The ongoing struggle to control burgeoning and herbicide-resistant black-grass populations is well documented, and is fight that many UK growers are all too familiar with.
Unfortunately, black-grass isn't the only herbicide-resistant grass weed that growers are battling, with an increasing number of agronomists reporting difficulties in controlling ryegrass infestations.
We take a look at the current situation facing UK growers, and – in conjunction with our in-season updates (click here to subscribe) – highlight some key points of action growers can take to reduce the ryegrass pressure in arable crops.
Herbicide-resistant Italian ryegrass was first detected in the UK in 1990 and it is now present on more than 475 farms in 33 counties in England. In comparison, herbicide-resistant black-grass was known to be present on 20,000 farms in 35 counties in 2017, so although the ryegrass problem is relatively new, it is an increasingly significant cause for concern, especially as resistance to ALS inhibitors was confirmed in 2012.
As is the case in black-grass, ryegrass is affected by both target site and non-target site resistance (NTSR) or enhanced metabolism resistance mechanisms. The main threat seems to be from enhanced metabolism mechanisms which can reduce the efficacy of a wide range of herbicide actives.
ACCase (FOP, DIM and DEN) and ALS (e.g. sulfonylurea) target site resistance have both been confirmed in ryegrass, but at present this resistance is much less frequent compared to black-grass.
Although herbicide resistance is currently less of an issue in ryegrass compared to black-grass, there is no room for complacency because ryegrass species globally have shown cross-resistance to 10 different herbicide modes of action in at least 11 countries – more than for any other weed.
As the table below indicates, resistance can impact the efficacy of most of the main herbicide modes of action, with the severity and incidence of resistance varying depending on the mode of action.
Key
Green = not recorded
Light Amber = low incidence or less severe
Dark Amber = high incidence and / or poor control with that ai
Red = Common and / or control failure with that ai
In order to reduce the risk of further resistance delveoping, and to protect the efficacy of existing herbicides, growers should use a combination of active ingredients with different modes of action: these should be deployed in a programme that stacks products in a sequenced approach beginning at the pre-emergence timing. For more information about which actives to use and when, please subscribe to our in-season updates.
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