‘Powerful and convincing’ evidence for better integrated black-grass control, new AHDB report

The most of the black-grass control advantage came from delaying sowing of winter wheat from mid/late September to early/mid­ October and the use of pre-emergence herbicide at later drilling dates, a report by AHDB Cereals & Oilseeds has found.

The findings are based on a 52-month innovative programme of black-grass trials and modelling work, and have improved the credibility and relevance of black-grass reduction figures associated with non-chemical control approaches when combined with a robust herbicide programme.

Dr Paul Gosling, who manages weed research at AHDB, said: “The experimental design of the trials used in this work has rarely been used before and it allowed for a robust assessment of the effects of chemical and non-chemical approaches to be tested in both isolation and combination.”

In 2014, AHDB published a black-grass solutions publication that was based on a review by Peter Lutman and colleagues, and funded by Syngenta, of more than 50 field experiments. Dr Gosling continued that the latest research is in line with these earlier results but “the work has greatly enhanced our understanding and have produced powerful and convincing evidence of the overall benefits of delaying autumn sowing of winter wheat as part of a strategy to improve control of black-grass by chemical means.”

Key findings:

Field trials

• Most of the black-grass control advantage came from delaying sowing of winter wheat from mid/late September to early/mid­ October.
• Four out of five trials gave a positive result for reducing the number of black-grass plants and heads per plant as a consequence of delaying sowing.

However, there was a significant range in control across sites, once again highlighting there is no simple solution for black-grass control in the UK.

Herbicide performance

•  Consistent control advantage was associated with improved pre-emergence herbicide performance at later drilling dates.
• On average, pre-em herbicides gave 26% more control when drilling was delayed by three weeks, from mid/late September to early/mid­ October. Improved efficacy came from lower temperatures and increased soil moisture boosting performance.

John Cussans, who was part of the team at Niab, said: “Efficacy was maximised when pre-ems are applied at the actual pre-emergence stage.  If you’ve drilled early, don’t be tempted to wait until the early post-emergence stage to think you will get efficacy benefits associated with later application.  Our findings show that pre-em herbicides must be applied relative to crop and weed emergence. Holding off until the early post emergence stage simply doesn’t get the most out of this chemistry.”

Seed rates, crop yields and spring crops

• Increasing seed rate suppressed black-grass by up to 28%, which is similar to the 26% value presented in the black-grass solutions publication
•  A 1.08t/ha yield loss directly attributable to a 100 black-grass heads/ha on untreated early sown wheat plots.
• Spring-sown wheat resulted in a substantial (92%) reduction in black-grass plants emerging compared with September-sown wheat.

Richard Hull of Rothamsted Research said: “The reduction in black-grass plants emerging in the spring wheat plots was very consistent between all years and trials conducted. However, this substantial reduction alone is not enough, herbicides still need to be applied to spring-sown crops to gain full advantage.”

Modelling work

The modelling studies, which assumed an initial potential black-grass population of 100 plants/m2, were a particularly useful component of the project.

Dr Moss said: “Even where high seed rates were used and drilling was delayed, the models showed that at least 50% control was still required from post-em herbicides in situations with highly resistant black-grass populations. As we need to reduce our reliance on herbicides, I recommend that all people grappling with black-grass spend some time with the report, crunch the numbers and reassess their approached to integrated control.”

The report, along with all AHDB’s black-grass control information, can be downloaded from cereals.ahdb.org.uk/blackgrass