The benefits of combined black-grass strategies have been quantified in a new report from AHDB Cereals & Oilseeds.

Published following a 52-month innovative programme of black-grass trials and modelling work, the results have improved the credibility and relevance of black-grass reduction figures associated with non-chemical control approaches combined with a robust herbicide programme.

The work found that most of the black-grass control advantage came from delaying sowing of winter wheat from mid/late September to early/mid­ October.

The most important finding was the consistent control advantage associated with improved pre-emergence herbicide performance at later drilling dates.

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. Based on a Syngenta-funded review by Peter Lutman and colleagues of more than 50 field experiments, it put values on the control possible for a range of non-chemical approaches.

Dr Gosling continued: “The latest research findings are in line with these earlier results but the work has greatly enhanced our understanding of the power of integrating approaches to hit black-grass populations hard.

“The results 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.”

What was studied?

Led by Rothamsted Research and NIAB, five field trials and modelling work looked at the combined effect of varying drilling date (including the use of spring crops) and seed rates.

The work also looked at data from NIAB herbicide trials to assess the impact of varying the timing of pre-emergence herbicide applications.

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

The most important finding from the study was the consistent control advantage associated with improved pre-emergence herbicide performance at later drilling dates.

Dr Stephen Moss, who led the project, said:  “We analysed the results from our field trials and data from five additional herbicide efficacy trials, which also varied drilling dates and herbicide application.

“We found that, on average, pre-em herbicides gave 26% more control when drilling was delayed by three weeks, from mid/late September to early/mid­ October.

“This improved efficacy is probably due to lower temperatures and increased soil moisture boosting performance.

“This free extra power added to the herbicide programme is impressive, especially when you consider that adding an additional herbicide to the pre-em stack often only gives a modest 10–15% control increase.”

John Cussans, who was part of the team at NIAB, said: “Analysis of NIAB’s herbicide data showed that 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

In relation to seed rates, Dr Moss said: “Increasing seed rate suppressed black-grass by up to 28%, which is similar to the 26% value presented in the black-grass solutions publication.”

In one experiment, crop yields were assessed. This recorded a 1.08t/ha yield loss directly attributable to a 100 black-grass heads/ha on untreated early sown wheat plots.

Black-grass seed return in the earliest-sown untreated plots was up to 87,959 seeds/m2, highlighting the massive potential seed return from black-grass.

The field experiments also confirmed the tremendous value of spring crops. Spring-sown wheat resulted in a substantial (92%) reduction in black-grass plants emerging compared with September-sown wheat. This was in addition to a lower number of heads and seed produced per black-grass plant.

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.”

Key findings: 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