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The cost of potato haulm desiccation without diquat

According to AHDB in the UK, Spotlight and/or Gozai straights or combinations, along with flailing, can give virtually as quick desiccation as diquat. This was shown in trials and demonstrations across AHDB’s 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, and Mark Topliff from AHDB’s Farm Economics team crunches the numbers.

The withdrawal of diquat and the absence of like-for-like herbicide replacements available on the UK 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 Reglones 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 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 AHDBs 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 in the region of 13 15%

Source: Read the full report on the AHDB website.
Photo: PotatoEurope
Video: PotatoEurope – can also be viewed on YouTube.

Lukie Pieterse, Editor and Publisher of Potato News Today

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