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Controlling giant ragweed

Glyphosate-resistant weeds are not a new problem in Canada, but producers must be proactive to keep these weeds from getting out of control.

There are now five glyphosate-resistant weeds found in Canada: giant ragweed, common ragweed, water-hemp, Canada fleabane and kochia (which is currently the only glyphosate-resistant weed not found in Ontario).

Giant ragweed, the first glyphosate-resistant weed found in Canada, is an aggressive weed that can cause substantial yield losses in field crops if left unchecked. Although it’s not a new problem – giant ragweed was first discovered in Canada in 2008 in Essex County, at the tip of southwestern Ontario – it’s a growing issue, according to Peter Sikkema, a researcher at the University of Guelph’s Ridgetown Campus. He notes glyphosate-resistant giant ragweed has so far been confined to the six most southerly counties of the province. However, the weed is becoming increasingly prevalent in corn and soybean fields, and growers need to be vigilant in order to protect their fields.

Sikkema warns that if no action is taken to control giant ragweed (Ambrosia trifida L.), the potential yield loss is very high. His research has shown yield losses in corn from giant ragweed ranged from 63 to 82 per cent, with an average of 72 per cent. In soybean, the yield losses ranged from 19 to 96 per cent, with an average of 73 per cent.

In the past, giant ragweed was mainly found along roadsides and creeks, but a shift to no-till soybean production has allowed giant ragweed to gain a foothold in southwestern Ontario, according to Sikkema.

The annual weed reproduces by seed and grows up to four metres in height. According to the Ontario Ministry of Agriculture Publication 505: Weeds, “It is distinguished by its very tall stature, its large, lobed but not divided leaves, its long, slender spikes of pollen-producing flower heads and its large, angular seeds with spines around the upper shoulder.” For allergy sufferers, its pollen is a common allergen from August to September in southwestern Ontario.

When it comes to controlling glyphosate-resistant giant ragweed in corn, soybean and winter wheat fields, Sikkema says farmers have options. The first line of defense is to use good crop husbandry practices that keep weed populations in check. Using a diverse crop rotation of three or more crops and using herbicides with multiple modes of action is fundamental, Sikkema advises. Other good practices include seeding a cover crop after winter wheat harvest and using practices that give the crop a competitive advantage, such as seeding at higher populations, using narrower row spacing, and controlling insects and diseases, he adds.

Aggressive tillage in spring might be able to control giant ragweed, but Sikkema has doubts about this method of control, particularly the negative effects of aggressive tillage on soil structure and soil health. “I’m not sure that’s a practice that’s sustainable long-term,” he says.

When it comes to control of glyphosate-resistant giant ragweed with alternate herbicides, the options vary by crop. “We have good solutions in corn,” Sikkema says. “Marksman, Banvel and Distinct can be used post-emergence in corn.”

In winter wheat crops, 2,4-D, along with Target, Estaprop, Lontrel and Trophy give good control. In soybean crops, he has found Roundup plus 2,4-D tank-mixed applied pre-plant, seven days before seeding soybean, is very effective. “It’s important to have that seven-day interval to prevent injury to the soybean.”

With soybean, Sikkema notes it’s important to control glyphosate-resistant giant ragweed before the soybean comes up. There are no herbicides applied post-emergent that provide acceptable control of glyphosate-resistant giant ragweed in soybean, he says.

Giant ragweed seedlings initially emerge in early spring. They can be identified by their spatulate (spoon-shaped) cotyledons, which unfold from a hairless hypocotyl and an indentation at the base of the cotyledons. The first true leaves are entire and ovate with deep lobes.

Farmers are doing a good job of managing glyphosate-resistant giant ragweed, Sikkema says. However, he cautions that some giant ragweed biotypes have multiple resistances to both glyphosate and Group 2 herbicides. In the future, Sikkema says the Roundup Ready Xtend soybean, which are resistant to both Roundup and dicamba, will give farmers another tool for managing glyphosate-resistant weeds.  

December 31, 2015  By Helen Lammers-Helps