ALS-Resistant Pigweed
Bill Curran
Penn State
Weed Specialist
Lamar Witmar and Wes
Martin of Brubaker Agro
nomic Consulting spoke with
us about a pigweed control
failure in soybean that one of
their clients had experienced.
Although the specifics are
still a bit vague, the grower
had applied Pursuit herbicide
POST in soybean in 1998,
which failed to control the
pigweed. The grower then
followed this up with Classic
and perhaps some Pinnacle
that same season which still
did not kill the weeds.
We heard about the failure
that fall and were able to
obtain some seeds from the
uncontrolled pigweed. We
tested the seed in greenhouse
experiments and confirmed
that it was redroot pigweed
and resistant to a number of
ALS-type herbicides. In fact,
the pigweed was resistant to
the six ALS herbicides tested
belonging to the imidazoli
none, sulfonylurea, and sul
fonamide herbicide families
(Table 1).
The treated pigweed
varied in response to ALS
herbicides. The triazine her
bicide atrazine was the only
herbicide to kill the pigweed
in the greenhouse test, while
Confirmed In Pa.
Raptor and Pursuit slowed it
down (50 to 69 percent con
trol) and Beacon showed
almost no activity (Table 1).
We have obtained additional
pigweed seed from these sites
to do further greenhouse
studies and will continue to
monitor the situation in the
future.
The farm in question is in
Lancaster County and has
raised corn, soybean, wheat,
and forages and several dif
ferent types of animals, in
cluding chickens. The
pigweed problem was rela
tively new on the farm and is
a particular problem in a
field directly behind the
animal containment build
ings.
Although we still haven’t
determined the cause, the
grower did explain that he
had used Pursuit before in
soybean and had used other
ALS-type herbicides in corn
such as Exceed. It appears
that this grower may have
used ALS-type herbicides for
about five years in a row in
this field. In addition, while
visiting the ALS-resistant
pigweed farm, I also in
spected a field that was eight
to 10 miles away on a differ
ent farm that had a redroot
pigweed control failure this
year (2000).
It was a soybean field that
had received an ALS herbi
cide program for STS soy
bean that hadn’t worked. The
grower apparently resprayed
it with another ALS herbicide
program that still didn’t kill
the pigweed. Could this be
number two?
The real question, “did
four to five years of back-to
back ALS herbicide use select
for resistant weed popula
tions in a corn-soybean-small
grain rotation? Or did the
grower bring the resistant
pigweed onto the farm in
purchased feed, forage, or
seed? The grower did admit
to frequently spreading live
stock manure in the problem
field, which was adjacent to
the animal confinement area.
Could the ALS-resistant
pigweed be related to feed
and manure?
As a side note, I purchased
some pigweed seed from a
Mississippi (MS) supplier last
spring for a research experi
ment. We were using the seed
in a small weed control ex
periment at Rocksprings. We
happened to have an ALS
herbicide treatment in this
small experiment, which
failed to control the MS
pigweed. I observed both
dead and alive pigweed in the
experiment. The healthy
Corn Talk, Lancaster Farming, Saturday, October 14, 2000—Page
(OOIM Ml 11WS
PENNSYLVANIA MASTER CORN GROWERS ASSOC., INC.
pigweed were almost defi
nitely ALS-resistant. This
certainly suggests that ALS
resistant weeds are common
enough in some areas of the
country that we easily could
import them into our area.
Although we still don't
know exactly what happened
at the Lancaster County
farm, the take-home mes
sage: ALS-resistant pigweed
(and maybe other weeds) will
likely become more common
place in the future in Penn
sylvania, especially in corn. If
we were not using ALS herbi-
I .iblc 1 Effect of several herbicides on rcdrool pigweed control and dry weight reduction
in the greenhouse Means aic the average of two experimental urns
Treated
Untreated check
Beacon 73DF
Firstßatc 84DF 0 3 oz
Classic 23DF
Classic 25DF
Pinnacle 23 DF
Pm suit 2AS
Puismt 2AS
Raptoi 2AS
Alia/mc 90DF
"Percent contiol and dry wt reduction values within a column followed by the
aic not significantly different from one another according to Fishers Pi elected
3% level
cides, they would not be po
tential problems.
With Roundup Ready soy
bean, the importance of ALS
resistance is probably less im
portant than it was five years
ago. However, it is important
to remember that no herbi
cide is completely resistant to
failure from resistance, toler
ance, and weed shifts.
So, the bottom line
rotate potential problem her
bicide modes of action annu
ally and pay attention to
where you purchase forages
and feed to insure that resis
tant weeds are not the norm.
% Control- 1
Rate
0 73 o/
0 3 oz
1 1)0/
0 23 oz
4 fl oz
8 fl 07
4 II oz
2 lb active
% Di y wt
reduction 1
Oa
0 a
0 a
9 ab
Oa
2 ah
46 he
67 cd
61 cd
100 d
Oa
12 b
13 be
22 c
18 be
30 d
63 c
69 c
100 f
same letter
LSD at the
199