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Finding better ways to fight superweeds

Adding alfalfa to crop rotation one option

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At a time when soil erosion is recognized as one of the biggest threats to the world's ability to continue feeding itself, it's disturbing to now see U.S. weed scientists suggesting tillage to address invading "superweeds."

There is no question addressing the lengthening list of weeds that have developed resistance to glyphosate must be a top priority for researchers and extension agronomists advising conventional farmers.

As reported by Reuters, resistance to glyphosate has now reached the point where row-crop farmers in the Midwest are unable to control weeds such as Palmer amaranth infesting their fields, prompting weed scientists to suggest farmers resort to tillage.

These weeds were a problem limited to the deep south a few short years ago, forcing farmers in Arkansas to resort to hands and hoe to bring them under control. They are now "exploding" in the Midwest, Dallas Peterson, a weed specialist with Kansas State University, was quoted in a Reuters article.

The U.S. Department of Agriculture says 70 million acres of U.S. farmland had glyphosate-resistant weeds in 2013. Isolated occurrences of glyphosate-resistant kochia have been found on the Canadian Prairies.

You have to admire these plants' remarkable ability to adapt. It took scientists decades of effort to figure out how to modify crops to have resistance to herbicides. It took only a few years for nature to accomplish the same thing, albeit helped by farmers whose repeated use of the same products created intensive selection pressure.

It is only a matter of time before more of these invasive plants make their way to the Canadian Prairies. Despite the hype over the development of so-called stacked traits, which allow farmers to use herbicide combinations, it is really doing the same thing that caused this problem in the first place all over again, expecting different results.

Tillage is equally short-sighted.

So are farmers stuck with an either/or choice between two imperfect options?

What if there was a third option?

The University of Manitoba's long-term organic crop rotation study, now the longest-running study of its kind in Canada, compares four-year-long crop rotations for annual crops and annual crops mixed with perennial forages under organic and conventional production systems.

Those trials, which started in 1993, have shown the annual grain-based conventional rotation can be the most productive by way of yield, although it is not the most economic or energy efficient. And it is susceptible to resistant weed problems because of its reliance on herbicides.

But researchers have found adding a perennial alfalfa crop into the rotation for two years virtually eliminates the risk of herbicide-resistant wild oats. It's also very good at reducing other annual grasses, thistles and quackgrass, but not as effective at reducing others, including broadleaf weeds such as pigweed. But it does slow down their evolution toward herbicide resistance because the farmer is not spraying with herbicides during the alfalfa phase of the rotation. "In other words, we make the herbicides last longer," said the rotation study's overseer, Martin Entz.

Farmers accustomed to growing annual crops under conventional management are quick to argue they can't afford to "lose" two years of production to a crop such as alfalfa, for which they have no use unless they have livestock.

However, the economics of including perennial forages in an annual-cropping system are changing as the cost of controlling some of the unintended consequences of conventional systems, such as superweeds, rises.

Including alfalfa also reduces the amount of nitrogen fertilizer needed by as much as 40 per cent. And it provides other soil-building qualities, such as biological drainage; its deep tap roots aerate the soil and draw out excess moisture.

No one is suggesting alfalfa is the panacea for invasive weeds or any other issue plaguing a farming system. Picking one approach as THE answer is a mistake that is made all too often in farming.

But it's an option, another tool, farmers can consider. And thanks to long-term trials such as these, they have accurate, scientifically valid metrics to help them assess how it fits on their farm.


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 August 9, 2014 B9

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