Diquat Ban May Require Wider Agronomy Changes – Agronomist & Arable Farmer – Andrew Cromie, John Keer

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Andrew CromiePotato growers will have to start planning future desiccation strategies before the next season's crop is even sown.

Trials at the Hutchinsons’ Fenland demonstration site near Mildenhall in Suffolk this autumn have illustrated how the loss of diquat is likely to increase desiccation costs and put more focus on other ways to help manage canopies towards the end of the season.

Variety choice and fertiliser strategy will be two key elements to consider, alongside the alternative chemical desiccants and mechanical flailing. 

Various desiccation strategies were put to the test in an 18ha field of Markies on black fen soil at the Friesland Farm site, hosted by the A L Lee Farming Company.

The work focused primarily on different timings and sequences of the two main chemical alternatives: Spotlight (carfentrazone-ethyl) and Gozai (pyraflufen-ethyl), with and without flailing.

Results across all treatments were generally much better than expected, although agronomist Andrew Cromie stresses this was mainly due to conditions this year being so conducive to effective desiccation. “Unusually for this variety, canopies were less bulky and already starting to senesce by September, while warm, dry weather was ideal for chemistry to work quickly," he said.

The situation is likely to be very different in a more normal year, with larger, growing canopies, that will be harder to knock-down, Mr Cromie pointed out.

Growers will, therefore, have to think carefully about whether growing a long-season variety such as Markies is the right option on certain fields and soil types if it is going to be harder to manage the canopy before harvest. In some cases, Mr Cromie suggested planting such varieties earlier in the season, or switch to a more determinate type that senesces earlier.

In some cases it may be possible to reduce nitrogen rates slightly or apply it earlier. “It's all about working towards helping the crop die off naturally before harvest, without compromising yield. This is not something you can start thinking about in September; planning must begin before the crop goes into the ground in March or April," he commented.


Targeted approach

While variety choice, planting date and nitrogen use may help with canopy management, growers also need to carefully plan how alternative chemistry and flailing will fit into their system. The Hutchinsons trials indicate that generally the most effective strategy is to apply a desiccant to open up the canopy and follow this with mechanical topping, leaving enough stem attached for effective uptake of any subsequent herbicide application that is required to control regrowth.

Mr Cromie 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.

Results from plots where flailing was not 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.

The 'belt and braces' treatment featuring two applications of both chemical alternatives 10 days apart shows the maximum that can be legally applied and performed best overall, but at a cost, Mr Cromie said. At 160-170/ha, that approach is double what might have been spent using a diquat-based programme.

John Keer, from Richard Austin Agriculture, said 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. He added: “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."

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.