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Glyphosate resistance a challenge for weed control

Ray Mueller
In this June 1, 2010 photo, central Illinois corn farmer Jerry McCulley sprays the weed killer glyphosate across his cornfield in Auburn, IL. A handful of hardy weeds have adapted to survive glyphosate _ sold as Roundup and a variety of other brands _ which many scientists say threatens to make the ubiquitous herbicide far less useful to farmers.

FREEDOM – Don't expect to cure herbicide resistance by weeds with more herbicides. A whole system of other practices is required instead.

That was the message from Vince Davis, who is a BASF technical services representative for Wisconsin and northern Illinois, to attendees at the spring meeting of the Outagamie County Forage Council that was held at Wichman Farms. From 2011 to 2015, he was a weed management specialist with the University of Wisconsin Extension Service.

About 10 elements, including weed control, are factors in the record yields that farmers have achieved with corn and soybeans in recent years, Davis stated. In the scenario, he predicted weed control, particularly as herbicide resistance increases, will the “top management challenge” to crop production within the next five years.

Herbicide resistance history

Although it's relatively new in Wisconsin, resistance by one or more weed species to herbicides dates to the 1950s, Davis pointed out. Those resistances developed against herbicides in the PPO and ALS mode of action categories, he noted.

Since 2002, during time spent at Purdue University and the University of Illinois, Davis has been tracking weed resistance to glyphosate (original trade name Round-Up). At the time, most of the attention focused on herbicide resistance by common farm field weeds such as horseweed (marestail) and waterhemp, he said.

Compared to the other modes of action provided by herbicides, Davis described glyphosate, which is Round-Up's active ingredient, as “a unique tool.” Round-Up, which was introduced in 1974, became a very popular seller in the 1980s.

Glyphosate resistance

By 2004, resistance to glyphosate had been documented in states such as Missouri, Ohio, and Arkansas, Davis noted. A decade later, many states had joined that list with resistance by at least seven weed species, including giant ragweed and Palmer amaranth by then, he added.

When Davis arrived in Wisconsin six years ago, waterhemp was a little known farm field weed in the state, he observed. Within four years, it became the most troublesome weed, by now having been checked for possible glyphosate resistance in 19 counties, he pointed out.

Vince Davis

Davis was on hand at the Extension Service in Madison when glyphosate resistance by waterhemp was documented in Outagamie County. By 2016, five weed species had been tested for glyphosate resistance in Wisconsin, he said.

Current challenge

When first introduced, Round-Up served to fix the resistance that had developed against other modes of herbicide action, Davis explained. “There is still resistance to those modes but there is no substitute for glyphosate.”

It's fairly safe to assume that waterhemp which is resistant to glyphosate also has ALS resistance, Davis remarked. Resistance to PPO, which is the active ingredient for pre-emergent herbicides, has been documented in three Wisconsin counties while in Illinois there are 60 counties with waterhemp having both PPO and glyphosate resistance, he reported.

One problem in dealing with that challenge is the tendency of too many farmers to blame weed control failures on reasons other herbicide resistance, Davis stated. Those cited reasons include weather conditions, application rate errors, and improper timing, he noted.

In the cases where those denials are not accurate, the problem is amplified, Davis warned. That's because a resistant bank of seeds is being grown, he explained. “That's a true selection danger because the plants which survive are resistant.”

Solving the problem

A while new system is needed to address glyphosate resistance, Davis declared. It involves crop rotations, tillage, agronomic practices, and the approach to herbicide applications.

On the latter point, Davis strongly advocates the use of herbicides with residual traits for both pre and post-emergent treatments. This would serve to deter the growth of early emerging weeds, reduce the competition to crops (particularly soybeans), helping them to form a canopy more quickly and thereby limiting weed growth, saving the water reserves that would be used by weeds, and reducing the production of weed seeds, he pointed out.

In addition, the use of herbicides with residual traits also offers more flexibility in the weed control strategy, Davis observed. He said the Extension Service has frequently used such herbicides with good success.

Having the residual trait is effective with certain weeds, Davis pointed out. He noted that ragweed germinates after about 150 growing degree units while waterhemp doesn't do so until 350 GDUs – about a difference of three to four weeks in most cases.

Soybean specifics

Soybeans provide the greatest challenge for the choice of herbicides because they are a broad leaf plant, Davis noted. A related reason for supporting the early season growth of soybeans by controlling weed competition is the fact that sunlight is directly related to the production of oil, protein, and starch, he explained.

With the sunlight peaking at the summer solstice in the 3rd week of June, it is important to have vigorous growth of soybeans established by that time, Davis emphasized. With flowering beginning about two weeks later, yield differences of up to 10 bushels per acre – worth well more than the $30 some per acre extra cost of his recommended herbicide strategy – are at stake, he indicated.

By making two applications of herbicides which have a residual in the mix, Davis referred to the benefit of having overlapping timetables of residual controls. In effect, a residual is reinforced once it has reached half life and its later “breaking point” for having biological action, he explained.

Soybean row spacing

Davis cited research results on the link between row spacing and the timing for weed control for enhancing soybean yields. The plants should be free of weed competition by their V1 growth stage in 30-inch rows, at V2 in 15-inch rows, and at V3 in drilled rows of about 7 inches, he advised.

The key to boosting soybean yields is to obtain production from the growth of branches, Davis pointed out. Plants will develop those branches if they don't face a challenge of obtaining sunlight by having to grow upward in order to reach above weeds, he explained. “This is not a one-year phenomenon.”

Davis called for “planning and stewardship” for dealing with the overall challenge in coping with herbicide resistance by multiple weed species.