Potato growers search for new weed control options

As farmers wait for an announcement by the Pest Management Regulatory Agency as to whether or not they can continue to use linuron to control weeds in their potato crops, the search is on for alternatives

Linuron’s days could very well be numbered. This active ingredient has been a trusted weed management tool for Canadian potato growers for years. Now, growers are awaiting a decision by the Pest Management Regulatory Agency (PMRA) as to how, or even if it can be used going forward. With an announcement about the chemistry’s future expected sometime in 2017 or 2018, potato growers are asking, what’s next?

“We’ve known about the possible phasing out of this product since 2012, so we’ve had some time to explore other options,” says Clarence Swanton, a weed scientist in the Department of Plant Agriculture at the University of Guelph. “But linuron has been critical for the control of early season broadleaf weeds in particular, and potato growers have been reluctant to look at using other products.”

Linuron is the only Group 7 chemistry for early weed control in potatoes, which makes it an excellent rotational tool for a crop that has been plagued by increasing weed resistance. Linuron is one of the best pre-emergent herbicides on the market targeting both annual grassy and broadleaf weeds. But it’s not without its detractors. On top of increasing linuron resistance, the herbicide fell under PMRA’s review process due to potential health and environmental concerns.

“I don’t think the loss of linuron is a crisis for potato growers,” says Mario Tenuta, professor of applied soil ecology at the University of Manitoba. ยป

“The issue is more about our alternatives. Potatoes may not have the same range of weed control alternatives as other crops, but we do have options. The greater concern is that we are losing a unique chemical group which makes managing resistance more challenging.”

Early weed control is crucial for potato production. During the critical early period, competition from weeds can cause dramatic and rapid yield loss. Given this sensitivity to weeds, there is no cultural option that will effectively eliminate the need for herbicides in potato production. Weed control is generally added to the soil so the crop is protected from the moment of germination.

“If weeds emerge in the first week after the potatoes first emerge, you could get up to 50 per cent yield loss,” says Andrew McKenzie-Gopsill, a weed scientist with Agriculture and Agri-Food Canada in PEI. “If those weeds are held back to three weeks after emergence, that yield loss drops down to 15 per cent or less. So, growers need to focus their efforts into that very early weed control.”

One of the challenges for weed control in potatoes is that hills large enough for potato seed piece planting are surrounded by flat furrows. “If those hills get packed with weeds in the early season, the crop may not compete,” says Tenuta. “Potatoes need weed free conditions at emergence.”

McKenzie-Gopsill says that more management will be required with loss of linuron in order to get the same yields without increasing the incidence of herbicide resistance. “Growers will have to make sure they have a three-year crop rotation to try to promote soil building activity and eliminate some weed seed,” he says. “We would also recommend a preplant, or a post plant, pre-emergent burn down again to strive for that weed free crop.”

One new option on the horizon for growers to include in their weed management plan is Sencor STZ. The herbicide recently received registration and will be available for pre-emergent weed control in potatoes for growers in eastern Canada for the 2018 growing season. Bayer is still conducting trials for use in western Canada.

Sencor STZ is a co-pack containing Sencor (metribuzin) and STZ (sulfentrazone), and will add to Sencor solo by targeting triazine-resistant weeds such as lamb’s quarters and pigweed. Its Group 5 and Group 14 chemistries will also tackle glyphosate and linuron resistant weeds.

“We have been working to develop a complete package for potato growers, and finding something new in herbicides can be a challenge,” says Jonathan Weinmaster, crop and campaign marketing manager, horticulture and corn at Bayer. “The STZ portion of Sencor STZ is essentially replacing the linuron in the popular Sencor plus linuron tank mix that’s being used today, and will provide equal or better weed control. We will be conducting field scale demonstration trials this summer to show growers the control they can get from this product.”

Adding a second active ingredient to any herbicide helps eliminate a broader range of weeds, reduce selection pressure and slow down existing resistance to that chemistry to provide potato growers, in this case, with the early season weed control they need, says Weinmaster.

Swanton encourages the potato industry to become more proactive in developing new options for growers, whether or not linuron is removed from the shelves. “Linuron has been a very effective treatment for potatoes because of its broad range in controlling broadleaf weeds,” says Swanton. “We always hope there is another silver bullet readily available but as there have been no new modes of action for weed control in potatoes in over 20 years. We are working to use existing technology in different ways so we can still get a good result.”