Sensitivity Test

These crops are most likely to suffer if dicamba and 2,4-D drift.

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Image by Pamela Smith

You read the label. You know the rules. Never spray dicamba or 2,4-D where it might drift onto crops that are sensitive to auxin herbicides.

But, which crops are sensitive?

University of Georgia (UGA) Extension weed scientist Stanley Culpepper has gathered data on which specialty crops (and row crops) suffer serious visual damage when small amounts of these herbicides drift.

Culpepper’s lists are not comprehensive or final. UGA scientists are testing more crops every year, and ratings for some crops could change with more data. For example, red oak and white oak trees were found to be extremely sensitive last year in Missouri when dicamba moved off-target, University of Missouri weed scientist Kevin Bradley says.

The new formulations of 2,4-D and dicamba (XtendiMax, FeXapan, Engenia, Enlist One and Enlist Duo) are designed to be less volatile, which means they should not evaporate as easily. They are not designed to be less potent to sensitive crops, which are numerous.

2,4-D-Sensitive Crops:

Here are the sensitivity results for 2,4-D:

• Extreme Sensitivity: cotton, grapes, sweet potato and tobacco

• Severe Sensitivity: pepper, tomato and watermelon

• Moderate Sensitivity: cantaloupe, canola, cucumber, peach, peanut, pecan and squash

• Lower Sensitivity: broccoli, cabbage, kale, mustards, onion and turnips

Many of these vegetables, such as tomato, squash and watermelon, would only fall into the “Lower Sensitivity” category for glyphosate, Culpepper points out.

“People who tell growers that this will be just like spraying Roundup are seriously misleading them,” he says. “Part of the reason we did these studies is to prove that some of these crops are 10 to 20 times more sensitive to auxin herbicides than to glyphosate.”

Get more information about sensitive crops on Culpepper’s website at bit.ly/2oymHj2.

Dicamba-Sensitive Crops:

Stanley Culpepper used four levels to rate the herbicide sensitivity of crops: Lower Sensitivity Crops (visible symptoms after exposure to more than 1/75 the labeled rate); Moderate Sensitivity (1/75 to 1/300 the labeled rate); Severe Sensitivity (1/300 to 1/800 the labeled rate); and Extreme Sensitivity (less than 1/800 the labeled rate).

Here are the sensitivity results for dicamba:

• Extreme Sensitivity: grapes, lima beans, Southern peas, snap beans, soybeans, sweet potatoes and tobacco

• Severe Sensitivity: cotton, pepper, tomato and watermelon

• Moderate Sensitivity: cantaloupe, cucumber, peach, peanut and squash

• Lower sensitivity: broccoli, cabbage, kale, mustard, pecan and turnips.

It only takes tiny amounts of active ingredient. According to Culpepper’s data, just 10 to 16 drops from an eyedropper of the newly labeled dicamba herbicides, dispersed over an acre, could cause visual injury symptoms to “Extreme Sensitivity” crops such as soybeans and grapes.

Regardless of the yield response to these levels of drift, a fruit or vegetable crop often becomes worthless once it shows visual herbicide damage, Culpepper says.

“In the specialty-crop world, perception rules,” he says.

Herbicide tolerances have been established for 2,4-D for some specialty crops. However, EPA is still evaluating herbicide residue tolerances for dicamba in crops such as grapes, tomatoes and melons, says BASF technical marketing manager Chad Asmus.

For now, end users such as grocery stores and processors will not accept any level of dicamba herbicide exposure for fruits and vegetables.

One tomato processor, Red Gold, has been so concerned about potential drift that it issued letters in 2017 asking farmers not to make dicamba applications within one-half mile of a tomato field. A similar campaign is planned for 2018.

“If an off-target event occurs, it will not just be a yield loss, but the crop will need to be destroyed, and the total value of the crop for your neighbor and the total processing value of the crop to us as the processor will be claimed,” the letter states. “This can result in a loss well in excess of $10,000 per acre, as well as a potential fine from the regulatory agency in your state.”

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Emily Unglesbee