Richard Wilkins knows about glyphosate-resistance. He farms next to the Delaware farm that had America's first confirmed case of glyphosate-resistant marestail, also called horseweed.
The Greenwood, Del., grower knows how to "put down" those horseweeds. Also vice president of the American Soybean Association, he was in Washington, D.C., recently talking to ag media about the rising problem of herbicide-resistant weeds.
Wilkins noted that the resistant marestail arose because of continuous cropping of no-till soybeans and only one application of glyphosate. "We started seeing problems in no-till even before glyphosate-resistant marestail was confirmed. If you don't kill it at the right point, the growing point will divide."
For Wilkins, the answer was simple – rotate, rotate and rotate. He intensively rotates crops, which is no problem. He grows corn, soybean, small grain, vegetable and hay crops on about 1,200 acres. Depending on the crops, he rotates between no-till and minimum tillage to incorporate manure.
He also rotates herbicides, relying on atrazine for effective marestail control. With burn-down and pre-emergent herbicides, "we've been able to clean up the problem at a low cost," he added.
There's nothing like weed-free fields to draw landowner interest. And that has earned this farmer new rental farms – to clean up.
Resistance troubles threaten conservation gains
Also tackling the weed resistance issue at the North American Ag Journalists meeting was Bill Curran, Extension weed specialist at Penn State University. "Resistant weeds are erasing gains we've made in conservation tillage over the last 15 years," he reported. Wilkins alluded to the same phenomenon.
Newer multi-mode herbicides may help the problem, They're similar but may not be the same as tank mixes.
The Weed Science Society of America and agrochemical companies are making herbicide selection easier for controlling weed resistance. A grouping system has been devised for products with similar modes of action, explained Curran.
When you know the mode, it's much easier to select the herbicide combinations and rotations to combat or at least delay resistance. Then rotate the modes from one year to another. Many herbicide labels now carry a group number.
Select or combine products with at least two different modes of action against the same weed and rotate modes of action, advises the Penn State agronomist. That's especially important on weeds typically prone to resistance such as pigweed, lambsquarters, ragweed, horseweed and foxtail species.
Group numbers can be found on many herbicide product labels. Glyphosate, for instance, is a group 9 herbicide, so this notation will be found on product labels such as Roundup, Touchdown, Credit, and many other products that contain glyphosate.
Labels with multiple numbers, such as this one taken from Halex GT, show that it contains herbicides with three different modes of action: 15, 9, and 27.
Download a chart of the modes by clicking on from: Know_the_mode.
Curran also recommends useful downloadable publications on controlling resistance of a number of weeds from: Controlling_resistant_weeds