Hold back oilseed rape herbicides at your peril

Growers are warned not delay their early herbicide applications in oilseed rape through fear of losing their crops to cabbage stem flea beetle attack.

This autumn sees the first plantings of winter rape without neonicotinoid insecticide seed dressings for a number of years.

Without the protection they provide against the pest, some may be put off investing in a broad-leaved weedkiller at an early stage, especially as there are reports of high levels of beetles on volunteers already.

See also: Campaign aims to save key oilseed rape herbicides

BASF’s Steve Dennis tells Farmers Weekly that going in with the mindset that a crop is going to fail is the wrong approach.

“People underestimate the potential yield losses from broad-leaved weed competition, especially when using low seed rates,” he adds.

Early OSR hebicides

  • Apply at expanded cotyledon to one-true-leaf stage
  • Later applications compromise weed control and yield
  • Consider pre-em if cleavers and hedge mustard a problem

Mr Dennis points towards Adas research that shows yield losses of 1.1t/ha at higher seed rates and up to 2.3t/ha at lower rates. At current prices, that equates to £250/ha and £575/ha, respectively.

Blackgrass burden

While the strength of metazachlor-based products is in broad-leaved weeds, they can give a useful boost to blackgrass control if applied before the weed emerges.

“There’s high seed return in the worst areas this season, so getting a metazachlor product on to your rape crop as early as possible will help,” explains Mr Dennis.

He says the ideal timing is when the crop is fully emerged at expanded cotyledon stage, but says 15cm of settled soil and adequate moisture is necessary.

“If you have cereal volunteers you may want to delay to one-true-leaf to accommodate a graminicide, but any later you start to compromise the treatment.”

With the likely need for multiple pyrethroid sprays this season to control flea beetle, Suffolk independent adviser, Mike Warner, prefers the one-true-leaf timing for metazachlor.

“I’ll be applying a pyrethroid insecticide at the expanded cotyledon stage, followed by a second when the first true-leaf appears and include the herbicide with that,” he says.

He notes that this will cut the need for an extra pass for a true pre-emergence. “But where cleavers and hedge mustard are a problem, a clomazone application pre-emergence might be advisable.”

When applying any metazachlor-based products avoid using on drained land after the end of September to avoid water contamination.