Dicamba Injury Study

Reproductive Stage Soybeans More Sensitive to Dicamba

Matt Wilde
By  Matt Wilde , Progressive Farmer Crops Editor
Kevin Bradley, University of Missouri weed specialist, informs farmers and agronomists about the latest studies on dicamba and weeds during the university's Pest Management Field Day on July 9 at the Bradford Research Center in Columbia, Missouri. (DTN photo by Matthew Wilde)

COLUMBIA, Mo. (DTN) -- University of Missouri (Mizzou) research has shown that soybeans entering the reproductive phase are most vulnerable to injury from dicamba.

That reproductive time is now across the major production areas, according to the latest U.S. Department of Agriculture (USDA) Crop Progress and condition reports. Dicamba applications are ongoing in many states due to a late planting season and rule changes.

Weed scientists urge farmers and chemical applicators to communicate and follow label instructions to mitigate possible production losses associated with dicamba drift. The herbicide and weed management were dominant topics Tuesday at Mizzou's Pest Management Field Day at the Bradford Research Center in Columbia, Missouri.

"When (soybean) plants start to flower, they are most susceptible to yield loss (from dicamba)," said Mandy Bish, University of Missouri Extension weed specialist. "There's still a lot of dicamba being sprayed.

"Neighbors need to communicate with each other to find out what's planted where and come up with a game plan," she continued. "Applicators must follow the buffer and wind speed rules and even shut down spraying earlier than the label requires. Two hours before sunset may not be sufficient during weather that favors temperature inversions."

Studies show non-dicamba-tolerant soybeans in the R1 to R3 reproductive stage -- beginning flowering to beginning pod set -- sustain the greatest yield loss from drift. Multiple exposures exacerbate losses, research indicated.

University of Missouri testing last year showed soybeans injured by dicamba in the V3 stage produced 1 bushel per acre (bpa) more than non-injured beans after a single drift event. Injured soybeans in the R1 and R3 stages yielded 11 bpa and 7 bpa less, respectively, after a single drift event than non-injured soybeans. Injured soybeans in stage R5 only suffered a 2 bpa loss compared to non-injured soybeans. If soybeans sustained drift injury during the R1 and R3 stages, yields were half as much compared to non-injured soybeans.

"There's a difference getting drifted on once compared to many times," said Kevin Bradley, a Mizzou weed specialist. Tests show production losses escalate in soybeans sensitive to dicamba if drift injury occurs multiple times.


Soybean planting occurred well into June or even July in some cases. Only 7% of Iowa's soybean crop was in bloom as of Sunday, July 7, according to USDA. That's 10 days behind the five-year average.

Six percent of Missouri's soybean crop is in bloom, according to the latest crop progress and condition report. Illinois is even less at 2%. Soybean progress is below average in most states.

Dicamba labels allow over-the-top soybean applications prior to beginning bloom (R1) or no more than 45 days after planting, or whichever comes first, for XtendiMax, Engenia and FeXapan. Tavium in-crop applications are allowed through V4 or 45 days after planting, whichever comes first.

Some states, like Iowa, follow federal label rules dealing with application timing. States with earlier dicamba application cut-off dates extended the time to spray due to the lack of suitable spraying days and delayed planting. For example, Illinois extended its June 30 cutoff date to July 15 for June-planted soybeans. Oklahoma filed a special local needs label to allow for in-crop dicamba use for 90 days after planting cotton and 60 days after seeding soybeans.

"If soybeans are flowering or in a later stage, people need to understand the yield loss due to injury can be much greater," Bradley said.

Dicamba injury symptoms tend to take at least 10 to 14 days to appear. DTN previously reported dicamba off-target injury complaints have started coming in to state agencies.

"It is important to note that not all dicamba injury results in yield loss," Bish said.


Bradley said early studies indicate new 2,4-D choline formulations are not as volatile as reformulated dicamba products.

Bradley expects more herbicide options and flexibility for farmers in the future, including additional soybean traits with three-way tolerance.

A new University of Missouri study reveals farmers need to consider their cereal rye cover crop seeding rate in order to achieve the most effective weed control. Research shows seeding rates of at least 50 pounds of cereal rye per acre are required to suppress waterhemp, and preferably even higher.

When planting into green cover crops, research indicates farmers should wait to apply residual herbicide until cover crops start to deteriorate after burndown and soybeans reach to the V2 to V3 stage to be the most effective.

Dicamba-injured soybeans do not attract more insects, so there is not a greater increase in yield loss, research indicated.

University of Missouri entomologist Kevin Rice said Japanese beetle populations won't increase this year due to extensive rainfall and flooding, but injury thresholds could be reduced due to short plants.

Stink bugs may cause more problems this year since they like later-planted soybeans and late-maturing pods, Rice said.

Matthew Wilde can be reached at matt.wilde@dtn.com

Follow him on Twitter @progressivwilde


Matt Wilde