Proactive management is required to help manage ALS-herbicide resistant weeds in sugar beet, especially when planting Conviso Smart varieties, claim experts.

The Conviso system allows the use of Conviso One which contains two ALS-inhibiting active substances (foramsulfuron+ thiencarbazone-methyl) that would otherwise kill the sugar beet crop.

Up to a quarter of the sugar beet area is likely to be grown using the system this season, says British Sugar’s weed control expert, Pam Chambers. “That will likely include some where it might be grown for a second time in a field.”

Resistance issues

But resistance in broadleaf weeds such as poppy, chickweed and mayweed, is becoming much more common to ALS-inhibiting herbicides, classified by the Herbicide Resistance Action Committee as ‘Group 2’, as well as in grassweeds such as blackgrass and Italian ryegrass, she adds.

Pam believes that’s been partly driven by the widespread use of ALS-inhibiting herbicides – such as sulfonylureas – across the rotation, with the mode of action available in virtually all arable crops.

Furthermore, in most crops, using herbicide actives in mix or sequence with ALS-inhibiting herbicides has somewhat masked issues, but growing Conviso sugar beet has potentially exposed weeds to programmes only containing that mode of action, says Pam.

“We’ve seen some cases of weeds not being controlled in fields across all arable areas where beet is being grown.”

A joint British Sugar, BBRO and ADAS screen of poppy and chickweed seeds collected from such fields has shown that while not all the cases of poor control are caused by resistance, some will survive subsequent Conviso One herbicide application and are capable of setting seed, explains Pam.

“Those findings highlight that the entire rotation should be reviewed when considering ALS-inhibiting herbicide use, with the aim of using alternative modes of action.”

Proactive approach

In the sugar beet crop, proactive weed management is vital, especially where possible ALS-resistant weeds are suspected, says Stuart Jackson, UPL’s head of technical services.

“Don’t leave weed control until the last minute – if there are problems after a Conviso One application, weeds will already be at least 2-4 true leaves and competing with the crop for resources. At that size, they’ll be harder and more expensive to control with other chemistry.”

Instead, Stuart advises growers use either pre-emergence or early post-emergence herbicides containing actives such as metamitron, ethofumesate and the post-emergence only phenmedipham, before applying Conviso One. “Be in the driving seat to manage weed populations at the front end rather than fight a rear-guard action,” he suggests.

Options include products such as Bettix Flo (metamitron), Efeckt (ethofumesate) and Betasana SC (phenmedipham). Combinations of these actives are the bedrock of conventional sugar beet herbicide programmes, comments Stuart.

Poppy control

Another option which could help with poppy control containing quinmerac and dimethenamid-P is likely to be in short supply – only product already in distributor stores or on farm is likely to be available to use. Whereas using combinations of metamitron, ethofumesate, and phenmedipham will be especially important for controlling poppies while still small, adds Pam.

She says ethofumesate is ‘strong’ at controlling chickweed and could be useful for those which have survived a Conviso One application. “Be mindful of the restriction of a maximum of 1000g active/ha over a three-year period and check labels for supported rates as they can vary.”

Metamitron and ethofumesate will also provide additional control of grassweeds such as blackgrass to help boost control from Conviso One. “It’s very unwise to rely solely on Conviso One for grassweed control,” warns Pam.

Using an earlier spray will also potentially help with the timing of Conviso One follow-up sprays. “The trigger for applying Conviso One is fat hen at four true leaves, but that weed can grow quite fast compared with other weeds, especially weed beet,” she says.

“Applying conventional chemistry first should make it easier to put Conviso One on at a good time to make sure you catch all of the emerging weed beet, assuming that’s your main target.”

Conviso Smart merits

In the longer term, sugar beet growers should carefully consider the merits of the Conviso Smart system versus conventional beet varieties, advises Pam. “Where weed beet is the driver, you can easily justify it and also next year, when there’ll be a stacked trait variety with tolerance to both Conviso One and beet cyst nematodes.

“But if you’re using it for other reasons, then it’s sensible to compare conventional versus Conviso Smart weed control systems, looking at costs and ease of use. One way to help you to compare is to spray an area of conventional herbicide on Conviso Smart beet to evaluate the cost and efficacy against Conviso One treatment,” she concludes.