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Use Every Tool Available

Reliance on the PPOs to control Palmer amaranth may lead to the next round of resistance.

By Amanda Huber print email

Eliminating herbicide-resistant weeds would be easy, that is if money were no object. But, making a profit on the farm is essential to continuing, and it’s working within the economical parameter that makes combating herbicide resistance such a challenge.

For instance, tillage and cultivation are very effective methods of controlling weeds. As Eric Prostko, University of Georgia professor and Extension weed specialist likes to say, “there are still no steel-resistant weeds.”

It’s the economics of returning to the use of tillage that would make it difficult.

“Many growers are farming more acres than ever before, which would make it difficult for some to start doing more tillage and cultivation,” Prostko says.

Resistance Take-Home Message:

1. Use tillage, cultivation, cover crops and crop rotation in the re- sistance programs when practical and profitable.

2. Apply a residual herbicide on EVERY acre!

3. Remove escaped weeds from a field before seed production through hand weeding or the use of non-selective herbicide applicators.

4. Know what modes of action are being used in the field, and make every effort to use alternatives when possible (i.e. metribuzin in soybean instead of Valor or Reflex).

Source: Eric Prostko, Extension Weed Specialist, University of Georgia

Carefully Consider The Economics
Another tool to help with herbicide-resistance is crop rotation. Here again, if a producer is going to grow an alternative crop, it must be profitable.

“It is hard to expect a grower to plant a rotational crop just for resistance management,”

Prostko says. “Additionally, a rotational crop would only be beneficial for herbicide-resistant weed management if herbicides with alternative modes of action are used in that crop. For example, corn growers can use atrazine.”

Herbicides are another tool to use against weeds, but given the resistance situation facing producers, a well thought out plan for weed management and herbicide use needs to be employed.

Have A Plan
Although effective, characteristics of the herbicide, itself, have influenced the development of resistance. Those characteristics are as follows: 1) herbicides with a single site of action; 2) herbicides used multiple times during the growing season; 3) herbicides used for consecutive growing seasons; and 4) herbicides used without other weed control strategies.

According to researchers, it has been documented that the repeated use of a single product for only two years could lead to the development of a herbicide-resistant weed problem.

Based on several of these points, many Extension weed specialists are greatly concerned that the next wave of resistance will come in the mode of action known as PPO inhibitors or protoporphyrinogen oxidase herbicides.

Commonly Used PPO Herbicides In Southern Crop Production

Mode of Action:    Crop: Product Name:
PPO Inhibitor Peanut Valor
Ultra Blazer
  Cotton Valor
  Soybeans Valor
Ultra Blazer


Some products may be used as a burndown preceding the crop, while others may be used during the crop production season and, in the case of cotton, as a defoliant. However, it is all the same mode of action being applied. Consider this when choosing rotation crops and herbicides to use in those crops

So Many Herbicides: One MOA
Many common herbicides have this mode of action, such as Ultra Blazer, Cobra, Goal, Reflex, Resource, Aim, Valor, Storm and Spartan.

“Most growers in Georgia are now relying on Valor, for soybean, peanut and cotton, and Reflex, for soybean and cotton, in many crops,” Prostko says. “These herbicides are their own worst enemies since they are so good in controlling Palmer amaranth.”

“Over-reliance on any chemistry will lead to resistance,” says Jay Ferrell, University of Florida Extension weed specialist. “The best strategy is to combine our chemistry and use every tool we have.

“Resistance starts with one seed that makes one plant. If we can have another herbicide come along and knock that one weed out, then the clock restarts and millions of chances are required before that one lucky mutation occurs again.

“This is why a multi-pronged strategy is so important.”

Herbicide-Based Production Systems
Prostko says he and Stanley Culpepper, University of Georgia Extension weed specialist in cotton, are also concerned about Palmer amaranth developing resistance to Ignite as there is a lot of interest in Ignite-based cotton and soybean production systems.

Make sure that weed control strategies based on a herbicide contain other approaches and tools to weed control, including cultural practices and herbicides with different modes of action.

The key to preventing herbicide resistance in weeds to is be pro-active instead of reactive, and preventing resistance is easier to do than controlling weeds after resistance has infiltrated the weed spectrum. PG

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