Weighing up potato desiccation options without diquat – CPM – Andrew Cromie, John Keer

Various desiccation strategies were put to the test at Hutchinsons Fenland trials site ...

The site was situated at A L Lee Farming Company’s Friesland Farm, near Mildenhall in Suffolk last September.

The work focussed on different timings and sequences of the two main chemical alternatives, Spotlight and Gozai, with and without flailing (see table below) in an 18ha field of Markies on black fen soil.

Results across all treatments were better than expected, but agronomist Andrew Cromie stresses this was mainly due to conditions favouring desiccation. “Unusually, canopies were already starting to senesce by Sept, while warm, dry weather was ideal for chemistry to work quickly.

“We saw 85% burn-down within a week, which is extremely quick, and I doubt it would be repeated in another year with a big, growing canopy.”

Generally, the most effective strategy was to apply a desiccant to open up the canopy and follow this with mechanical topping. Andrew acknowledges flailing has challenges, especially in wetter years than the one just gone. Wheeling damage, erosion of potato beds, soil compaction and the potential for surface crop residues to increase disease risk in tubers must all be considered, as does weed pressure.

“Fortunately this Sept was very dry and our trial was in a very clean field, but if bindweed is present, flailing’s not an option,” he says.

Topping can also trigger regrowth from the plant’s axil, so growers should leave a reasonable amount of stem (at least 15-20cm) exposed and not covered with mulch, to ensure any follow-up herbicide can work effectively.

Results from plots where flailing wasn’t used suggest good canopy destruction and skin set can still be achieved through chemical applications alone, although the situation could be very different in other seasons and soil types, he warns.

The ‘belt and braces’ treatment (number six) shows the maximum that can be legally applied and performed best overall, but at a cost, Andrew points out. At 160-170/ha, that approach is double what might have been spent using a diquat-based programme.

John Keer from Richard Austin Agriculture says the number of treatments applied is as important as the products used, so the key is to start desiccation programmes earlier and knock-down canopies in stages.

“Sprays only kill the part of the plant they hit. Use the first application to open up the canopy, then alter subsequent applications according to the amount of green material left in the crop. Include a fungicide wherever you’ve still got green material to reduce the risk of late blight coming in,” he says.

Without flailing, three or four herbicide applications may be needed to achieve full desiccation, although it depends on the variety, soil type and weather, so decisions must be on a field-by-field and season-by-season basis, he says.

Regardless of the approach taken, desiccation is likely to take one to two weeks longer than with a diquat-based programme, so more focus is also needed on other ways of managing canopies to help speed-up the process, adds Andrew.

Planning starts before the crop is sown, with variety choice and soil type being key considerations. Growers may question whether growing a long-season variety like Markies is the right option on certain fields. In some cases, it may be better to go for a more determinate type that senesces earlier. Reducing nitrogen applications to avoid over-large canopies will also be worth considering.

Other novel desiccation options trialled at Friesland Farm included pelargonic acid, which is currently only available for amenity use in the UK, so pricing is prohibitive for agriculture (cost 250-500/ha at the 50-100/ha rates). Salt solution was also investigated (sodium/potassium chloride) and was found to be slower acting than diquat and not as effective as herbicides. It also requires high water volumes, which adds logistical challenges.

Treatment options on trial

9 September +7 days +2 days
1 Untreated +/-Flail Untreated
2 Untreated +/-Flail Gozai (0.8) + Phase II (1.5)
3 Untreated +/-Flail Spotlight (1.0)
4 Gozai (0.8) + Phase II (1.5) +/-Flail Gozai (0.8) + Phase II (1.5)
5 Spotlight (1.0) +/-Flail Spotlight (0.6)
6 Spotlight (1.0) + Gozai (0.8) + Phase II (1.5) +/-Flail Spotlight (0.6) + Gozai (0.8) + Phase II (1.5)

All treatments applied in 300 l/ha of water and included 0.5 l/ha Ranman Top (cyazofamid) for blight control.

Source: Hutchinson Fenland trials, Suffolk (2019).

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