In world of weeds, nature laughs last

While an invasive plant called hogweed has been grabbing headlines in Canada of late, a different plant nicknamed "pigweed" has been gaining notoriety in the U.S.

The official name is Palmer amaranth, a biological cousin to the red-root pigweed that is common in Western Canada. Arkansas cotton farmers also used to call it "careless weed," because if you allowed even a few escape the hoe or plow, these plants reproduced so quickly they could soon take over a field.

The days of farmers worrying about that seemed over when herbicides came along, followed by the development of glyphosate-tolerant crops in the late 1990s.

Farmers were ecstatic when Monsanto released Roundup Ready cotton and soybeans in the south as well as corn and canola in the north; it meant they could use one herbicide -- glyphosate -- during the growing season to control a wide spectrum of weeds without harming their crop.

In the world of chemical weed control, glyphosate is a bit of a wonder. Discovered by bio-scientist John E. Franz while working for Monsanto 30 years ago, it is known to kill up to 140 species of plants without leaving toxic residues.

What's more, it allowed farmers to control weeds without tilling the soil and exposing it to erosion. But the chemical hailed as the "world's greatest herbicide" because it made reduced or "conservation" tillage economical has become the reason many southern U.S. farmers are once again plowing their fields and importing hoeing crews from Mexico.

Palmer amaranth is among 10 weed species in 20 states and one in Ontario that have become resistant to glyphosate due to overuse.

The problem arises because there is a lot of genetic diversity in weed populations and there are occasional genes that become tolerant of, or even immune to, a herbicide. With all the susceptible plants knocked out of the competition for survival, they are free to reproduce and take over the field.

Until recently, Monsanto claimed glyphosate was immune to these evolutionary forces. But nature always has the last laugh.

Soybean and cotton farmers, who, for a brief few years, enjoyed the simplicity of spotlessly clean fields using one type of weed control, are now thrust into a complex realm of integrated pest management for which they are poorly prepared. Glyphosate has gone from the only weed control they thought they'd ever need to being next to useless.

"We've got a generation, not just farmers, but a generation of weed scientists, a generation of consultants, of seed dealers that all don't know anything but Roundup Ready weed control," Arkansas crop consultant Ford Baldwin told a recent tour hosted by Bayer CropScience.

Bayer has introduced an alternative, the Liberty Link herbicide-tolerant cropping system, which allows farmers to use the herbicide glufosinate in a similar manner.

The fear is, however, that farmers with glyphosate-resistant weeds will make a wholesale switch to glufosinate without making other changes to their management. The outcome of that is predictable.

So the company is co-ordinating a "Respect the Rotation" initiative with public extension workers encouraging farmers to use its products as part of a combination of weed-control strategies.

This message is not new to Manitoba farmers. In fact, some of the earliest work on identifying and managing resistance was done by university extension and government staff here in the 1980s after wild oats and green foxtail became resistant to the herbicide trifluralin.

Manitoba farmers are well aware they must rotate herbicide types and grow a diversified crop mix to delay the evolution of herbicide resistance.

Whereas farmers in the U.S. were in the habit of sowing the same crop into the same field year after year, farmers here typically rotate their fields between several crops. As well, the weeds that have developed glyphosate resistance in the U.S. are not found in this province, at least not yet.

It is still a cautionary tale, however, and one Manitoba farmers would do well to heed.

The irony in all this is that the pigweed causing so much consternation in the U.S. is a type of amaranth, an ancient grain that is highly valued in other parts of the world for its nutrition and versatility. The amaranth's leaves, stems and seed are all edible, leading some to call it the "crop of the future."

The seed contains 30 per cent higher protein than wheat and is considered an alternative for people with wheat allergies. It has achieved naturally many of the qualities biotechnology companies are spending millions trying to insert into conventional crops -- drought tolerance, enhanced nutrition and the ability to adapt to changing conditions.

And now, it's even Roundup Ready.

Laura Rance is editor of the Manitoba Co-operator. She can be reached at 792-4382 or by email: laura@fbcpublishing.com

Republished from the Winnipeg Free Press print edition July 31, 2010 B9