ROCKVILLE, Md. (DTN) -- Reports of the characteristic cupped leaves associated with dicamba drift have put soybean growers on high alert this summer.
While suspicious symptoms have been reported, what is less certain is how much yield loss famers will actually face when combines roll across these fields in the fall.
Growers planted 20 million acres of Roundup Ready Xtend dicamba-tolerant soybeans this year. They had three options of new dicamba formulations -- Monsanto's XtendiMax, DuPont's FeXapan and BASF's Engenia -- all of them engineered to be less volatile than older dicamba herbicides.
While the physical symptoms of dicamba damage are fairly distinctive, the actual yield loss growers face is far more difficult to diagnose, said University of Missouri weed scientist Kevin Bradley.
Scientists from multiple universities have run experiment plots with driftable amounts of dicamba on soybeans, but field situations are far less precise, he said.
"The problem is we have no idea what rates of the herbicide contacted that field and the concentration they received," he said. "Was it 1/100th of the labeled rate? 1/1,000th? We don't know."
What we do know is timing matters, and so does dose.
"If it's early in the season, and you see cupped soybean leaves around V2 to V4, then our data and a lot of others' data say the field will probably recover and suffer little to no yield loss," Bradley said. "But once you hit R1, the flowering stage, there's a pretty good chance you'll suffer some yield loss," he added. "We can say that with confidence."
YIELD LOSS STUDIES
Bradley's recent dicamba drift experiments in Missouri make clear how incredibly sensitive soybeans are to dicamba.
When V3 soybeans were sprayed with trace amounts of dicamba (1/20,000th the labeled rate, or 0.000025 lb./acre --- a tiny fraction of a teaspoon per acre), the soybeans cupped and curled and showed obvious damage. But by the time harvest rolled around, no yield loss could be detected.
As the dose increased and soybeans grew, so did the yield losses in Bradley's experiments. Less than half a teaspoon of dicamba sprayed per acre (1/200th the labeled rate or 0.0025 lb./acre) on R2 soybeans produced a 14% yield loss.
When the dose was increased to nearly a tablespoon per acre (1/20th the labeled rate or 0.025 lb./acre) at R2, the yield loss swelled to 68%.
Ironically, the later, more serious damage is actually harder to spot for growers, Bradley noted. In the reproductive stages, soybeans tend to show damage with curled or aborted pods, and dead flowers, which are not as visible as the cupped leaves of earlier injury.
USDA researchers from Penn State University crunched seven decades' worth of data on simulated dicamba drift injury to soybeans in 2014 for more of a bird's-eye view on yield loss.
They concluded that when flowering soybeans are exposed to dicamba "vapor drift," or 1/1,000 of the use rate, 1% yield loss is likely. When exposed to "particle drift," or 1/100 the use rate, flowering soybeans lose closer to 9% yield.
That's a lot of numbers. Bradley sums up the chances of yield loss this way: "If the injury occurs early in the season, as long as you don't have a drought or some big stress later, you'll probably suffer more in the 1-to 3-bushel range," he said. "If it's later, during flowering or beyond, you're looking at much bigger numbers."
Soybean growers producing for seed increase have additional worries.
University of Arkansas weed scientist Tom Barber led a 2015 study examining the impact of dicamba injury on non-tolerant soybeans produced for seed. That research showed seeds from plants exposed to dicamba from R1 to R6 "had reduced germination, vigor and increased injury."
Barber told DTN that low rates of dicamba that come into contact with susceptible soybean at later growth stages such as R3-R5 can result in carryover that causes plants produced from that seed to exhibit dicamba injury symptoms.
See Bradley's studies here: http://bit.ly/….
See the Penn State meta-analysis here: http://bit.ly/….
See Barber's study here (pg. 266): http://bit.ly/….
Emily Unglesbee can be reached at Emily.firstname.lastname@example.org
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