Sugar beet growers are advised to take a proactive weed management approach to help manage the growing threat from acetolactate synthase (ALS) inhibitor herbicide resistant weeds when growing herbicide-tolerant varieties.

The Conviso Smart system relies on the use of Conviso One, which contains two ALS-inhibiting active substances in foramsulfuron and thiencarbazone that would otherwise kill the sugar beet crop.

Its introduction, in particular, has been helpful for growers with weed beet issues.

Up to a quarter of the sugar beet area is likely to be grown using the system this season, according to British Sugar weed control expert Pam Chambers.

“That will likely include some where it might be grown for a second time in a field.”

See also: 6 new varieties added to 2026 sugar beet variety list

But resistance in broad-leaved weeds, such as poppy, chickweed and mayweed, is becoming much more common to these herbicides, classified by the Herbicide Resistance Action Committee as “Group 2”.

This is also the case in grassweeds such as blackgrass and Italian ryegrass, she says.

That has 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.

In most crops, using herbicide actives in mix or sequence with ALS-inhibiting herbicides has somewhat masked issues.

However, growing Conviso sugar beet has potentially exposed weeds to programmes only containing that mode of action, Pam says.

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

Weed screen

A joint British Sugar, British Beet Research Organisation and Adas screen was conducted of poppy and chickweed seeds collected from such fields.

It 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, Pam says.

Those findings highlight the need to look across the entire rotation when considering ALS-inhibiting herbicide use, she says, and look to use alternative modes of action.

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

“Don’t leave your weed control until the last minute,” he stresses.

“If there are problems with weed control after a Conviso One application, weeds will already be at least two to four true leaves and competing with the crop for resources.

“At that size, they will be harder and more expensive to control with other chemistry.”

Recommended approach

Instead, he advises growers use either pre-emergence or early post-emergence herbicides containing actives such as metamitron, ethofumesate.

Post-emergence herbicides containing only phenmedipham should be used before they apply Conviso One.

“Be in the driving seat to manage weed populations at the front end rather than fight a rear-guard action.”

Another option that 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.

Using combinations of metamitron, ethofumesate, and phenmedipham will be especially important for controlling poppies while still small, Pam adds.

Ethofumesate is “strong” at controlling chickweed and could be useful on any that have survived a Conviso One application, Pam suggests.

“Be mindful of the restriction of a maximum of 1,000g 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.”

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,” Pam says.

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

Looking forward

In the longer term, sugar beet growers should carefully consider the merits of the Conviso Smart system versus conventional beet varieties, Pam adds.

“Where weed beet is the driver, you can easily justify it, and also next year, when there will be a stacked trait variety with tolerance to both Conviso One and beet cyst nematodes.

“But if you’re using for other reasons, then it is sensible to compare conventional versus Conviso Smart weed control systems, looking at costs and ease of use.

“One way to help you compare is to spray an area of conventional herbicide on Conviso Smart beet to evaluate cost and efficacy against Conviso One treatment,” she concludes.