MULTIPLE HERBICIDE ACTIVES & FAVOURABLE WEATHER REQUIRED TO OUTFOX RESISTANT RYEGRASS4 October 2023
Despite increasing concerns regarding the spread of herbicide-resistant ryegrass, effective control can still be achieved where a stacked and sequenced programme of herbicides is applied accurately and in favourable conditions.
That’s according to ProCam’s Rob Adamson, who explains that success is reliant on a careful balance of cultural controls, correct herbicide choice and timing, and adequate soil moisture.
“Effective levels of ryegrass control have become harder to achieve since the first herbicide-resistant strains of Italian ryegrass were detected in the UK in 1990,” Mr Adamson explains. “Since then, growers have been facing an uphill struggle, with an increasingly diminished toolbox of effective herbicides, the efficacy of which is extremely condition dependant.
“For example, 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. 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 such as Essex and South Yorkshire.”
Even in a good year ryegrass can still be tricky to control, not only because resistant strains are becoming increasingly common, but also because ryegrass produces a huge number of highly dormant seeds which germinate continuously throughout the year.
“But there is hope,” Rob explains, “with ProCam trials showing that even high populations of ryegrass with tough resistance profiles can be managed successfully in winter wheat.”
The trials, which purposely targeted the challenging ryegrass populations of Essex and South Yorkshire (to determine how various herbicide stacks perform under the worst pressure possible), confirmed that control is achievable, but also reinforced the importance of herbicide stacks and the use of a diversity of herbicide modes of action.
“Historically, flufenacet has been the core component of ryegrass programmes, however there are increasing instances of poor efficacy,” Rob explains. “A diversity of other molecules used in mixture or sequence is therefore essential, and a 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,” Rob continues. “And 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, and 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 a range of other modes of action.
“Our trials in 2022 demonstrated that when conditions are dry, the strongest tool in the box should be preserved until moisture is available,” Rob advises. “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.”
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 had been deployed first, the three-way combination of chlorotoluron, diflufenican and pendimethalin in Tower also provided a robust follow-up. In both scenarios, the programmes benefitted from the deployment of five or six actives.
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,” Rob continues.
“But 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. 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.”
Rob is keen to point out that the chemical toolbox is only part of the story for successful ryegrass management. “Success is judged on the number of grass ears and subsequent seed return in the summer,” he explains. “With this in mind, preventing any ryegrass weeds that do successfully establish from tillering too heavily is also essential to the protection of subsequent crops. This comes down to ensuring the current crop is as competitive as possible, with seedbed preparation, crop nutrition and drilling date all important considerations.
“The protracted germination of ryegrass means that, unlike in a blackgrass situation, delayed drilling is not as useful,” Rob continues. “Although this tactic will still give an opportunity for a stale seedbed to be established, it could be counter-productive if the crop is drilled too late, as this would allow competitive ryegrass plants to overpower the weaker and thinner wheat seedlings.
“Thankfully, recent weather conditions mean there’s good soil moisture available this year, which means there should be the opportunity for growers to remove the first flush of ryegrass and to drill wheat into a clean seedbed. Any subsequent weed growth should then come closer to the pre-emergence application. If that’s the case, in the worst affected regions it will make sense to use the strongest active, cinmethylin, as a pre-emergence treatment to get ryegrass control off to the strongest possible start.”