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Wet weather sparks slug warning for early OSR crops

Early drilled oilseed rape crops will be established over the next fortnight and growers are urged to be proactive in their defence against slug attack after recent rains heightened risk.

While this summer has delivered some warm and dry spells, much of the country has experienced regular showers and, in some regions, these have been heavy and thundery.

Kent-based independent crop consultant James Rimmer of CCC Agronomy says his area of the southeast has been particularly hard hit by storms in late July and post-harvest stubbles are now the wettest he has seen for several seasons.

His clients either drill oilseed rape early during the first two of August or wait until early September, trying to avoid peak adult cabbage stem flea beetle migration.

While this abundant soil moisture is ideal for rapid germination of the early drilled crops across his area, Mr Rimmer says it also provides the ideal situation for slugs.

“We’ll be putting out traps to assess pressure before drilling, but I already get the feeling it could be a slug year this year because of soil conditions,” he adds.

Early treatment

Where slugs are present – the treatment threshold for oilseed rape is just one slug per trap between sowing and the four true-leaf stage – he advises a slug pellet treatment at or very soon after drilling.

With establishment already difficult due to flea beetle, Mr Rimmer emphases the need to avoid additional problems that might hold seedlings back – including slugs grazing off cotyledons or true leaves as they emerge.

“I believe you need to apply a minimum of 50 baiting points/m2 early on. Last year I used [mini ferric phosphate pellet] Menorexx to achieve that.

“It’s also more cost effective because Menorexx gives you those 50 baiting points with a slightly lower dose than a standard sized equivalent,” he explains.

Mr Rimmer adds that typical weather in early August means that even a mini pellet should give a reasonable duration of protection before reapplication.

However, if the weather deteriorates significantly while young oilseed rape crops are still susceptible to slug damage, he recommends switching to a standard sized pellet like Sluxx HP.

Accurate application

Certis’ northern technical specialist Harry Raley agrees that standard pellets are advisable if more persistent rain arrives but reminds growers of the importance of re-calibrating applicators if switching between products.

“Each will have different ballistic properties, so to ensure they are still being spread accurately to the desired distance, applicator settings may need to be changed.

“It’s not practical to conduct a tray test every time, particularly in busy periods, so Certis developed the Calibration Wizard to help.”

The online tool allows operators to enter pellet and spreader type, desired spread width, and application rate. It then provides the correct forward speed, disc speed, disc flute number and aperture setting to achieve those goals.

“In a sensitive crop like oilseed rape drilled into high slug pressure, it is vital application is done right to protect plant numbers and the Calibration Wizard provides a quick and easy way to keep things on track,” adds Mr Raley.

Slug control in OSR

  • Showery weather has heightened slug risk for early OSR crops
  • Trap to gauge pressure before drilling
  • Treat early with mini pellets to achieve a high number of baiting points
  • If weather deteriorates switch to standard size pellets
  • Re-calibrate applicators between products to ensure accurate spreading