Resistance — know your herbicide families

It is doubtful that you can pick up an agricultural publication worthy of the price of soybean in the ink used for the print without finding at least one article lamenting the curse of herbicide-resistant weeds.

We read, write, and talk a great deal about the various biotypes that have modified their genetics to outsmart modern herbicide science and warn of others to come.

Resistant weeds seem to be proliferating at a faster rate than modern herbicide development.

We could go into a long dissertation as to why and how herbicide companies have merged together and reduced their emphasis on new herbicide discovery and development. It might make interesting reading, but it would not change our current situation of a dwindling herbicide arsenal available to provide weed control in the various crops.

As each new resistant biotype evolves, we attempt to devise control remedies that usually end up costing more money, restricting our options and/or adding complexities to our operation. For these reasons, herbicide-resistant weeds demand more than our passing attention.

Ignoring the situation or adding a Band-Aid here and there is not a solution to this dilemma. We must take herbicide resistance seriously and learn how to manage the weeds we have with the herbicides available on a long-term basis.

There is a lot said about rotating herbicides and diversity in our herbicide programs. A farmer friend complained, “All this herbicide resistance is running together, and I can’t remember what weed is resistant to what herbicides.” He is not alone. I work with it every day and it sometimes gets confusing to me also.

He was likely struggling more with the herbicide families than with resistance to individual herbicides. For instance, greater than 60 percent of the pigweed populations in the Mid-South are resistant to the ALS family of herbicides. This means they are resistant to Staple and Envoke in cotton; Classic, Canopy, Firstrate and Pursuit in soybeans; Resolve, Exceed, and Steadfast in corn; and Grasp, Regiment, Newpath and Permit in rice. This makes rotating herbicides in different crops somewhat more challenging.

Horse traders and bird dog trainers (and cotton breeders) are notorious for citing pedigrees and linage. “This fine specimen is out of Speeding Bullet by Steady Hand and grandson of Leaps Tall Buildings, etc.” is common jargon. And any cotton breeder with a hand lens and lab coat can talk 30 minutes about the number of hairs on a cotton leaf. It is easy for me to make light of their passion, but this type information is vital to their business and they must take it very serious.

Hopefully, not to the linage extreme, but knowledge and understanding of herbicide families are also important to our farming decisions. Although Arkansas, Tennessee and Mississippi have more herbicide-resistant weeds than any other three states in the United States, it is not too late to initiate management strategies. Using herbicides with different modes of action is critical to our sustainability of weed management.

One question that might be asked: “Are we leaving more weeds in the field each year than the year before?” I fear the answer to this question is “yes” on many fields. If so, common thought tells us this is not a sustainable system over time. At some point, the weed infestation will become too great to farm.

Each year the number of weed escapes increases, the more difficult it will be to reverse the trend. We must not only take the time to select herbicides with different modes of action, but also to learn how and when to use each mode of action for maximum benefit.

For instance, a long residual PPO herbicide applied early in the season may be of more benefit than the same herbicide applied later in the season. And yes, this does seem to run together without some commitment to the issue. Maybe it is time we took a page from the horse traders and dog trainers and began to study and talk herbicide families and mode of action.

We have attempted to list most herbicides by family in our Arkansas MP-44 weed control guide ( Other universities have similar information in their guides. Hopefully, farmers and consultants will utilize this information as herbicide programs are planned for each crop to devise resistance management plans that will be effective over multiple years to insure sustainability in our greatest resource — agriculture. And now, I promise to get off this soap box and get back to weed control situations.

Five quick thoughts:

• LibertyLink soybean may be the best option as chances for activation of preplant and pre-emergence herbicides diminish with dry weather.

• Don’t forget where the bullet hoods are parked. They can be very valuable if pigweed control breaks in six- to eight-leaf cotton.

• One last glyphosate plus Callisto application in corn at the 16- to 24-inch growth stage can eliminate late-season morning-glory and solve a lot of harvesting problems.

• Running water furrows break herbicide barriers and allow weeds to come with a vengeance. Residual herbicides behind the water furrow plow are essential.

• Relying on Newpath alone for barnyardgrass control in rice is extremely risky. Resistance to Newpath has been documented in Arkansas. It is impossible to differentiate between a late germinating escape and a resistant plant. One resistant plant is too many.

TAGS: Soybeans
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