Hard-To-Control Weeds Need a Mix of
Early growth stage of water-hemp, a weed that is becoming resistant to certain
Farmers and land managers will have to become proficient at long-range
planning for weed controland also learn to use a mix of different
techniquesto control weeds, say Agricultural Research Service scientists.
Hemp dogbane, waterhemp, leafy spurge, cocklebur, and thistle are becoming
more difficult to control with one-step approaches such as using a single
tillage method, annual crop rotations, or seasonal herbicide applications. The
ARS scientists advocate use of integrated weed management systems that increase
cost efficiency for farmers, promote more ecologically sustainable production,
and conserve soil and water resources.
Weeds and weed control cost U.S. farmers about $15 billion each year.
Part of the problem is that the long-term use of specific herbicides has led
to development of weeds with herbicide resistance. Another problem is that
shifts in tillage practices make it easier for different types of weeds to get
established. And some cropping patterns have discouraged competition among weed
species, causing certain ones to spread and crowd out crops.
One scientist tackling weed problems in new ways is ARS range scientist
Robert A. Masters. He is in the ARS Wheat, Sorghum, and Forage Research Unit at
Masters has combined herbicides, controlled burning, and replanting of
native warm-season grasseswithout tillageto supplant leafy spurge,
a noxious weed that threatens Northern Plains grasslands.
"The productive capacity of Great Plains grasslands has been reduced
greatly by invasive weeds like leafy spurge," he says. "These weeds
have displaced desirable native forages as well."
To prepare a site for planting native forage grasses without tillage,
scientists in Lincoln, Nebraska, practice controlled burning in the spring to
remove plant residue remaining after herbicide treatments killed weeds the
Leafy spurge was introduced into the northern Great Plains from Eurasia in
the late 1800s. It has no natural enemies in this country, and herbicides
provide only short-term control. Unlike sheep and goats, cattle and horses
won't graze land infested with leafy spurge, and they avoid eating forage
grasses growing next to the weed.
In field tests conducted between 1992 and 1995 in cooperation with
scientists at the University of Nebraska, Masters applied a three-pronged
strategy to fight leafy spurge. The result was a 60-percent reduction in spurge
populations and a surge in production of warm-season grasses.
First, Masters applied a combination of the herbicides Arsenal and Oust to
kill leafy spurge plants in the fall. He burned the dead plant residue the
following spring. Then he planted native prairie species such as big bluestem,
switchgrass, and indiangrass.
"Two years after planting," says Masters, "switchgrass and
bluestem hay yields increased to over 4,000 pounds per acre, and indiangrass
produced more than 3,000.
"Our goal is to develop economical, integrated weed management
strategies that enable land managers to convert marginal cropland and degraded
grasslands to high-value grasslands that are resistant to noxious weed
invasion," he says. "In our current research, we're refining this
strategy by using a combination of the herbicides Plateau and Roundup, in place
of Arsenal and Oust, to promote establishment of mixtures of native grasses and
Loyd Wax, an agronomist at the ARS Crop Protection Research Laboratory in
Urbana, Illinois, is taking a similar approach to thwart waterhemp, a species
similar to redroot and smart pigweed.
Waterhemp biotypes have become increasingly resistant to ALS-inhibiting
herbicides that block production of branched-chain amino acids, the building
blocks of protein. Wax found super-resistant waterhemp biotypes that withstood
up to 520 times the labeled rate of certain ALS-inhibiting herbicides.
To combat these ALS-resistant biotypes, Wax conducted studies showing that
growers could markedly improve waterhemp control by combining
non-ALS-inhibiting herbicides with cultivation. This strategy affords the
grower two very different methods to control ALS-resistant waterhemp
"We controlled established waterhemp populations with non-ALS
herbicides and then tilled the soil at a depth that was not favorable to
waterhemp seed germination and emergence," says Wax.
"We also found that we could combine sequential applications of
non-ALS-inhibiting herbicides with the rapid canopy closure of close-drill
soybeans for very good waterhemp control in no-tillage systems," he says.
Continuous production of corn and soybeans creates an environment favorable to
weeds such as giant foxtail, which is beginning to choke out the corn on the
right. The foxtail on the left has been treated with a postemergence herbicide,
relieving the corn from competition.
"Farmers will greatly benefit from knowing which species of weeds are
established in their fields and using this information when they plan crop
rotations and farming operations. They should use a variety of farming
techniques to control weeds. As farming systems change, weeds and weed
populations change. So farmers can no longer expect one-shot solutions to weed
They Fall Into Two Types
Annual weeds are plants that reproduce by seed and generally germinate each
year to reproduce. This type of weed includes velvetleaf, cocklebur, and
foxtail. The seeds of annual weeds have a better opportunity to germinate and
grow into mature plants if they are buried in the soil. Seeds not buried tend
not to germinate.
Perennial weeds regrow each year from roots or rhizomestubal
extensions located below the soil surface. Perennial weeds include thistle,
quackgrass, and milkweed. These weeds are usually controlled by tillage
operations such as disking or cultivating.
A shift in tillage techniques is the culprit behind changes in the behavior
of annual weeds such as velvetleaf, cocklebur, and foxtail, according to ARS
agronomist Douglas D. Buhler of Ames, Iowa. He's seeking the right balance of
tillage and cropping practices to knock down both annual weeds and perennials
like thistle, quackgrass, and dogbane.
"Shifts in tillage practices bring about rapid shifts in weed
populations," Buhler explains.
His research shows the population of hemp dogbane remained constant over 14
years when land operators disked or plowed their soil. Plowing disturbed the
dogbane root systems and prevented the weed from spreading. When researchers
switched to no-till farming systems on adjacent acreageor to systems
using no tillage before plantingdogbane populations exploded by 400
percent over the same 14-year period.
On the other hand, no-till tends to discourage
growth of annual weeds like velvetleaf and cocklebur. Buhler's research
showed velvetleaf and cocklebur thrived in cultivated fields but were reduced
by up to 80 percent in fields that were not tilled. By
Dawn Lyons-Johnson, Agricultural
Research Service Information Staff, 1815 North University Street, Peoria, IL
61604, phone (309) 681-6534.
Robert A. Masters is in the
USDA-ARS Wheat, Sorghum, and
Forage Research Unit, 344 Keim Hall, P.O. Box 830937, Lincoln, NE
68583-0937; phone (402) 472-1546, fax (402) 472-4020.
Loyd Wax is in the USDA-ARS
Unit, N-325 Turner Hall, University of Illinois, 1102 S. Goodwin, Urbana,
IL 61801; phone (217) 333-9653, fax (217) 244-7703.
Douglas D. Buhler is at the USDA-ARS
National Soil Tilth Laboratory, 2150 Pammel
Dr., Ames, IA 50011-4420; phone (515) 294-5502, fax (515) 294-8125.
"Hard-To-Control Weeds Need a Mix of Measures" was
published in the April 1998 issue of Agricultural Research magazine.
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