How stacked spray programme can tackle resistant ryegrass

A stacked and sequenced spray programme applied accurately in favourable conditions can help combat resistant ryegrass, says an agronomist.

Rob Adamson of Procam says a careful balance of cultural controls, herbicide choice and adequate soil moisture will help growers get on top of the troublesome weed – despite increasing concerns of it spreading.

Ryegrass can be tricky to control even in a good eyar. Resistant strains are becoming increasingly common and ryegrass produces a huge number of highly dormant seeds which germinate continuously throughout the year.

Worrying levels

“Last year’s warm and dry conditions, paired with a migration back towards early drilling, meant many pre-emergence treatments ran out of steam and were ineffective against later germinating seeds,” says Mr Adamson.

“Across the country, we saw worrying levels of ryegrass because of a lack of soil moisture which prevented herbicides from working as they should. This is a perfect storm in areas worst affected by resistant ryegrass, including Essex.”

But Mr Adamson says there is hope. Despite a diminished toolbox of effective herbicides, ProCam trials show that even high populations of ryegrass with tough resistance profiles can be managed in winter wheat.

The trials purposely targeted challenging ryegrass populations to determine how various herbicide stacks perform under the worst pressure possible. They reinforced the importance of stacking herbicides with different modes of action.

“Historically, flufenacet has been the core component of ryegrass programmes – but there are increasing instances of poor efficacy. A diversity of other molecules used in mixture or sequence is therefore essential.”

Diverse mixture

Mr Adamson says diverse mixture can effectively overcome the resistance. The registration of Luxinum Plus (cinmethylin) last year provides a useful alternative and gives growers and agronomists a reprieve from the reliance on flufenacet.

“Our trials – which are also being carried out next year – clearly showed that cinmethylin is inherently the strongest molecule on ryegrass. However, a lot still depends on the availability of soil moisture to enable cinmethylin to be effective.

“Because ryegrass germinates continuously, one application of one active ingredient will not provide season-long protection. An over-reliance on cinmethylin will also expose it to the risk of resistance developing, which we must work hard to avoid.”

Growers should therefore consider a sequence of treatments – utilising both this new molecule alongside other modes of action. And when conditions are dry, Procam trials suggest preserving the strongest tool in the box until moisture is available.

This doesn’t mean that pre-emergence treatments should be excluded, but that actives with good levels of persistence such as diflufenican and alconifen (Proclus) should be deployed at this timing instead.

“That way, the stacked programme will get off to an early start, with cinmethylin still available to be used when soil moisture is more readily available,” says Mr Adamson.

At post-emergence, when moisture levels were improving, the trials indicated that Luxinum Plus and Parade (pendimethalin and picolinafen) gave good levels of control as a follow-up to Liberator (flufenacet and diflufenican) and Proclus.

Alternatively, where Luxinum is deployed first, the three-way combination of chlorotoluron, diflufenican and pendimethalin in Tower provided a robust follow-up. In both scenarios, programmes benefitted from using five or six actives.

Pre-emergence

Avadex was also shown to have a good benefit at the pre-emergence timing, taking the total number of active ingredients available to seven.

“Even in high-pressure scenarios, deploying a stacked and sequenced programme of cinmethylin used alongside a diverse range of other modes of action enabled the two trials to achieve 94% and 99% control of ryegrass populations,” says Mr Adamson.

“A lot hinges on how the season unfolds, with the decision on when and which actives to use dependent on when rain falls and when subsequent flushes of weeds are likely to emerge,” he explains.

If the season starts off dry, the strongest active should be preserved. On the other hand, if conditions are wet from the outset and weed germination is likely to happen sooner rather than later, the strongest option should be used first.

“Either way, a stack and sequence is critical to maximise the duration of activity, and minimise later flushes of troublesome grass weeds.”