True minimal tillage aids blackgrass control in winter rape

Concerns that grassweed herbicide efficacy could be compromised by continuous minimal tillage in oilseed rape are misplaced, says a leading crop and soil advisor.

In minimal tillage systems organic matter rises and the pH drops. Combined with straw and other trash there have been fears that residual herbicides may not be reliable.

“Some cereal herbicides can be affected by these factors, but Kerb Flo (propyzamide) and other soil acting residual herbicides can be used with confidence in winter oilseed rape and winter beans,” says independent crop systems specialist Steve Townsend. “In my experience, in continuous minimal tillage systems, Kerb Flo has consistently produced excellent control of blackgrass and bromes.”

Residual herbicides are washed off the straw and chaff onto the underlying soil where they are activated. Where minimal tillage has been practiced for several years the soils have a higher surface humus content and better soil structure and tilth favouring good herbicide activity.

These conditions are not favoured by blackgrass, and populations fall. Each year seed continues to germinate in the top few inches of the soil, ideal for residual herbicides especially Kerb Flo that stays close to the soil surface.

“Heavier soils, where blackgrass is worse, have a high buffering capacity that prevents a big drop in pH. And with continuous minimal tillage comes an increase with the worm population bringing calcium as well as other nutrients such as phosphate to the surface. Worms spread organic matter through the top 15 cm of soil, so it is rare for farms that had been under minimal tillage for many years to have surface organic matter levels above 10%,” explains Mr Townsend.

Mr Townsend advises keeping cultivations as close to the surface as possible to get the best from Kerb Flo and similar soil acting herbicides. Adding phosphate and nitrogen in the seedbed are important to counter the effects of bacteria that temporarily use up large quantities of both nutrients in straw degradation.