9 weeds show glyphosate resistance in the U.S.

This morning I called Mike Owen, weed specialist with Iowa State University. He had been quoted in an article in Dakota Farmer magazine, and I wanted to get a little more information about the topic. The article was about glyphosate-resistant weeds becoming more common in the U.S. Nine weed species with glyphosate resistance have been documented in the U.S. as of 2008. These are rigid ryegrass, horseweed (Canada fleabane), common ragweed, Italian ryegrass, giant ragweed, common waterhemp, Palmer amaranth (a relative of redroot pigweed), hairy fleabane and Johnsongrass.

I asked Owen what crop production practices were in use in the areas where most of these resistant populations developed. He said continuous Roundup Ready cotton and continuous Roundup Ready soybeans. As you can imagine, the selection pressure for resistant weeds is very high in these circumstances.

With a much more diverse rotation in Western Canada, we should be somewhat immune to the development of glyphosate-resistant weeds. One would think. But when I asked Bruce Murray (in the photo), weed specialist with Manitoba Agriculture, if this is a fair statement, he said, “They will get it faster. But it’s not impossible for us to get it slower.”

And while Roundup Ready canola is by far the most common Roundup Ready crop grown in Western Canada — and usually on a one-in-three or one-in-four rotation — glyphosate is used pre-seed, pre-harvest and post-harvest in many other fields. Glyphosate is a very important product for most Western Canadian farmers. Granted, as Murray points out, these various uses are targeting different weeds and at different times of the year. But he says the one weed that “scares him a little” because it flushes and is often present in small sizes at many of these stages is kochia. He has “no scientific proof to suggest kochia is more susceptible to glyphosate resistance,” but he does think it’s a leading contender.

The myth is "five holed"

In talking with Owen, I said, “I guess the myth that weeds would not develop resistance to glyphosate has been shattered.” He responded, “Yeah, slam dunked, five holed.” (He knew I was Canadian, so he used a hockey analogy.)

Owen encourages all farmers to be proactive to prevent glyphosate resistant weeds from occurring on their farms. You know the steps to prevent resistance for other pesticides — rotate groups, use other cultural methods of weed control, etc. — and Owen encourages growers to use the same practices to maintain the effectiveness of glyphosate.

If you do suspect a resistant population, clean the equipment — especially the combine — after you’ve combined that area. “In three or four years, a combine can take a weed patch and spread the infestation over the entire field,” Owen says. “As a weed scientist, the combine is my job security.” No other piece of equipment is better at spreading weeds around the countryside, especially as farmers get bigger and bigger and combines travel more and more miles.