Page 10—Corn Talk, Lancaster Farming, Saturday, March 18, 1995
Herbicide Resistance: A Challenge To Agriculture
W. S. Curran
Assistant Professor
Weed Science
Penn State
Weeds that resist an applica
tion of a once effective herbi
cide arc not a new problem for
northeastern fanners.
We have managed or at least
attempted to control triazine
resistant (TR) weeds in a num
ber of crops for the last 15
years. Although triazine resis
tance continues to spread into
new areas, most producers have
resigned themselves to dealing
with the problem by simply
Table 1. Plan Rotation Strategy - Think Mode of action. This table elassihes Ireibicides hasul on mode ol
action and commodity use In planning a weed management program identil's opportunities for iorating lietueen the
dilferent herbicide classes In particular concentrate on lotalion ol the ALS and the PSI classes
Herbicide class Corn Sos beans Wheat/barley Alfalfa
AI S(sulfonylureas Accent Beacon Broadslnke Hannnm \ Pursuit
itnidazolinones Broadstnke Canopy Classic
sullonamrdes) Exceed Permit Pinnacle Pursuit
Puisuit (IMI conn Scepter
ACCase (lipid s\nthesis None Assure Fusilade Hoelon Poast
inhibitoisi Pusion Poast
Select
EPSP (glyphosatei Roundup Roundup Roundup Roundup
PSI rlnazines phenyl Atrazme Blades Hasagian loros Buctrl Hiiclnl I exotic
ureas uracils misc i Buctnl Basagran l.oros Sensor Smbar
Loros Pnncep \ tlpar
Root (dimtroanalines) Prnul Pioul Sonalan None Balan
T reflan
Shoot Dual Frontier Dual Frontier None r-plain
rchloroacetamrdes Harness Micro Micro-Tech
thiocarbamatesi lech Surpass
Eradrcane Sutan-
Cell membrane Ciraniosone Clramosone Gramosone Uramosone
idiphenyl ethers Resource ’Blazer Cobra
paraquat) Resource Tackle
PORstphenosy’s Banvel Clanty None Banvel 24 D Butsrac
benzoics pyndmesi 2 4-D Stinger MCPA stmgei
Pigment (clomazonei None Command None None
Green Land Corn
1994 PENN STATE TRIALS
The Ohio Seed Company Peter Johnson
West Jefferson, Ohio 43162 PA/DE/MD Rep.
Mifflinburg, PA
800-879-3556
switching to or including non
triazine herbicides in their con
trol programs.
Producers with triazine resis
tance have fewer effective her
bicide options, require more
tank-mixing, and make more
trips over the field, all resulting
in increased costs, potential
poor control, and possibly grea
ter yield or quality loss.
Wouldn’t a program targeted at
prevention be more cost effec
tive in the long run?
It may very well be too late
for many producers and
triazine-resistance. However,
there is still time to take action
Early Medium Season (Zone 2)
new GL 226 # 1 Hybrid
. Avg. 42 Entries
. GL Advantage
Advanced GL 223 #1 Hybrid
. Avg. 30 Entries
. GL Advantage
Late Medium Season (Zone 3)
Advanced GL 262 #5 Hybrid
. Avg. 36 Entries
. GL Advantage
Late Season (Zone 4)
Advanced GL 345 #2 Hybrid
• Avg. 27 Entries
. GL Advantage
on other heibicides that are just
beginning to show resistance
problems.
Some newer herbicide fami
lies of greatest concern to pro
ducers in the Northeast include
the sulfonylureas, imidazoli
nones, and sulfonamide herbi
cides, all collectively known as
the ALS inhibitors because of
the site or enzyme for which
they attack in susceptible
plants. The ALS inhibitor class
includes a number of products
commonly used in com, soy
beans, small grains, and forage
or hay crops (Table 1).
176.7
153.5
+23.2 bu/ac
186.3
162.9
+23.4 bu/ac
198.2
187.9
+ 10.3 bu/ac
195.5
179.1
+ 16.4 bu/ac
Exparianca
Resistance to this class of
chemistry is currently not a
major problem in the midwest
or northeastern U.S. However,
in dryland wheat-producing
areas of the western U.S. and
Canada, ALS resistance is a
problem.
There are several reason why
this class of herbicides pose
such a threat for resistance
development. First, these herbi
cides all attack a single site
within susceptible plants
(ALS). Resistant weeds have an
altered form of ALS which is
no longer inhibited by the her
bicides. Scientist estimate that
peihaps one in a million weeds
that are traditionally suscepti
ble to this class of herbicides
may have an altered ALS. Her
bicide families that act at multi
ple sites within plants are less
likely to select for resistance.
(The photosynthesis inhibitors
including the triazine herbi
cides also act at a single site.)
Secondly, the ALS herbi
cides are highly effective on a
number of weed species. This
means that few susceptible
weeds escape treatment, so the
selection pressure for resistant
types is much greater. Weed
a*
©(DIEM Ml MSWO
PENNSYLVANIA MASTER CORN GROWERS ASSOC., INC
species are not all equally sus
ceptible to selection for resis
tance. In the Northeast, pig
weed and cocklebur are very
sensitive to several ALS herbi
cides and could become a prob
lem more quickly (pigweed and
lambsquarters were highly sen
sitive to the triazines). Third,
some of the ALS inhibitor her
bicides provide season-long
control and in fact require
plant-back restrictions because
of their residual properties.
Longer residual herbicides
will continue to control highly
susceptible weeds late in the
growing season and potentially
select for the resistant types.
Finally, the effectiveness and
sheer number of ALS inhibitor
herbicides in the marketplace
make it easy for producers to
use one in virtually every crop,
regardless of rotation.
In addition, within the next
five years, more than a half
dozen new ALS inhibitor herbi
cides may be introduced in the
com, soybean, and small grain
markets. On the positive side,
resistance to one ALS inhibitor
herbicide does not automatical
ly guarantee cross resistance to
(Turn to Pago 11)