Palmer amaranth

Palmer amaranth is known for its rapid growth, prolific seed production and its ability to overcome different herbicide families.

Editor’s note: The following was written by Nevin Lawrence with the University of Nebraska, Albert Adjesiwor with the University of Idaho, and Andrew Kniss with the University of Wyoming for the University of Nebraska CropWatch website Oct. 29.

There are dozens of “target sites” to which herbicides can bind to disrupt plant growth, each referred to as a site of action. To reach the site of action, all herbicides must first be absorbed into a plant and then move through the plant to reach the target.

Weeds in a field are not identical, and even neighboring plants of the same species can vary in how they absorb, move or interact with a herbicide. There are often many millions of weeds present in one field, so we should expect that within the field there is a high likelihood that a single weed will be capable of surviving a herbicide application.

Herbicides don’t cause plants to be resistant. They remove all the susceptible weeds from the field, leaving only the resistant plant(s). A surviving plant or two may not be noticed, but it may be capable of producing thousands of seeds. And once several thousand herbicide-resistant seeds have entered the soil seedbank, the farmer has a problem.

One of the most successful strategies for managing herbicide-resistant weeds is applying multiple types of herbicides, known as mixing modes-of-action, at the same time. The logic of this approach is that the probability of a weed within a given field being resistant to a herbicide might be one in a million, and there are millions of weeds in a field. But the possibility of a weed being resistant to two different types of herbicides at the same time may be one in a billion.

This approach works, and a large body of field research and population modeling backs it up. However, depending on which crops a farmer grows, what weeds are in the field, and the economics of adding extra herbicides to a spray tank, using multiple modes of action can be a nearly impossible solution to implement.

The Herbicide Resistance Risk Calculator was developed to help farmers make these challenging decisions.

This is an interactive online web application at bit.ly/3mSQxtc which allows a farmer to choose a weed they are currently managing, or are concerned about, enter a four-year crop rotation, and select herbicides for each crop. The application then estimates:

=the effectiveness of each herbicide program,

=the risk of resistance developing to each herbicide mode of action used in the rotation, and

=the cost of each herbicide program.

How is the program used?

The web-based calculator is based upon some simple models, and therefore results should not be thought of as a scientifically exact prediction of what will happen based on the herbicide choices a farmer makes.

Rather, the program assesses how effectively a herbicide controls a weed, how many times the herbicide is used in a rotation and whether effective mixtures are being used.

Risk scores are estimated on a scale of 0 to 4. A minimum score of 0 means the herbicide site of action was never used during the four-year period (and thus those sites of action are not presented in the table). Each year during the rotation an effective herbicide is used on the target weed, that herbicide site of action is initially given a score of 1; however, this score is reduced if a second effective site of action is applied in the same year. If an effective site of action is applied alone in each of the four years, it would result in the maximum risk score of 4.

The risk score is reduced by an amount that depends on the efficacy of the second SOA. For example, if a second herbicide site of action provides excellent control of the target weed, the resistance risk score is reduced more than if a second herbicide provides marginal control.

The lower the risk score, the less likely it is the weed population will become resistant.

Example 1

A four-year rotation of continuous corn is chosen. In this example a single POST application of glyphosate + dicamba is chosen.

The Palmer amaranth in this example is already resistant to glyphosate, so a resistant risk score isn’t calculated for that site of action, and glyphosate does not contribute to the estimated weed control efficacy. The estimated weed control efficacy is calculated exclusively from the control estimated from dicamba and is greater than 85%.

In this case, only having one effective herbicide being used gives an herbicide resistance risk score of “1” for each year for dicamba-

resistance developing.

As dicamba is the only herbicide used for all four years, the cumulative resistance risk is “4”, which is the highest possible score. There is a very high likelihood of Palmer amaranth developing dicamba-resistance in this scenario.

Example 2

This example is for continuous corn rotation, POST only weed control program, and two effective herbicide sites of action.

Using the same resistant weed population and crop rotation we now replace dicamba alone with Diflexx Duo, which is a premix product containing dicamba (Group 4) and tembotrione (Group 27).

The first thing to note is that both herbicides do not provide the same level of control. Tembotrione does not control Palmer amaranth as well as dicamba alone. When multiple modes of action are applied at the same time, the web app selects the herbicide which provides the greatest amount of control, in this case dicamba, and uses that rating to estimate the level of control.

Therefore, control did not increase even though an additional mode of action was added compared to the first example. In this way, the weed control estimates from herbicide mixtures are fairly conservative.

However, the cost per acre and the herbicide resistance risk score did change.

In the first scenario, dicamba was applied without any other effective mode of action, resulting in the highest possible score of “4”. Now that dicamba is being applied with another mode of action each year, that risk was reduced by half to “2” over four years — still greater than the ideal score of less than 1, but an improvement over dicamba used alone.

Example 3

This example involves management of herbicide-resistant palmer amaranth in a corn / dry bean / corn / sugarbeet rotation.

In this crop rotation example, herbicide options exist to manage herbicide-resistant Palmer amaranth in every crop but sugarbeet. In the sugarbeet crop, cultivation is used for weed control, and assigned a control value of around 70% with the assumption that in-crop cultivation will eliminate about 70% of the weeds.

There are no herbicide resistance risk scores given for cultivation, and herbicide groups 9 (glyphosate) and 5 (atrazine in Acuron) are marked as already resistant in the herbicide resistance risk score. But with a four-year rotation of three different crops, there is a greater number of herbicide modes of action used compared to previous examples with continuous corn.

Having a more diverse cropping system is lessening the cumulative use of particular herbicide sites of action, and consequently decreases herbicide-resistant risk scores. In general, as cropping system diversity increases, the number of herbicide resistant weed populations decrease.

We recommend using the Herbicide Resistance Risk Calculator as a tool to assess your current weed management plan and to plan for the future. We envision farmers experimenting with different products prior to purchasing herbicides for the spring, and hopefully aiding users in thinking about weed management as a multi-year plan.