This season’s protracted dormancy and germination makes correct timing of post-emergence oilseed rape herbicides critical to control resistant blackgrass, according to Dow Agrosciences.

The firm highlights a warning from ADAS grassweed expert James Clarke.

“The right timing for herbicide applications in oilseed rape this autumn will be critical for optimum control of blackgrass,” he said.

“For best results herbicide properties need to be matched with likely emergence patterns of weeds and soil conditions.”

Kerb Flo (propyzamide) works best when used on cool and moist soils once blackgrass seeds have germinated.

“Our work over several years at ADAS Boxworth showed that best application timings of propyzamide were after soils had wetted up,” said Mr Clarke.

“The combination of residual activity and good control of plants with shallow roots means applying in November typically gave the best results.

“Applications at the full dose gave the most reliable control of resistant blackgrass.”

Deeper-rooted blackgrass needed a contact graminicide added to maintain control, he noted.

Kerb Flo can be applied from 1 Oct to 31 Jan when soils are moist and cool.

Optimum timing varies from year to year, so Dow issues regular bulletins to agronomists. Where resistant blackgrass is a threat, Kerb Flo should be applied at 2.1litres/ha – otherwise 1.75litres/ha can be used.