Off-label dicamba use spurs drift injury and complaints

dicamba injury
Dicamba injury on non-resistant soybeans typically ‘cups’ the leaves — photo courtesy University of Arkansas

Heading into this season, Extension weed specialists were concerned about Environmental Protection Agency registrations not keeping pace with herbicide-resistant traits. Their worries, in particular, focused on Xtend soybeans and cotton from Monsanto, which have been genetically modified to resist dicamba herbicide.

Although several Xtend soybean and cotton varieties have been approved to plant, the EPA has yet to register any over-the-top use of dicamba for soybeans or cotton. Monsanto has submitted a registration packet for Xtend Herbicide, a premix of glyphosate and  a low-drift formulation of dicamba.

Complaints about dicamba injury to thousands of acres of crops in Arkansas and Missouri are mounting, according to a news release.

University of Arkansas Extension weed scientist Tom Barbera says injury reports have been concentrated in northeast Arkansas as well as in Lee, Lonoke and Phillips counties.

“What appears to be happening is that growers planted this technology, then decided to make off-label applications of dicamba over the top for weed control prior to the product receiving a full herbicide label,” Barber said in the news release.

“This new technology was approved for export to China in the early spring, and released for cotton and soybean growers to plant without a formulation of dicamba herbicide labeled for use. Given the resistant pigweed situation we’ve had, growers who planted this technology felt like they didn’t have an option for pigweed control, and they needed to spray something.”

Between 150,000 and 200,000 acres of Xtend soybeans were planted throughout the Delta region this season, according to estimates from University of Arkansas Extension soybean agronomist Jeremy Ross.

Due to growing weed resistance, glyphosate no longer controls pigweed, and several pigweed populations in Arkansas also are resistant to PPO herbicides that contain fomesafen, such as Flexstar.

The dicamba injury to non-Xtend soybeans is likely coming from three sources: drift, tank contamination and volatility — the ability of some dicamba formulations to vaporize after application, become windborne and deposit on sensitive crops away from the original target.

Injured soybeans exhibit typical “cupping” symptoms as well as blistering and pickering.

Yield losses depend on when injury occurred. Plants injured during the productive stage, or R1, can have 10 percent yield losses even after very low levels of exposure.

Soybeans typically have significantly less yield losses if exposed to the herbicide during the vegetative stages.

The Arkansas Plant Board has received more than two dozen formal complaints of dicamba drift from the state’s northeast counties as well as several other communications from growers unwilling to file formal complaints.

Susie Nicols, board manager, says it’s unusual when more than 20 of the board’s approximately 200 open cases deal with one specific herbicide.

Although the Plant Baord doesn’t have the authority to file criminal charges in cases of off-label pesticide application, it does have the authority to fine violators a maximum of $1,000. Because the current fine system appears to have failed as a deterrent, Nichols says administrators are considering raising the fine to $10,000 for “egregious violations.”

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