HARROW, Ont. — Ontario farmers have two reasons to be concerned when it comes to giant ragweed.
The weed, which can grow up to three metres high and produce prodigious amounts of seed, has become glyphosate resistant in the far southwestern corner of the province.
It’s also moved out of the ditch banks and into fields over the past couple decades.
Eric Page with Agriculture Canada said giant ragweed is now among the top five most troublesome weeds in Essex County and is the second most difficult to control.
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“From the 1950s until the 1990s, giant ragweed was never a problem in southwestern Ontario,” Page said.
“If you look at the literature, giant ragweed is usually described as a ditch weed.”
His research has led him to believe that giant ragweed became a problem because of changes in crop rotation rather than the use of glyphosate. Resistance developed only after the species began appearing in farmers’ fields.
“This study looks at how cropping practices have changed in the last 60 years. Glyphosate is part of that change, but it’s a late part.”
Sixty-five percent of Ontario was planted to wheat in 1950. There was a significant amount of corn, but soybeans were little more than a minor crop. Corn became the dominant crop by 1980, and soybeans had grown to become a major crop.
The soybean acreage has since risen dramatically. In 2011, half the farmland in Essex County was planted to the oilseed, and Page suspects the acreage was even higher last year.
The problem is that soybeans compete poorly with giant ragweed.
“The increase in acreage of soybeans is the single biggest factor that let giant ragweed out of the ditch and into the farm fields,” he said.
Page’s research involved planting corn varieties popular in the 1950s, 1980s and today to assess their respective ability to suppress giant ragweed. He found that corn was able to reduce the weed’s seed production by 90 percent when pre-emergent herbicides were used.
As a result, he recommended that farmers with a giant ragweed problem include a greater percentage of corn in their rotation and plant a slightly higher population.
He also said glyphosate resistance can be managed or avoided by using a pre-emergent herbicide program and rotating chemistries from year to year.
Page said the yield impact from giant ragweed in corn is a fraction of what it is in soybeans. A wider rotation also promotes soil health and reduces disease and weed pressures.
