URBANA, Ill. — Agronomist and weed expert Larry Steckel has long thought mixing different herbicides together helps prevent herbicide resistance. A recent study published in Pest Management Science provides evidence his hunch was correct.

USDA agronomist Jeffrey Evans and several colleagues completed the study, and had it published in March. It was conducted at 105 central Illinois farms. The goal in part was to find out if different mechanisms of action (MOAs), often called sites of action, applied at the same time would help prevent glyphosate-resistant strains of waterhemp, a species of pigweed. Put simply, MOA is the manner in which a given herbicide works to kill a target weed.

The mixing worked. "You really need to use two different herbicides to be effective. That’s really the main takeaway" of the study, said Steckel, an extension specialist and associate professor at the University of Tennessee-Knoxville.

Researchers did their study by growing plants from seeds taken from waterhemp from farm fields between 2004-06. The parent plants had survived treatment with glyphosate in the fields.

The plants derived from the field weeds were raised in a greenhouse under controlled conditions and sprayed with a commercial formulation of glyphosate. Fourteen days after being sprayed the plants were examined and then labeled as either sensitive (meaning dead) or resistant.

In order to establish what herbicides had been used on the parent plants, researchers analyzed farm management records from the custom retail applicator. They also analyzed soil samples for chemical and physical properties. Other variables in the study that were considered included field perimeter length, distance to nearest road, field margin composition and proximity to waterways.

They found that management practices were most predictive of glyphosate resistance among a total of 66 variables analyzed. Frequency of glyphosate resistance among seeds produced was highest in those fields with at least 57 percent rotation of herbicides. This result ran contrary to popular wisdom, the paper said.

"The rotation of herbicides didn’t help as far as glyphosate resistance was concerned," Steckel said. "That was a little surprising. It’s not exactly clear why it did that."

The other results were more expected, however, and it’s those that seem significant: namely, the presence of glyphosate-resistant waterhemp in fields was greater where fewer herbicide MOAs per year were used. They also found that use of glyphosate in more than 75 percent of the years increased the likelihood of glyphosate-resistant waterhemp.

The researchers also made a point of distinguishing between average MOA applications per year and number of MOAs per year achieved through mixing of MOAs at each application. Herbicide mixing was "strongly linked" with reduced selection for glyphosate resistance, the paper said.

A field that received an average herbicide treatment of 2.5 MOAs per application during the study period was 83 times less likely to produce glyphosate-resistant waterhemp 4-6 years later, than one that received herbicide treatments of 1.5 MOAs per application.

Further, researchers found seeds from fields that received an average of three MOAs per application were 51 times less likely to be glyphosate-resistant than those from fields with two MOAs per application.

The researchers concluded that mixing MOAs is an effective strategy for dealing with glyphosate-resistant waterhemp, even though there could be some pitfalls – for example, broad cross-resistance could develop. The mixing of MOAs is not a "silver bullet" for glyphosate-resistant waterhemp, the researchers explained. For one thing, the use of more chemicals is expensive.

Farmers might be tempted to mix MOAs at a smaller dose to save money. But if that’s done, the chances could increase that non-target sites would develop resistance or that broad cross-resistance could develop.

Finally, researchers said the use of more MOAs and mixing only delays development of resistance – it doesn’t prevent it. Growers have to use other strategies in addition to chemicals to control weeds over the long term. Still, the authors describe mixing as an effective resistance mitigation strategy.

The paper, Managing the Evolution of Herbicide Resistance, is available online at http://onlinelibrary.wiley.com/doi/10.1002/ps.4009/full