This year marks the second year of sales for Clearfield sunflowers,
with U.S. growers forecast to have planted 350,000 acres of the herbicide-resistant
crop amid expectations it will streamline the job of weed control.
Clearfield sunflowers owe their herbicide resistance to a wild sunflower
(Helianthus annuus) accession that Kansas State University weed
physiologist Kassim Al-Khatib collected from a soybean field in 1996.
Agricultural Research Service
plant geneticist Jerry Miller learned of Al-Khatib's wild sunflower
collection from John Nalawaja, a colleague at North Dakota State University-Fargo,
and requested seed specimens. Miller germinated eight-leaf seedlings
inside an herbicide-spray chamber, and Nalawaja helped test them. They
sprayed the seedlings with imazethapyr and imazamox at 2 to 15 times
the herbicides' label-recommended rates. Their goal: To identify the
healthiest survivors and then transfer the plants' resistance to cultivated
sunflower, eventually furnishing growers with a crop that could withstand
direct spraying while nearby weeds withered and died.
At the time, 1998, "This prospect was very exciting because the
list of broadleaf weeds and grasses controlled by these herbicides was
extensive," says Miller, who's with ARS's Red River Valley Agricultural
Research Center, Fargo. "Also, for sunflower to expand into no-till
acreage, planting resistant hybrids was the only alternative for postemergence
weed control," he adds.
From the start, Miller favored conventional plant breeding over biotechnology
as a means of passing H. annuus's herbicide-resistance genes
into cultivated sunflowers. "We knew that Roundup Ready technology
for sunflowers was unlikely to be approved for use in the oilseed and
confection crop," he says. "I'd tell growers: 'This is as
close to Roundup Ready technology as sunflower will ever get.'"
(Roundup Ready refers to Monsanto's herbicide product line and its genetically
modified Roundup-resistant soybeans and corn.)
From 300 wild sunflower specimens, Miller identified 28 that were the
most resistant. From those, he chose six to cross with cultivated sunflower,
producing five generations of crossbred progeny in 1 year. Each time,
he sprayed the sunflower crossbreeds and backcrossed the most robust
specimens with the original cultivated parent plants. Backcrossing eliminated
traits Miller didn't want, like branching, which yields multiple flowerheads
rather than just one.
Normally, the seed of a pollinated plant needs time to form and pass
through a dormancy stage before germinating. But Miller saved time using
embryo rescue, a technique that skips such processes by regenerating
whole plants from cultures of embryonic tissue. The embryonic tissue
produced new seedlings in just 30 days after pollination.
In 2002, Miller and colleagues released two germplasm lines of imazamox-resistant
sunflower for commercial seed companies to use in developing their own
hybrids. A few companies now market Clearfield sunflowers, which are
registered for use only with Beyond, an imazamox herbicide manufactured
by BASF Corporation.
The germplasm releases, USDA lines HA 425 and RHA 426, were never intended
for production as a stand-alone commercial hybrid. Rather, they (and
the hybrid produced by crossing the two lines) serve as benchmarks by
which a candidate Clearfield hybrid's resistance is compared en route
to being approved.By Jan
Suszkiw, Agricultural Research Service Information Staff.
This research is part of Crop Production, Product Value, and Safety,
an ARS National Program (#301) described on the World Wide Web at www.nps.ars.usda.gov.
Jerry F. Miller
is in the USDA-ARS Sunflower Research Unit, Red
River Valley Agricultural Research Center, P.O. Box 5677, Fargo,
ND 58105; phone (701) 239-1321, fax (701) 239-1346.
"Clearfield Sunflowers Stand Tall Against Herbicides"
was published in the January
2005 issue of Agricultural Research magazine.