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Increasing glyphosate resistance in Ontario

Every year, the number of glyphosate-resistant weeds in Ontario gets larger, a lot larger. Dr. Peter Sikkema, a professor of field crop weed management at the University of Guelph’s Ridgetown Campus, is mapping these hot spots. “In 2008, we found one site in Essex County confirmed to have glyphosate-resistant giant ragweed,” he says. “In 2009, we found an additional 18 sites (12 in Essex County and six on Pelee Island).”

In 2010, the numbers jumped again; 29 additional sites were confirmed, with the majority in Essex, but for the first time one site was confirmed in both Kent and Lambton counties, giving a total of almost 48 sites for Ontario. That year, Sikkema also found eight sites with glyphosate-resistant Canada fleabane, all located in the southern half of Essex. 

Glyphosate resistance in the US is nothing new. In Indiana and Ohio, they have been battling glyphosate-resistant Canada fleabane for the past 10 years. The latest disturbing findings are of waterhemp. University of Illinois extension weed specialist Dr. Aaron Hager says that in Illinois waterhemp is showing resistance to five herbicide groups. “Resistance in a single plant to two groups is common and resistance to three will become more common,” he says. “We’ve come across one population of plants resistant to four of the groups.”

Based on recent surveys, 87 percent of plants were resistant to at least one herbicide; 66 percent of the samples were resistant to glyphosate, about 33 percent to PPO (protoporphyrinogen oxidase) inhibitors, and almost all waterhemp  to ALS (acetolactate synthase) inhibitors. “There is the potential for waterhemp to become an unmanageable problem with currently available post-emergence herbicides in conventional or Roundup Ready soybeans,” Hager says. “With weeds like waterhemp, the male plants create pollen and female plants make seed and both seeds and pollen that contain resistant genes can move from field to field.”

Multi-pronged approach
Experts such as Sikkema and Hager advise the use of an integrated approach in both managing resistance that may exist already, and preventing further resistance. These include:

• Crop rotation – John Urquart, a cash crop farmer in the Kirkton, Ontario area, uses a three-year rotation of corn, Roundup Ready soybeans and wheat, and believes it is an important strategy in keeping herbicide resistance at bay. Hager agrees. “We don’t see a lot of farmers in Illinois growing soybean-after-soybean, but two-year corn, one-year soybean is used by many,” he says. What is worse, a substantial majority of corn hybrids planted in the US are Roundup Ready, which means some farmers are using Roundup Ready corn and soybean cultivars and therefore the same mode of herbicide action every year.

Sikkema notes, however, that some of the sites he is found with glyphosate-resistant Canada fleabane are on farms that “have had great rotation and the use of different modes of action. So for these farmers, it’s been an unlucky matter of windblown seed.”

So, while rotation is important, other actions must also be taken.

• Tillage – Urquart plants his soybeans using no-till, but occasionally uses minimum-till. “We mostly do it because it warms up the soil for planting, but you do also get a small amount of weed control,” he says. “Spring tillage is very effective for the control of glyphosate-resistant Canada fleabane,” adds Sikkema, “and will dramatically reduce the stand of glyphosate-resistant giant ragweed.”
• Effective use of herbicides – Good pre-plant weed control is imperative. In the spring, one to three weeks before planting, to control giant ragweed, Sikkema recommends Roundup plus one of the following four herbicides: 2,4-D (Group 4), Eragon (14), Lorox (7) or FirstRate (2). For Canada fleabane, Sikkema says growers can achieve excellent pre-plant burndown control with Roundup plus one of six options: Eragon, Integrity (14 & 15), Amitrol (11), Sencor (5), Broadstrike (2) or FirstRate.
• Choosing different modes of action over time – “Growers are advised to use non-Roundup Ready hybrids or cultivars in their rotation periodically to reduce the selection intensity for glyphosate- resistant weeds,” says Sikkema.

For a grower using LibertyLink soybeans, Sikkema says, “It’s still very important to start clean, so for giant ragweed, use Roundup plus 2,4-D pre-plant, then apply Liberty in-crop before weeds are four inches in height, and again when the next weed flush is three to four inches in height. Apply it during the warmest, sunlit hours of the day with 15 to 20 gallons of water per acre.”

For Canada fleabane, he recommends Roundup plus Eragon pre-plant, with Liberty applications in-crop as for giant ragweed above.