An effective stale seedbed allows the main flush of black-grass to germinate, typically in early October, before spraying off with Roundup (glyphosate) then sowing the wheat crop. In early drilled crops, the weeds seen in the stale seedbed would germinate in the crop putting huge pressure on selective herbicides for in-crop control.
Effective control with herbicides is much easier with a lower starting population. The overall aim is to control enough weeds to minimise seed return and reduce the size of the seedbank. Even very good control levels of 85% or 90% are of little value with huge numbers of weeds because the remaining plants will still produce lots of seed. Delaying drilling results in fewer weeds germinating in the crop so herbicides can feasibly control enough black-grass to reduce populations.
Conditions in October are typically better than September for performance of pre-em herbicides. AHDB Research (Project Report No. 560 Sustaining winter cropping under threatfrom herbicide-resistant black-grass (Alopecurus myosuroides)) showed a consistent improvement in herbicide performance by delaying drilling by three weeks from September into October. A mix of Liberator (flufenacet + diflufenican) + prosulfocarb gave an average of 47% control at early sowing dates but 73% at later sowing dates. The improvement was seen across all years and trial sites.
One of the main reasons for better herbicide activity is more soil moisture in October because of cooler weather and longer nights. Soil moisture allows flufenacet to move in the soil and reach the germination zone where it is taken up by the roots of seedlings. Farmers can also promote good efficacy from flufenacet by producing a good quality seedbed and applying at the true pre-em timing within 48 hours of drilling.
Later drilling and herbicide application dates encourage better residual protection.
Partly because later sowing reduces the number of days the crop is in the ground requiring protection. But also, because residual herbicides have better persistence in cooler weather. The new active Aclonifen found in Proclus may be of particular interest for growers looking to maximise residual protection from the pre-em spray.
At later drilling dates, crop and weed development will be slower meaning the weeds remain smaller for longer. At earlier growth stages, black-grass is more vulnerable to post-em treatments such as Atlantis OD (mesosulfuron + iodosulfuron). So late drilling could improve the chances of an effective post-em spray.
Delayed drilling also helps control Barley Yellow Dwarf Virus (BYDV). In cooler weather it takes longer for the second generation of aphids to emerge which tend to spread the disease. This means that fewer insecticide applications are needed during the autumn.
Controlling black-grass is a year-round task, requiring cultural and chemical controls. Our definitive guide explains the building blocks of a successful control strategy, covering everything from mapping populations to the integrated controls available to tackle the problem.
Drilling a crop with minimal disturbance is often the aim for farmers dealing with black-grass, but short of changing the drill, what practical steps can they take to reduce disturbance?