Multiple strategies outlined for battling herbicide resistant weeds

Ray Mueller
Waterhemp is a prolific seed producers and has developed resistance to herbicide treatments.

FOND DU LAC – The battle to control herbicide resistant weeds in corn and soybeans needs to be fought on several fronts and with multiple strategies, according to presenters at the Extension Service's 2019 pest management updates.

Waterhemp and giant ragweed, both of which are prolific seed producers, are the two major weeds which have developed the most resistance to herbicide treatments with glyphosate or imazethapyr as the active ingredients, Extension Service weed scientist Nick Arneson pointed out.

Greenhouse testing results

Arneson thanked the growers and crop consultants who, in 2018, submitted 88 samples of seeds from possibly resistant plants for greenhouse testing in Madison. That testing found that 70 to 95% of the weed seeds were resistant to various doses of one or both of the herbicides, he reported.

Burying the resistant seeds in the soil via tillage isn't a solution because the seeds remain viable – up to 15 percent after even four years for giant ragweed and up to 50% in the next growing season and up to 15% after four years for waterhemp, Arneson indicated.

In addition to how seeds are spread within a field during harvesting and tillage, they can also be carried to new sites in feed, manure, and bedding or by waterfowl and by equipment such as combines, planters, and tillers, Arneson cautioned.

Cleaning of combines

Because of the many places that resistant weed seeds can collect on combines and be spread from field to field and farm to farm, the Extension Service is emphasizing the cleaning of combines, according to southwest regional Nutrient and Pest Management program specialist Daniel Smith.

A leaf blower and air compressor, and perhaps a knife, are the best tools for completing a thorough cleaning of a combine in about 30 minutes, Smith said. For safety, the equipment operator should wear eye and ear protection along with a top of the line respiratory mask, he advised.

Cleaning combines helps prevent the spread of herbicide resistant weeds such as ragweed.

In addition to tending to the obvious points on a combine where weed seeds can collect, be sure to open all the covers and hatches, Smith stressed. To preclude the spreading of seeds to new sites as a result of the cleaning, set up in a farmyard or in an already infested field and pay a custom operator to clean the combine before coming onto one's farm, he suggested.

During some recent combine inspections, waterhemp and lambsquarter were found germinating in a combine near Green Bay after the winter wheat harvest, Smith reported. For any biomass that resists a blower, use a knife to loosen it, he said.

To ease the cleaning of the grain tank, run wood shavings (not chips) through the auger, Smith advised. Other parts that need to be cleaned include the rock trap, feeder house, and rotor along with mudded tracks, he added.

Ongoing research projects

In addition to those practical measures, Extension Service weed scientist Rodrigo Werle continues to oversee testing projects with multiple herbicide formulations and application timing strategies at the state's research plots near Lancaster and Janesville along with a 5-acre plot at O'Brien Farms near Brooklyn, where the density of waterhemp plants has reached 100 per square foot.

Control of giant ragweed or waterhemp at the desired 90% goal is not being achieved with most of the up to 30 herbicide products that are being evaluated although many of them provide 50 to 60% control, Werle reported. He enjoys the support and cooperation of soybean and corn grower and promotion organizations and of the chemical companies for five ongoing studies.

Rodrigo Werle

While the overall control results are falling short of the goal in most cases, Werle explained that the benefits of those research ventures include the identifying of significant differences between the products along with the importance of application timing, multiple modes of action, and the combination and pre and post emergent applications. Failures could often be traced to improper timing or the presence of resistant weeds, he noted.

Specific findings

Werle has been pleasantly “surprised by the synergies” between various burndown and residual products, especially at the O'Brien Farms site near Brooklyn. The Liberty herbicide has also been performing when mixed with other products and the mixes with two or three modes of action are performing better than those with a single mode of action, he pointed out.

In particular, Werle stated that a pre-emergent application is not adequate for controlling waterhemp in soybeans, that a post-emergent should be applied when the waterhemp height is two inches, that a two-pass application is often appropriate on corn (allowing the second application at the V6/V7 growth stage, and that single treatment on corn should be at the V2/V3 stage.

On other points, Werle advises targeting treatments to known points of infestation rather than entire fields in order to reduce costs and selecting herbicides known to be effective with the particular species, which could be lambsquarter or pigweeds in some cases. A pre-emergent is not as effective with giant ragweed as with waterhemp because of the large ragweed seeds, he pointed out.

A booklet detailing all the results of the recent studies is available from the Extension Service. Werle can be reached by e-mailed to