Sensitivity Test

6 Apr 2017

By Emily Unglesbee
DTN Staff Reporter

ROCKVILLE, Md. (DTN) -- The best way to avoid drift damage from newly labeled auxin herbicides this year is never to spray near crops that are sensitive to dicamba and 2,4-D -- and that might be harder than you think.

For the past five years, University of Georgia Extension weed scientist Stanley Culpepper has been gathering data on which specialty crops (and row crops) suffer serious visual damage when small amounts of these two herbicides drift onto them.

This list is not comprehensive, nor final, Culpepper warned. Don't assume that a specialty crop is safe just because you don't see it here. UGA scientists are in the process of testing more crops every year and ratings for some crops could change as more data come in. Keep in mind that sensitive non-crop plants abound, as well, University of Missouri weed scientist Kevin Bradley added. For example, trees such as red oak and white oak have shown extreme sensitivity to dicamba.

DICAMBA-SENSITIVE CROPS

Culpepper divided the herbicide sensitivity of crops into four groups: lower sensitivity crops (visually detectable injury symptoms when exposed 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 dicamba sensitivity levels of crops tested so far by Culpepper and his colleagues:

-- 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.

Keep in mind that regardless of the yield response to these levels of drift, a vegetable crop often becomes worthless once it shows visual herbicide damage, Culpepper said.

"In the specialty crop world, perception rules," he said. For now, end users such as grocery stores and processers will not accept any level of dicamba herbicide exposure to fruits and vegetables. Some herbicide tolerances have been established in specialty crops for 2,4-D, but EPA is still evaluating "inadvertent residue tolerances" for dicamba in crops such as grapes, tomato and melons, BASF technical marketing manager Chad Asmus told DTN in an email. "An EPA decision is expected later in the 2017 growing season," he said.

One tomato processor, Red Gold, is so concerned about potential dicamba drift that they issued a letter to commercial farmers who operate near tomato growers urging them not to make any dicamba applications within a half mile of a tomato field. "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."

2,4-D-SENSITIVE CROPS

Here are the sensitivity levels of various crops to 2,4-D, according to Culpepper's studies:

-- Extreme sensitivity: cotton, grapes, sweet potatoes and tobacco.

-- Severe sensitivity: pepper, tomato and watermelon.

-- Moderate sensitivity: cantaloupe, canola, cucumber, peaches, peanut, pecan and squash.

-- Lower sensitivity: broccoli, cabbage, kale, mustards, onion and turnips.

For some context, many of these vegetables such as tomato, squash and watermelon would instead fall into the lower sensitivity category for glyphosate, Culpepper noted.

"People who tell growers that this will be just like spraying Roundup are seriously misleading them," he said of the newly labeled dicamba and 2,4-D herbicides. "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."

See more information on sensitive crops on Culpepper's site here: http://bit.ly/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

(PS/AG)