.

Mark says: “Three of the crops grown in England, Jelly, Georgina and Maris Piper, were fully skin-set after three to four weeks. However, Royal and Titan were not fully skin set, even at the end of four weeks.

Yields were also unaffected by treatment type, he added, pointing out that there was typically a 10t/ha yield increase in undefoliated crops over the three weeks after initial treatment. In addition, there were no significant differences on vascular browning or stem-end necrosis across any of the chemical or mechanical treatments.

“One area we found some variation was stolon detachment; while there were no problems with Jelly, Georgina or Maris Piper, there was some adhesion in Royal and in the crops in Scotland, but overall virtually no stolon plug removal in any trial.”

Cost analysis – expect increases

Mark Topliff, Lead Analyst in AHDB’s Farm Economics team said: “Those growers moving from desiccating with diquat only to a flail and PPO inhibitor will see a significant increase in costs per ha – anything from 150 – 210%*”

“If the grower already uses a flail to top the haulm and continues to use in combination with a PPO inhibitor, then the cost increase is less significant – the region of 13 – 15%”

**Assumption sources: AHDB; DEFRA; Farmers Weekly; John Nix Pocketbook 2020; Mark Taplin agronomist.

*Costs could be lower if grower has good access to a specialist contractor

**Assumption sources: AHDB; DEFRA; Farmers Weekly; John Nix Pocketbook 2020; Mark Taplin agronomist.

Saltex – effective, but not yet approved

The loss of diquat is a massive blow to the potato industry, according to Will Gagg, farm manager for  RJ & AE Godfrey.  

Many of the farms who used the active as a pre-emergence herbicide or a desiccant are finding they may have to start to use Roundup (glyphosate), which is a higher risk chemical than diquat, he says.

Opting for flailing before desiccating may also create a nightmare for harvesting on heavy  soils in wet autumns, such as 2019, too.

The farm, which is the current SPot Farm North, has been hosting some of the trials done by Mark Stalham of NIAB CUF on desiccating in a post-diquat world.

Will says: “We have been impressed by the trials, surprised and pleased with some results but disappointed by others.”

He had been expecting more from pelargonic acid, but despite observing differences in the haulms within just twenty minutes of application, desiccation did not then go any further.

On the other hand, Saltex performed better than he had expected.

“This is good news as it is readily available and has limited effects on the next crop. This is an area we certainly want to follow up next year.”

However, he cautions that there are no official recommendations for Saltex, and as yet we do not have approval from the Chemicals Regulations Directorate (CRD).

He notes that the trials showed that other desiccants such as Gozai and Spotlight worked efficiently, although Spotlight does not open the crop up, so, on his farm, Saltex could take its place.

“We need to do more research into later applications of desiccants, perhaps repeating the same trials next year but desiccating indeterminate varieties in September or October.”

Moving on to talk about the general herbicide trials, he says that there were no outstanding results from any one application. Moreover, despite timing being perfect, some bindweed and fat hen was left behind.

“Costs were similar to what we are doing now, so we will not be altering our average on-farm base line for herbicide.”

Trials protocol

  • N fertilizer determined by RB209
  • AZO + Tee-jet OPOZ flat fan 110° nozzles
  • 400 l/ha (except Saltex 1123 l/ha)
  • 3 bar pressure
  • 3.6 m sprayed width

SPot North flail demonstration day

Our Strategic Farm hosts RJ & AE Godfrey were considering investing in a haulm topper ahead of the 2020 season. In September 2019 we hosted a 'Flail Demonstration Day' at the farm in North Lincolnshire, so that local growers could visit and see the results of four potential options.

Farm Manager has subsiquently invested in a three-row system for the coming season.

'Topper of the Pops'?

Below are the machines that were on show on the day:

Further reading

Strategic Farm results presentation Pre-planting decisions for desiccation Latest potatoes podcast - desiccation and storage in 2020