February, 1999
Diphenylether Herbicide Crop Response Study
by Donald E. Null, Regional Agronomy Specialist
University Outreach and Extension, NW Region
Situation: Three diphenylether herbicides are
commonly used in Missouri herbicidal weed control programs in
soybeans. The compounds are aciflurofen (Blazer/Status), lactofen
(Cobra) and fomesafen (Flexstar/Reflex).
The diphenylether herbicides are extremely efficacious on the
pigweed species. Since common waterhemp has become one of the most
troublesome weeds in north Missouri, the diphenylether herbicides
have been widely used to control it. Producers have shown
considerable concern over their use, however. The diphenylether
herbicides are contact-type, cell membrane disrupting compounds.
Soybean response to the diphenylethers can best be characterized as
"plant tissue burn". That is, soybean leaves respond to
diphenylether applications by exhibiting mild to severe necrotic
symptoms, depending upon the diphenylether herbicide used,
environmental conditions, rate applied and tank mix partner.
Producers commonly refer to "fried beans" as a response to
diphenylether herbicide applications.
Many soybean producers are fearful that soybean yields are reduced
due to the necrosis that accompanies the use of this class of
chemistry. Research, under weed free conditions, needed to be
conducted to determine the effect of diphenylether herbicides on
soybean yield.
Objective: The objective of this study was to
determine if soybean yields were adversely affected by applications
of diphenylether herbicides. We hoped to be able to answer the
questions relative to the differences in soybean yield response to
two commonly used diphenylether herbicides; differences in soybean
yield relative to diphenylether rate used; differences that
application timing might have on soybean yields; differences that
planting date might have upon soybean yield response to
diphenylether herbicides; and differences that row width might have
upon soybean yield response to the diphenylether herbicides.
Methods: A Split Block study, with six replications,
was planned and planted during the springs of 1997 and 1998. Plot
size was 6' wide by 30' long. A total of 36 treatments were used
in the study. They included combinations of the following:
1. Lactofen or fomesafen at one-half and full labeled rates
with additives (additives were Sunnit II, 1% v/v and UAN,
1% v/v).
2. Two herbicide application times. One early and one late
(planned for V3 and R1 soybean growth stages).
3. One early and one late planting date (May 20 and June 28).
4. Two row widths (10" drilled and 36" wide rows).
The study was designed to be conducted without weed competition
to the soybeans. Asgrow AG3701RR was selected as the variety of
choice for the experiment. It was planted at the rate of 228,000
seeds per acre in 10" rows and at the rate of 196,000 seeds per acre
in 36" wide rows. Soybeans were planted on May 20 to one-half of
the plot area. The same variety, rate and row width were used to
plant the other one-half of the plot area on June 28. The late
plantings were no-tilled into a stale seedbed. A 6' wide, Tye
Pasture Pleaser, no-till drill was used to plant the 10" wide rows
and a John Deere 7300 vacuum planter was used to plant the 36" wide
rows. Roundup at 1 quart per acre was broadcast applied with a field
sprayer over the plot area on July 1 and July 31, 1998, to provide
complete weed control.
The diphenylether herbicides were applied with a CO2 backpack
sprayer delivering 20 gallons of spray solution per acre.
Even though the study was designed as a Split Block, treatments were
controlled rather than completely randomized. In actuality, the
study could best be described as four individual Randomized Complete
Block studies. This allowed for RCB statistical analysis of the
treatments. Plot layout was as follows:
Block 1 Early planted soybeans Narrow rows 8 DPE treatments and 1
untreated
Block 2 Early planted soybeans Wide rows 8 DPE treatments and 1
untreated
Block 3 Late planted soybeans Narrow rows 8 DPE treatments and 1
untreated
Block 4 Late planted soybeans Wide rows 8 DPE treatments and 1
untreated
Study Results: The data for all 36 treatments were
collected and analyzed to provide a complete split plot factorial
AOV table with an LSD significance level of 5%. There was a 9.9
bu/acre difference between the early and the late planting date.
This was highly significant (LSD=4.1 bu/acre). For a second year,
the value of narrow rows to soybean yield was higher with early
planting than with late planting. This is contrary to what would
have been expected. Figure 12 at right illustrates the crop yield
response to row width and planting date. Crop yield for these "no
DPE treatments" is expressed in percent of maximum untreated yield.
Figure 12
The maximum yield was 58.9 bu/acre for the early planted, drilled
soybeans. The LSD at the 5% probability level was 6.1%.
The yields of late planted soybeans were more adversely affected by
the lactofen treatments than by the fomesafen treatments. Figure 13
shows that, compared to the untreated check, both the one-half and
full rates of lactofen adversely impacted soybean yields when it was
applied to soybeans at either V3 or R1 growth stage in wide rowed
soybeans. Only the R1 applied full rate of lactofen exhibited a
significant yield decline in the late planted, drilled soybeans.
Figure 13 Effect of lactofen and fomesafin on late planted,
wide-rowed soybeans
Conclusion: We hoped to answer five questions with
this research. The following is an attempt to do that based on
the data produced from the two-year study.
Was there a difference in soybean yield response to the two
commonly used diphenylether herbicides used in the study? The
1997 & 1998 experimental data each suggest that lactofen treatments
had more of an adverse effect upon late planted soybean yields
than fomesafen. There was no adverse crop response to any of the
fomesafen applications. There was no significant adverse crop
response to the lactofen or fomesafen applications made to early
planted soybeans.
Was there a difference in soybean yield response to either
one-half or full labeled rates of each of the two commonly used
diphenylether herbicides? There was no significant difference
in soybean yield between application rates for either of the
diphenylether herbicides.
Did application timing have an effect upon soybean yield response
to the diphenylether herbicides? We had anticipated that any
adverse crop yield response would occur on the latest planted beans
that were treated with a diphenylether at the R1 growth stage. This
hypothesis was supported by the soybean yield response to the lactofen
treatments in both 1997 & 1998.
Did planting date have an effect upon soybean yield response to
the diphenylether herbicides? Both the 1997 and 1998 data
suggest that there was no early planted crop yield response to the
diphenylether herbicide treatments in the experiment. However,
lactofen applications did provide an adverse soybean yield response
to the late planted soybeans. In 1998, applications of the full
labeled rate of lactofen to either the V3 or R1 stage soybeans
adversely affected late planted soybean yields.
Did planting row width have an effect upon soybean yield response
to the diphenylether herbicides? An analysis of both the 1997 &
1998 data suggests that late planted, wide rowed soybeans were more
likely to be damaged by DPE applications than when the same
treatments were applied to late planted, drilled soybeans.
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