August 2022 - 12

FOOD SAFETY
Study shows safe use of dicamba in sweet corn
By Lauren Quinn
University of Illinois
Many agronomic weeds are
developing resistance to available
herbicides, making them harder and
harder to kill. With few effective
chemicals left and no new herbicide
classes on the horizon, farmers are
going back to older products that still
offer the promise of crop protection.
Dicamba has been on the market
since the 1960s, but the herbicide
is used only on about 17% of corn
acres in the U.S. It still appears to
be effective on waterhemp and its
troublesome weedy relatives, but
dicamba isn't currently labeled for
use in sweet corn because of known
sensitivity issues in the crop.
" Twenty years ago, herbicide
sensitivity was the number-one pest
management concern in the sweet
corn industry. There were a lot of
important hybrids that had adverse
responses, " said Marty Williams, a
USDA-Agricultural Research Service
ecologist and adjunct professor in the
Department of Crop Sciences at the
University of Illinois. Williams coauthored
a study in Weed Science.
But scientists know a lot more
now about the genes that help corn
safely metabolize dicamba and
other herbicides. So it was time for
Williams and his research team to
take another look at the risk of sweet
corn injury from dicamba. Their
recommendations offer practical
guidance beyond sweet corn.
" The gene that confers tolerance to
dicamba and other herbicides in sweet
corn is the same as in field corn, so our
study system is representative of other
types of corn, " Williams said.
The gene in question - Nsf1 - is
a cytochrome P450 involved in
detoxification of multiple herbicide
families in plants. With two
functional copies of the gene, corn
From left: Marty Williams, Daljeet Dhaliwal and Chris Landau. Photo: University of Illinois
fights off dicamba's cellular attacks
before they can cause injury. But
mutant versions of the gene also
exist in some corn lines. Plants with
two mutant copies of the gene are
highly sensitive to dicamba, while
mutant-functional gene pairings offer
intermediate protection.
Chris Landau, a postdoctoral
researcher working with Williams,
confirmed these patterns in sweet corn
by applying dicamba to three hybrids
representing functional, mutant, and
intermediate genotypes. He applied the
herbicide at three growth stages (V3,
V6, and V9), and mixed the herbicide
with the safener cyprosulfamide in half
the treatments.
" Ours is the first study in corn
to simultaneously evaluate the
combination of genotype, application
timing and safener on dicamba injury, "
Landau said.
As expected, sweet corn with mutant
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and intermediate Nsf1 genes showed
more dicamba injury than corn with
two copies of the functional Nsf1
gene. The study also showed dicamba
application at the latest timing, V9,
caused injury regardless of genotype,
suggesting earlier applications are safer
for all sweet corn lines. The safener
eased symptoms somewhat, but didn't
erase injury altogether.
" For almost every injury metric we
looked at, including ear breakage, ear
length, total ear mass, kernel mass, and
others, the safener helped, " Landau
said. " It also consistently lowered
injury at the earlier timings, V3 and
V6, but that effect wasn't as profound
as the V9 application. "
The results indicate dicamba could
be used safely in sweet corn, given a
few caveats (and approved labeling
by the Environmental Protection
Agency): Apply with safeners before
V9, and avoid applying in sweet corn
with mutant Nsf1 genes, if possible.
" This work really establishes what's
needed for more utility of dicamba
in sweet corn, " said co-author Aaron
Hager, associate professor and
Extension specialist in crop sciences.
" That includes working with breeders
in their continued efforts to get rid of
the sensitive alleles.
" Also, we know application timing is
going to be important, as it is in field
corn, " Hager said. " We know now that
safeners can bring some margin of
increased selectivity in sweet corn.
The work lays the foundation for the
industry to use a tool that has not
been widely used in this particular
cropping sequence. "
The article, " Significance of
application timing, formulation, and
cytochrome P450 genotypic class on
sweet corn response to dicamba, " is
published in Weed Science [DOI:
10.1017/wsc.2022.5]. Co-authors
include Chris Landau, Mark Bernards,
Aaron Hager and Marty Williams. VGN

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August 2022

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