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Glyphosate-resistant kochia calls for changes in weed war

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News that the troublesome weed kochia has developed resistance to yet another herbicide descended on the province's farmers last week like the first snowfall of winter.

They knew it was coming but dreaded its arrival.

A weed survey last fall confirmed there are populations of this highly prolific tumbleweed that are no longer controlled by glyphosate, the active ingredient in Monsanto's Roundup and in a host of generic versions.

The good news is these populations are still small. The survey team collected seed from 283 different kochia populations and grew them in greenhouses. Kochia plants from two sites didn't die when sprayed with glyphosate. Both sites are located in the Red River Valley.

Earlier surveys in Alberta, Saskatchewan, North Dakota, South Dakota and Minnesota have all identified glyphosate-resistant (GR) kochia in recent years, so it was only a matter of time before it showed up here too.

"As GR kochia has been found in less than one per cent of the sites sampled, Manitoba farmers have an opportunity to minimize the spread of this weed," said provincial weed specialist Nasir Shaikh in a release.

If they don't?

"If GR kochia populations become more common in the province, it will result in added management skills and expense for Manitoba farmers," the province says. Farmers might find themselves turning to pre-seed and pre-emergent products, which are older chemistry and in many cases less environmentally benign due to their residual nature.

The bad news is preventing its spread calls for some fairly major adjustments in how farmers manage weeds, changes history suggests they are reluctant to make.

Approximately 85 per cent of the kochia growing in Manitoba is already resistant to Group 2 herbicides, a class of weed-killers once routinely used to control it. The first populations of Group 2-resistant kochia were discovered in in 1988, and despite the advice of extension agronomists, farmers couldn't kick the Group 2 habit quickly enough to stop the resistant biotypes from becoming dominant.

Manitoba is also home to healthy populations of Group 3-resistant green foxtail and wild oats resistant to groups 2 and 3.

When farmers use the same products in the same way year after year, eventually there will be one plant somewhere that, for whatever reason, doesn't die as it's supposed to. It goes to seed. Its offspring don't die either and go to seed, too.

It was once thought glyphosate, one of the few herbicides that killed practically every plant it touched, would never succumb to selection pressure. It was perfect for replacing tillage to kill off early spring weed growth prior to planting and is credited for the rapid adoption of conservation tillage on the Canadian Prairies and elsewhere.

But then scientists came up with varieties of canola, corn, soybeans and cotton that could tolerate applications of this non-selective plant killer, which meant farmers could kill everything in the field except their crop.

So in addition to using it as a pre-seed burn-off, the herbicide was also used for in-crop weed control. The increased selection pressure has resulted in a growing list of weeds that have developed tolerance.

The problem has become so severe in parts of the southern U.S. that farmers are importing labourers with hoes to weed their soybean and cotton fields because they've run out of herbicide solutions.

Manitoba farmers are being advised they can avoid the same fate by reducing the times they use glyphosate in a season and by combining glyphosate with other weed-control products when spraying. "Farmers will also need to incorporate non-herbicidal measures like crop rotation, tillage and manual weeding, if necessary, to control populations," Shaikh said. In the context of Prairie history, tillage is one tool that should stay in the shed. But if weed resistance continues to spread, it will be one of the options farmers pull out.

Manitoba farmers can nip this in the bud. But will they?


Laura Rance is editor of the Manitoba Co-operator. She can be reached at 204-792-4382 or by email:

Republished from the Winnipeg Free Press print edition April 19, 2014 B10

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