WINTER wheat growers must begin to take a determined effort against the threat of resistant black-grass if losses in control are to be averted, industry experts are warning.
Accordingly, Rothamsted Research has conducted an in-depth review of the improvements in control achievable by changing cultural practices – while Syngenta has conducted a tandem project assessing how to boost control from pre-emergence herbicide use.
According to Rothamsted weed experts Dr Stephen Moss and Dr Peter Lutman, among herbicides applied post-emergence of black-grass, there is already a high incidence of resistance to ACCase herbicides, such as ’fops’ and ’dims’, and an increasing number of cases of resistance being recorded to ALS inhibitors, such as sulfonylureas.
Additionally, recent reductions in available herbicides will increase the risk of resistance to ACCase and ALS herbicides, they note, so in the absence of new herbicides farmers will need to look harder at alternative strategies – integrating cultural control with remaining chemical methods.
"Herbicide resistance has the effect of reducing the apparent dose of herbicide applied if you’ve got a build up of enhanced metabolism resistance," explains Dr Moss, "and target site resistance to both ACCase and ALS inhibitors, which gives a very high level of resistance, has been found widely.
"Black-grass can produce 1000 seeds per plant, so populations can build up rapidly, by over 10 fold per year. It only takes 12 black-grass plants per square metre to cause a 5% yield loss, and that is only an average figure – losses can be far higher.
"In the absence of new herbicides, there is a need for less reliance on post-emergence herbicides, increased reliance on pre-emergence herbicides and increased reliance on non-chemical control methods. We’ve found that you get 70% of the maximum yield response from the pre-emergence treatment, so these have real value as an insurance against poor control from a post-emergence follow up." Additionally, while individual cultural methods may give modest control, combined together they can contribute a lot, Dr Moss adds.
In line with this, the Rothamsted review examined the extra control achievable by making changes in cultivation method, drilling date, crop seed rate, variety choice, and by including spring cropping. Commissioned by Syngenta under its Grassweed Ultimatum initiative, a tandem Syngenta project examined how control could also be improved by targeting the weed using more pre-emergence active ingredients.
Analysing the latest two seasons of 10 independent and in-house trials, the Syngenta results revealed that tank-mixing two pre-emergence herbicide products, to target black-grass with three active ingredients, boosted pre-emergence control by around 10%, delivering a £50/ha margin over input cost, compared with using just two active ingredients in a single herbicide.
Specifically, tank-mixing the pre-emergence treatment Defy with reduced rate flufenacet/diflufenican took average control to 82.2% compared with 71.6% from a higher dose of flufenacet/diflufenican alone, says Syngenta technical manager, Stephen Williams. Similarly, Defy + reduced rate flufenacet/pendimethalin took control to 79.6%, he adds, compared with 71% from a higher dose of flufenacet/pendimethalin alone.
"These results suggest clearly that by tank-mixing treatments it’s possible to improve pre-emergence control and therefore ease the black-grass pressure on post-emergence chemistry," explains Mr Williams.
"As well as adding extra control, including Defy also reduced the variability of control, and it adds in extra activity against weeds such as cleavers, cranesbill and volunteer oilseed rape.
"Based on results, we are suggesting using pre-emergence mixtures of either 4.0 l/ha Defy + 0.4 l/ha flufenacet/diflufenican, 5.0 l/ha Defy + 0.1 l/ha diflufenican, or 4.0 l/ha Defy + 2.0 l/ha flufenacet/pendimethalin, depending on the black-grass situation.
"There’s little doubt that a pre-emergence mixture adds substantially to results. In trials, preceding a post-emergence sulfonylurea of mesosulfuron/iodosulfuron with Defy + flufenacet/diflufenican increased control from 68 to 91%. But this must be combined with sound cultural practices," he adds.
According to Dr Peter Lutman at Rothamsted, resistance and the loss of products is anticipated to make grass weed control in winter cereals increasingly difficult. The Rothamsted review examined published data from the last 30+ years, making it the most comprehensive review of non-chemical control of grass-weeds done in recent years, Drs Moss and Lutman add. Overall findings were as follows:
Impact of cultivations: Burial through ploughing can reduce black-grass by more than 60% compared to tine/disc cultivation, however direct drilling can increase the population by 35%. By comparison, shallow, post-harvest stubble cultivation was found to give only a 20% reduction.
Role of delayed sowing: Sowing cereals in early to mid October rather than September was found to reduce black-grass plant densities by only around 14%. However, delaying until November produced much greater reductions of around 70%. A greater benefit from later sowing may be from reduced black-grass head production, and therefore reduced seed return.
Increased crop density: Increasing crop density from 150 to 350 plants per sq m was considered likely to reduce black-grass numbers by 20%. Further increases were considered to provide an even greater reduction but also increase lodging risks, while populations below 100 plants per sq m were considered likely to decrease the crop’s ability to suppress weeds and so increase black-grass head numbers.
Choosing competitive varieties: In line with increased interest in the role of crop competition for weed control, cereal varieties that grow tall and produce good ground cover – due to tillering, leaf angle and leaf shape – were indicated as being able to reduce weed populations by 27-40%. However there was evidence that wheat is less competitive than barley and oats.
Spring cropping: Planting a spring crop was estimated to reduce black-grass by a potential 80%, but there is little detailed experimental data to support this.
Overall, the practical messages to growers to reduce black-grass and therefore ease the pressure on herbicides are as follows, Dr Lutman adds:
• Plough (at least once every 3-4 years, preferably more often)
• Don’t reduce wheat seed rates – sow to establish 150-200 plants per sq m
• Don’t sow too early
• Include a spring crop in the rotation
• Grow more suppressive varieties
