Problematic weed moving into Illinois

In 2010, a two-day conference centred in Memphis offered farm dealers from the U.S. and Canada, a glimpse of the effects of glyphosate resistance.

The "Respect the Rotation" event, sponsored by Bayer CropScience, provided a first-hand testament to the growing issue of glyphosate-resistant Palmer amaranth and its impact on soybean fields in the mid-South region along the Mississippi River.

It was determined that Roundup Ready technology, in both soybeans and cotton, along with increased reliance on glyphosate and insufficient rotation options with non-genetically modified rice, combined to create this perfect environment for Palmer amaranth to flourish. In one field, it was common to see the weeds towering to four and five feet in height.

In addition to promoting the availability of glufosinate (Liberty/Ignite), the overriding message to all during this tour was that this weed will migrate and become an issue to regions such as the U.S. Midwest, where simpler rotations of corn and soybeans could allow the weed species to flourish.

Now, early in 2012, Aaron Hager, an extension weed specialist with the University of Illinois, is sounding the alarm that Palmer amaranth has been identified as a weed species that could eclipse waterhemp, in terms of being the "weed to beat."

"Palmer amaranth is most common in the southern third of Illinois, but it may be expanding its range northward," said Hager, adding that the growth rate and competitive ability of this species exceed that of other Amaranthus species.

"Waterhemp can add close to one inch of new growth per day under good growing conditions, whereas Palmer amaranth can add multiple inches."

In the past few months, there has been one confirmed population of glyphosate-resistant Palmer amaranth in southern Illinois. Hager suggested this signals a turn in the environment under which Palmer amaranth can advance and flourish.

"This species has managed to drastically spread across the southern United States, so what says it won’t spread to northern Illinois as temperatures increase," Hager said. "This could become a huge problem, especially as glyphosate resistance is identified."

From a Canadian perspective, there are a number of conditions that may slow the advance of Palmer amaranth; however, there is no guarantee that such a northern migration would be halted merely by cooler climatic conditions.

Instead, there is the common three-crop rotation of corn-soybeans-wheat that breaks the cycle of glyphosate-tolerant crops. There is also the option of growing crops with the glufosinate-tolerant trait (in other words, rotating technologies as much as chemistries).

Another point of concern is identification and plant physiology. The leaves of Palmer amaranth are longer compared to other Amaranth species. The plant is also dioecious, meaning the plants are either male or female, which means the plant has more genetic diversity, giving it the potential for it being harder to control.

— Ralph Pearce is a field editor for Country Guide. 

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