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February 8-20, 2000
C.O.R.N. 2000-4
In This Issue:
A) ALS
Resistance Confirmed in Ohio
B) Lowering
the Potential for ALS Resistance
C) Herbicide
Options for ALS-Resistant Weed Populations
D) Nitrogen
Topdress on Wheat
E) Prosulfuron (Exceed and Spirit)
Bioassay now Available
OSU
has identified ALS-resistant weeds in a number of fields in Ohio, and we are in
the process of characterizing additional populations of giant and common ragweed
for resistance. We have predicted
for a number of years that ALS-resistance could develop based on the intense use
of ALS-inhibitors in Ohio and the identification of ALS-resistant weeds across
the United States in recent years. ALS-inhibitors include all sulfonylureas (Classic, Canopy,
Pinnacle, Accent, Beacon, Spirit, Exceed, Harmony Extra, Peak, Permit, etc),
imidazolinones (Pursuit, Scepter, Raptor, etc), and sulfonamides (Python,
FirstRate, etc). Some highlights of
our research on ALS resistance:
So
far we have confirmed ALS-resistance in: one out of 10 cocklebur populations
screened, one out of 3 Powell amaranth populations, one out of one waterhemp
populations, 13 out of 16 horseweed (marestail) populations, one out of 3
giant ragweed populations, and 7 out of 8 common ragweed populations.
Most
ALS-resistant populations are cross-resistant among all ALS inhibitors with
exception of the cocklebur population.
For example, ragweeds resistant to Classic are also resistant to
Raptor and FirstRate. Four of the 13 horseweed populations showed variable
response between FirstRate and Classic, indicating incomplete
cross-resistance.
We
sampled common and giant ragweed seed from 125 additional fields during 1999
where ALS resistance is suspected. We
will be screening these populations for ALS resistance in the greenhouse
this winter/spring.
Fields
where ragweed have been confirmed to be ALS-resistant have a history of
frequent ALS inhibitor use, in some cases, average one application per year.
In many of these fields, though, ALS inhibitors were used twice in
the same year in soybeans (example: Canopy PRE + Classic POST).
Use of ALS inhibitors on an every other year basis does not seem to
be sufficient to prevent the development of resistant populations.
When
a ragweed population is ALS-resistant, mixing low rates of another herbicide
mode of action with an ALS inhibitor (e.g. Flexstar + FirstRate), will not
provide adequate control. Labeled
rates of the alternative mode of action are generally necessary to ensure
control, and it will be most effective to rotate to an alternative mode of
action when a population is resistant.
Based
on our field sampling, we suspect a considerable number of fields across the
state will show resistant populations within the next several years unless
herbicide programs are altered now. Fields
with variable response to ALS inhibitors within the past several years are
possible candidates, especially where one observes dead plants next to
relatively unaffected plants.
We suggest that producers review field herbicide use histories. Fields where ALS inhibitors have been used several years in a row, or every other year with multiple applications per year, are most likely to be the first to develop resistance. However, even a single application every other year could result in resistance. Fields following this pattern of ALS inhibitor use should be rotated to an alternative mode of action. Rotating to Roundup Ready soybeans would be a good option for suspect fields, but other options are also available.
1)
Do not
use ALS inhibitors more than once every three years.
If in a corn/soybean rotation where ALS inhibitors would be used in
soybeans, switch to an alternative mode of action every other year of soybeans.
Use of ALS inhibitors has been increasing in corn and wheat (Spirit,
Accent, Harmony Extra, etc), so factor this use in when planning herbicide
programs to avoid resistance.
2)
Avoid
multiple applications of ALS inhibitors within the same year (e.g. Canopy PRE
followed by Classic POST). Where an
ALS inhibitor is applied preplant or preemergence and a postemergence
application is necessary to control escapes, use an alternative mode of action
for the POST application (Flexstar, Basagran, Cobra, etc).
3)
Applying
mixtures of ALS inhibitors with alternative mode of action herbicides can help
delay resistance from becoming a major problem. However, keep in mind that the rates of both products in the
mixture must be high enough to have substantial activity on a weed.
For example, a mixture of Banvel plus Spirit should have upwards of 4
ounces of Banvel per acre to have substantial activity on broadleaf weeds,
instead of just the 2-ounce rate often used in this combination.
Once a weed population is resistant, try to rotate to an alternative mode
of action.
COMMON
RAGWEED: variable preemergence activity from Sencor, Command, and Lorox.
Lorox may be the most effective on suitable soils.
Good postemergence control occurs with Roundup Ready, Cobra, Flexstar,
Reflex, or Blazer. Combinations of
a diphenylether with Basagran can improve control, but the rate of Blazer in
Galaxy is usually too low to provide consistent control.
GIANT
RAGWEED: Poor preemergence activity in the absence of ALS inhibitors.
Single or multiple postemergence applications will be most effective.
Best postemergence activity is from Roundup Ready or mixtures of Basagran
with Flexstar, Reflex, or Cobra. Application
of Flexstar or Cobra alone is more variable.
High populations will require multiple postemergence applications.
Effective control strategies that minimized herbicide costs in OSU research
included: 1) Roundup Ready - apply one quart when ragweed are 4 to 6 inches
tall, and make a second application approximately two weeks later; 2(Basagran +
Reflex or Basagran + Flexstar - apply half rate when ragweed are 3 to 4 inches
tall, and make a second application at the same rate in approximately two weeks.
A similar strategy can be used with a Cobra + Basagran mix.
This strategy maximizes control while minimizing cost and soybean injury.
Keep in mind that a labeled rate of Reflex or Flexstar can only be
applied once per season due to carryover concerns.
COCKLEBUR:
postemergence application of Basagran or Flexstar.
WATERHEMP:
This weed can emerge over extended periods of time. While a number of preemergence and postemergence herbicides
are effective, a planned preemergence + postemergence approach may be necessary,
especially in higher organic matter areas.
Preemergence herbicides with activity include Sencor, any chloroacetamide
(Dual, Frontier, etc), Treflan, Prowl, and Authority.
Postemergence herbicides with activity include Roundup, Cobra, Flexstar,
Reflex, Blazer, and Stellar.
HORSEWEED (MARESTAIL): Canopy + 2,4-D has been an effective preplant treatment, but will be much less effective on ALS-resistant biotypes. Application of 2,4-D alone can be effective, but primarily at the 1 quart per acre rate, which must be applied at least 30 days before planting. Mixtures of Roundup or Touchdown plus 2,4-D are effective, but activity can be reduced under cool conditions or if the Roundup/Touchdown rate is too low. Following this type of mixture with additional glyphosate postemergence in Roundup Ready soybeans should be adequate. Sencor has activity on horseweed, and mixtures of Sencor plus 2,4-D plus Gramoxone have provided excellent control in OSU research. If possible, tillage in the spring can usually control horseweed for the season.
Soon
it will be time to apply nitrogen as wheat fields begin to green-up.
When determining rates, a realistic yield goal is important. Yield
potential should be high since many wheat fields have suffered little winter
damage to this point. It is still
too early to determine how much damage may occur from heaving.
A general spring recommendation is one pound of nitrogen for every bushel
of expected grain. For most growers that would be rates between 80 – 100
pounds of nitrogen. You may want to
increase this rate 10-15% if your yield expectations are greater than 90
bushels. These rates are based on mineral
soils, which have both 1-5% organic matter and adequate drainage.
Nitrogen rates would need to be reduced on soils with high organic matter
to decrease the risk of lodging.
In
most years, nitrogen may be applied between early March and early April.
Research by the Ohio State University has shown mixed results in
splitting spring applications of nitrogen. In
years where nitrogen loss is high early in the season, a split application may
be a benefit. If you are following a
split program, it is important that the first application occurs soon after
initial green-up and the second application is at initial jointing (Feekes
Growth Stage 6). The time of application
is not as critical in a single topdress, but applications should be made after
initial green-up and before jointing.
Most
forms of nitrogen are satisfactory for wheat. Urea
and 28% solution (urea-ammonium nitrate) are often the most common.
Urea has the least potential to cause damage to the crop.
Damage is generally insignificant from broadcast applications of 28%
solution applied early, but the potential for damage increases as we approach
jointing. Dribble applications will
minimize damage from 28% solutions. Urea-ammonium nitrate solutions will have
some nitrogen available at application time; urea will have a short lag as it
converts to ammonium and nitrate forms of nitrogen.
Generally, 28% solutions have the greatest potential for nitrogen losses
since part of it is in the nitrate form. Urea
may have volatilization losses if temperatures are warm and if soil conditions
are dry.
For more detailed information, please refer to the Extension bulletin E-2567, Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat and Alfalfa.
The
Illinois Crop Improvement Association can conduct a prosulfuron bioassay (a
component of Exceed and Spirit) for growers who have a concern about carry-over
of this product. Prosulfuron soil
degradation is affected by lack of moisture and high soil pH.
Each of these can act independently to decrease soil degradation of
prosulfuron, but when combined degradation is slowed even more, thus causing a
greater carryover risk.
The
greatest risk for carryover is when soils are very dry, as stated in the label,
and soil pH is greater than 7.0. The
Exceed and Spirit labels states that "when less than 12 inches of rainfall
is receive within 5 months after application, or less than 1 inch of rain is receive within 4 weeks after
application, that only field corn, small grains or STS soybeans should be
planted 10 months after application".
The Exceed and Spirit labels further states that "when these
products are applied to soils with a pH greater than 7.8, then only field corn
or small grains should be planted the following season".
The Exceed and Spirit labels allow STS soybeans to be planted the
following season if the field was dry, as stated in the label, and/or soil pH is
greater than 7.8.
If
producers are interested in having a bioassay conducted for fields close to or
meeting the rainfall and pH conditions they need to contact Illinois Crop
Improvement Association for information. Soil
will need to be collected. You can
contact Illinois Crop Improvement Association by phone at 217-359-4053 or by
e-mail at
The bioassay cost is $125 with additional costs for containers and
shipping. Remember that bioassays
are not 100% accurate, but they will provide the best practical information
available for testing prosulfuron carryover.
Readers can subscribe electronically to this newsletter by sending an e-mail message
to: corn-out-on@postoffice.ag.ohio-state.edu.
A successful subscription message will receive by an automatic reply from the listserv.
Contact your local Ohio State University Extension Office or e-mail
Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/
C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.
Contributors to C.O.R.N. include: State
Specialists: Pat Lipps & Anne Dorrance (Plant Pathology), Hal Willson
(Entomology), Peter Thomison (Corn Production), Jay Johnson (Fertility), Jim
Beuerlein (Soybeans & Small Grain), Mark Loux (Weed Science), Jeff Stachler
(Weed Science);
District Specialists: Curtis Young (IPM), Ed Lentz (Agronomy) Jim Jasinski (IPM);
Extension Agents: Mike Estadt (Pickaway), Tom Krill (Van Wert), Roger Bender
(Shelby), Dave Jones (Allen), John Barker (Knox), Steve Bartels (Butler), Larry
Lotz (Fayette), Ray Wells (Ross), Clark Hutson (Seneca), Barry Ward (Marion),
Dennis Baker (Darke), John Hixson (Union), Bruce Clevenger (Defiance), Gary
Wilson (Hancock), Greg La Barge (Fulton), Howard Siegrist (Licking) and Steve
Prochaska (Crawford).
Editor: Greg LaBarge Web Editor: Nathan Watermeier
Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.
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