Front Cover
 Table of Contents
 Experimental procedure
 Results and discussion
 Summary and conclusions
 Literature cited

Group Title: Bulletin University of Florida. Agricultural Experiment Station
Title: Evaluation of herbicides for soybeans on central Florida organic soils
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00026869/00001
 Material Information
Title: Evaluation of herbicides for soybeans on central Florida organic soils
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 36 p. : ill. ; 23 cm.
Language: English
Creator: Scudder, W. T ( Walter T )
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1963
Copyright Date: 1963
Subject: Soybean -- Weed control -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (p. 24).
Statement of Responsibility: W.T. Scudder.
General Note: Cover title.
 Record Information
Bibliographic ID: UF00026869
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltuf - AEN9796
oclc - 18352845
alephbibnum - 000929032

Table of Contents
    Front Cover
        Page 1
    Table of Contents
        Page 2
        Page 3
        Page 4
        Page 5
    Experimental procedure
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
    Results and discussion
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
    Summary and conclusions
        Page 22
        Page 23
    Literature cited
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
Full Text


Technical Bulletin 650 February 196


INTRODUCTION .......... -..-.. ................................ 3

EXPERIMENTAL PROCEDURE .-.....-.. ---.......------------.................-........... 6

Primary Evaluation Trials -........ -------... ....--..-----..--...... 6

Secondary Evaluation Trials ........ ---...........-------- ........--......... 9

RESULTS AND DISCUSSION .............----.....-- ..-...- ..- ..-.......... ..... 11

Primary Evaluation Trials ...-........---- -. ....------...... -....-..-..- .... -- 11

Secondary Evaluation Trials ..................... ..-----....--..... -13

SUMMARY AND CONCLUSIONS ...-...-..-- ......-....---------------------............... 22

LITERATURE CITED ......-- ---------..... ........-----.....- ...--- ................. 24

APPENDIX ..........-.....--------......---------------... ..... -.... --.. ................. 25

Fig. 1. (cover photo).-Soybeans on left were chemically treated with
a pre-emergence CDEC spray; those on right received no treatment.



The problem of controlling weeds has been a major factor
prohibiting large-scale production of soybeans on the sawgrass
peat soils of Central Florida. With adequate weed control, good
yields can be produced on these soils when planted after early
sweet corn or other spring vegetable crops. Soybeans are grown
during the summer, at a time when the land is normally fallowed
or left to weeds. They show promise of becoming an important
self-paying summer cover crop when used as part of the veg-
etable rotation.
A weed-free crop of soybeans offers many advantages as a
summer cover. Fallowing, by plowing and harrowing repeatedly
throughout this off-season, controls the weeds, but is expensive
and tends to increase subsidence-a serious problem on peat soils.
Although a cover crop of weeds lessens subsidence and conserves
soil nutrients remaining from the fertilized truck crops, millions
of weed seed are borne to hamper production for years to come.
A well-cared-for crop of soybeans, on the other hand, not only
conserves residual fertility but also adds nitrogen to the soil.
In addition, a good bean yield will give a cash return much above
the cost of production.
Production of soybeans in Florida has increased considerably
during recent years. In 1958, 46,000 acres were harvested, pro-
ducing 1,150,000 bushels of beans at an average yield of 25
bushels per acre (8).2 Using the average government support
price of $2 per bushel, this crop was valued at approximately
$2,300,000. In spite of their higher productivity, Florida's or-
ganic soils were responsible for only a small percentage of this
total soybean production. Potential production on these soils
is considerable. Over 100,000 acres of "muckland" are devoted
to vegetables in Florida, much of which remains idle from June
until October.

1 Associate Horticulturist, Central Florida Experiment Station, Sanford,
"3 Numbers in parentheses refer to Literature Cited.

4 Florida Agricultural Experiment Stations

Staniforth and Weber (10) evaluated several herbicides used
as pre-emergence treatments in row-planted soybeans over a
seven-year period in Iowa. Although they obtained good re-
sponses with their treatments, yield comparisons from the ma-
jority of their tests showed that the herbicides did not greatly
enhance the effectiveness of cultivation, particularly when early
cultivations were done at shallow depths. They point out that
under different weather conditions, with more serious weed in-
festations, or with poorer cultivation procedures, benefits from
the use of herbicides are greater.
In Florida, during July and early August, while the soybean
plants are small, conditions are favorable for extremely rapid
weed growth. Many of the common weed species on these or-
ganic soils respond to high temperatures and rainfall with growth
in height in excess of 1 inch per day. Under these conditions,
row planting followed by the best mechanical means of cultiva-
tion available has failed to give practical weed control. For
example, a planting of nearly 1,100 acres of soybeans in the Zell-
wood area in 1955 was weeded repeatedly, using modern row-
crop cultivators. A good yield of beans was produced in the field,
but due to the high weed population in the rows, it was impos-
sible to recover more than a small proportion of the beans at
harvest time.
A review of the available literature has failed to reveal any
information on the control of weeds in soybean specifically in re-
lation to organic soils. Work has been done, however, with soy-
beans grown on several mineral soil types, using nearly all the
presently available chemical herbicides. Working on light sandy
soils in Virginia during 1955, Chappell (5) screened 17 different
chemicals for their herbicidal efficiency and effects on soybeans.
In these preliminary tests several treatments showed promise,
including the following: tris- (2,4-dichlorophenoxyethyl) phos-
phite (2,4-DEP) at 2 pounds per acre; 2-chloro-N,N-diallylaceta-
mide (CDAA) at 5 pounds; 2-chloroallyl diethyldithiocarbamate
(CDEC) at 5 pounds; sodium 2,4-dichlorophenoxyethyl sulfate
(sesone) at 3 pounds; sodium salt of N-1-naphthylphthalamic acid
(NPA, sodium salt) at 4 to 6 pounds; 2-methyl-4-chlorophenoxy-
propionic acid (MCPP) at 11/2 to 2 pounds; alkanolamine salts
of 4,6-dinitro-o-sec-butylphenol (DNBP, alkanolamine salts) at
6 pounds; sodium pentachlorophenate (PCP, sodium salt) at 16
pounds; and 2-chloro-4,6-bis (diethylamino) -s-triazine (chlora-
zine) at 6 pounds.

Evaluation of Herbicides for Soybeans 5

Burt (4) found that of over 20 chemical treatments applied
pre-emergence to soybeans on sandy soils at Gainesville, Florida,
8 pounds per acre of the sodium salt of PCP gave the most satis-
factory weed control without crop injury. Isopropyl N-(3-
chlorophenyl) carbamate (CIPC) also gave excellent control of
annual weeds at 9 pounds per acre, but presented the hazard of
occasional crop injury. Ethyl N,N-di-n-propylthiolcarbamate
(EPTC) at the 5 and 10 pound rates responded similarly. Pre-
viously, Burt (2) had tested both CDAA and CDEC. Although
he found they were well tolerated by soybean plants, they gave
an erratic weed control response on his light soils.
Peters et. al. (7) also obtained outstanding results with the
sodium salt of PCP on their silt loam soils in Missouri. In
their trials, CIPC and DNBP produced severe injury, while
CDAA failed to give satisfactory weed control.
On loam and loamy sand soils in Delaware, Indyk (6) ob-
tained good responses with the sodium salt of PCP, CDAA, and
CDEC, and with mixtures of PCP with CDEC and with the iso-
propyl ester of 2,4-dichlorophenoxyacetic acid (2,4-D). He
agreed with Burt that PCP should be recommended for commer-
cial use, pending the establishment of residue tolerances and
A research program to develop means of practical weed con-
trol in soybeans on Florida's organic soils was initiated at the
Zellwood Farm of the Central Florida Experiment Station during
1956. Use of herbicides, alone or coupled with mechanical cul-
tivation, was studied in all experiments. First stage tests con-
sisted of observational trials wherein several selective herbi-
cides, including the newer chemicals as they appeared, were
screened to detect their activity on the weeds and soybeans.
In the second stage trials, herbicides appearing superior in the
screening trials were used in replicated plots to study their in-
fluence on crop yields. The third stage trials involved large scale
testing in commercial fields. The results and conclusions from
these experiments are presented in this bulletin.

6 Florida Agricultural Experiment Stations


Primary Evaluation Trials

During the summers of 1956, 1957, and 1958, soybeans were
grown in five primary evaluation or screening trials for the pur-
pose of selecting the more promising herbicides among the sev-
eral being offered by the chemical industry. These experiments
are listed in Table 1 along with planting and treatment dates
and the total numbers of chemicals and individual treatments
included in each test.


Experiment Date Date No. of No. of
No. Planted Treated Chemicals Treatments*

56-1 June 19, 1956 June 22 15 30
56-2 July 13, 1956 July 13 15 41
56-3 Sept. 26, 1956 Sept. 27-28 17 34
57-1 May 7, 1957 May 8-9 15 34
58-1 May 22, 1958 May 22-23 23 50

Number of treatments excludes the untreated check plots which were interspersed re-
peatedly among the treatment plots of these trials.

For each of these trials, the beans were planted in single un-
replicated rows, usually adjacent to other agronomic or vege-
table crops. The plots were defined by crossing the crop rows
at right angles with the herbicide strips. In all cases, the
chemicals were dissolved, emulsified, or suspended in water
and sprayed over the soil surface after planting and before
emergence of both the soybeans and the weeds. During 1956,
the 5-foot wide plots were treated using a three-nozzle hand
sprayer. A tractor-mounted herbicide sprayer was used subse-
quently. The tractor wheels, 6 feet apart, marked the plot
boundaries. In all cases, the sprays were applied broadcast,
using flat-fan weed control nozzles spaced 10 inches apart. Gen-
erally, untreated check plots were located after every second
chemical plot. Since this placed each treatment either adjacent
to or near a weedy check, accurate comparisons were feasible.
This also made it possible to readily ascertain variations in the
natural weed populations of the experimental fields.
Eleven different varieties were planted and treated in the
first two tests in 1956, but due to poor seed quality, data were

Evaluation of Herbicides for Soybeans 7

recorded only for the variety CNS-24. This same Clemson line
was used for Experiments 56-3 and 57-1. Of the two varieties
planted in Experiment 58-1, data are reported only for Lee, a
grain type comparable to CNS-24. The other variety, Otootan,
was a forage type, but responded similar to Lee.
A total of 25 different chemicals were screened to evaluate
weed killing efficiency and to determine which might be sprayed
safely over the soil surface pre-emergence to soybeans. Herbi-
cides studied are listed in Table 2 along with information on the
formulation of each chemical tested. For convenience in the
discussions and tables, common names or experimental designa-
tions which are recognized and accepted by the Terminology
Committee of the Weed Society of America are used for chemi-
cals throughout this publication (9). There are, however, no
accepted common names for two of the herbicides included in
these tests. These are 2,5-dichloro-3-nitrobenzoic acid, referred
to by the company designation "Dinoben", and the mixture of
2-chloro-N,N-diallylacetamide and trichlorobenzyl chloride, here
designated as "CDAA-TCBC". In several cases, the designation
refers to the active agent although salts of these chemicals
were actually used. These include the sodium salts of NPA,
PCP, TCA, and 2,3,6-TBA. The 2,4-D formulation contained the
amine salt, and the DNBP consisted of a mixture of alkanol-
amine salts.
Herbicide treatment rates are specified in terms of pounds
per acre of the active chemical ingredients. In most cases, this
is the content of technical chemical, but acid or phenol equiva-
lents are used where more appropriate, as in the cases of 2,4-D
and DNBP, respectively.
The initial selection of herbicides and rates used in these
trials was based on suggestions supplied by commercial sources
of the chemicals and on information reported by other research
workers. Sometimes it was necessary to increase the chemical
rates used on this organic soil to two or more times those com-
monly suggested for weed control on light mineral soils.
Data from these primary evaluation trials were recorded in
the form of visual ratings. Since the tests were observational
and unreplicated, quantitative measurements were not made.
Ratings were determined by direct comparisons between treated
plots and untreated check areas. A 10 increment scale, ranging
from 0 to 10, was utilized for both observed factors, as follows:

8 Florida Agricultural Experiment Stations


Common Name
No. or Chemical Name Formulation*

1 amitrole 3-amino-1,2,4-triazole 50% S.P.
2 atrazine 2-chloro-4-ethylamino,6-
isopropylamino-s-triazine 50% W.P.
3 CDAA 2-chloro-N,N-diallylacetamide 4 lb/gal E.C.
4 CDAA-TCBC** 2-chloro-N,N-diallylacetamide 3 lb CDAA + 5 lb
+ trichlorobenzyl chloride TCBC/gal E.C.
5 CDEC 2-chloroallyl diethyldithio- 4 lb/gal E.C.
6 chlorazine 2-chloro-4,6-bis(diethyl- 4 lb/gal E.C.
amino) -s-triazine
7 CIPC isopropyl-N-(3-chlorophenyl)- 4 lb/gal E.C.
8 Dinoben** 2,5-dichloro-3-nitrobenzoic acid 80% S.P.
9 diuron 3-(3,4-dichlorophenyl)-1-1- 80% W.P.
dimethyl urea
10 DNBP 4,6-dinitro-o-sec-butylphenol, 3 lb/gal S.C.
alkanolamine salts
11 EPTC ethyl-N,N-di-n-propylthiol- 6 lb/gal E.C.
12 EXD ethyl xanthogen disulfide 5 lb/gal E. C.
13 fenuron TCA 3-phenyl-l,l-dimethylurea 3 lb/gal E.C.
14 ipazine 2-chloro-4-diethylamino-6- 2 lb/gal E.C.
15 monuron 3- (p-chlorophenyl) -1,1- 80% W.P.
16 NPA N-1-naphthylphthalamic 2 lb/gal S.C.
acid, sodium salt
17 PCP pentachlorophenol, sodium salt 75% S.P.
18 propazine 2-chloro-4,6-bis (isopropyl- 50% W.P.
amino) -s-triazine
19 sesone sodium 2,4-dichlorophenoxy- 90% S.P.
ethyl sulfate
20 simazine 2-chloro-4-6-bis(ethylamino)- 50% W.P.
21 TCA trichloroacetic acid, sodium salt 90% S.P.
22 trietazine 2-chloro-4-ethylamino-6- 50% W.P.
23 2,3,6-TBA 2,3,6-trichlorobenzoic acid, 1.5 lb/gal S.C.
sodium salt
24 2,4-D 2,4-dichlorophenoxyacetic 4 lb/gal S. C.
acid, amine salt
25 2,4-DEP tris(2,4-dichlorophenoxyethyl)- 2 lb/gal E.C.

*Formulations are given as follows: E.C.-emulsifiable concentrate; S.C.-soluble con-
centrate; S.P.-soluble powder; W.P.-wettable powder.
** Designations not accepted by the Terminology Committee of the Weed Society of Amer-
ica and the American Standards Association.

Evaluation of Herbicides for Soybeans 9

Weed control: 0 indicates no effect on weeds, no control;
8 indicates commercially acceptable con-
10 indicates complete control or weed kill.
Crop tolerance: 0 indicates no crop tolerance, complete
elimination of the crop;
8 indicates good tolerance with slight, usu-
ally temporary, crop injury;
10 indicates full tolerance of the chemical
with no harmful effect on the crop.

The weeds were classed into two groups, broadleaf weeds
and annual grasses. Separate control ratings were recorded for
each. The use of this scale for crop tolerance, the reverse of
crop injury, was selected to keep both scales positive in relation
to desirable characteristics. Thus, large numerical values rep-
resent favorable results on both scales. Conversely, low ratings
reflect poor weed control and serious injury to the soybean plants.
Intermediate levels describe partial control or crop tolerance
and may be interpreted as though each increment were roughly
equivalent to 10 percent. Thus each rating, multiplied by 10,
gives the approximate percentage weed control or crop toler-
ance, estimated in terms of both stand and growth of the plants.

Secondary Evaluation Trials
Secondary or advanced herbicide evaluation was accomplished
by replicated yield trials conducted during the 1957 and 1958
crop seasons.
In Experiment 57-2, the varieties Lee, Jackson, and CNS-4
were planted on July 2, 1957, using 1 bushel of seed per acre in
rows spaced 36 inches apart. The plots were arranged in a split-
plot, randomized-block design with four replications. Each of
the three variety plots within each block was subdivided into
10 treatment plots involving three rates of each of three herbi-
cides and a cultivated check. The individual treatment sub-plots
were four rows or 12 feet wide and 22 feet long. Chemical treat-
ments were as follows: CDAA at 6, 9, and 12 pounds per acre;
EPTC at 5, 10, and 15 pounds per acre; and PCP at 10, 15, and
20 pounds per acre. These were all applied broadcast as surface
sprays pre-emergent to the soybeans and weeds on July 3 and 4,
1957, using a tractor-mounted weed-control sprayer designed for

10 Florida Agricultural Experiment Stations

small plot work. The delivery rate, using Teejet number 8001
nozzle tips, 25 pounds per square inch pressure at the boom,
and 2.7 miles per hour tractor speed, was approximately 24 gal-
lons per acre.
During the first six weeks, until lay-by time, the check plots
were cultivated and hand hoed as needed to keep them free of
weeds. Treated plots were left untouched until that time, then
the whole experimental area was given a single cultivation. Soil
moisture during this period was moderately high. Rainfall,
shown in Table 4, averaged 1.46 inches per week for this six-
week period. Experiment 58-2 was designed similarly, using
only two varieties, Lee and CNS-4. The Jackson variety was
omitted, since it appeared to be poorly adapted to these organic
soils. The beans were seeded in rows 36 inches apart at the rate
of 1 bushel per acre on July 17, 1958. Each of the three repli-
cations were split into two variety strips. These in turn were
composed of nine randomized treatment sub-plots four rows (12
feet) wide by 21 feet long. Because of its favorable action in
the screening trials, CDEC was tested in addition to the three
herbicides used during 1957. Making up the nine treatments
were two rates of each of the four chemicals plus the check,
which again was hoed and cultivated. In this 1958 experiment,
CDAA was used at 4 and 6 pounds per acre, CDEC at 4 and 6
pounds per acre, EPTC at 8 and 12 pounds per acre, and PCP
at 8 and 12 pounds per acre. The treatments were applied one
day after planting pre-emergent to both crop and weeds. The
same equipment and procedures were used as in 1957 except that
the nozzles were assembled with number 8002 tips delivering
35 gallons of spray per acre.
Weed control and crop tolerance responses to the treatments
in these secondary tests were observed and recorded using the
same rating scales described for the primary evaluation trials.
In addition, single 16-foot plot rows were harvested and threshed
during late November of each year. The beans were then taken
to Gainesville, where they were cleaned, brought to a uniform
moisture content, and weighed to the nearest gram. The yields
were calculated in bushels per acre.

Evaluation of Herbicides for Soybeans 11


Primary Evaluation Trials
Each of the five primary evaluation trials was observed sev-
eral times during the few weeks following application of the
pre-emergence chemical treatments. The number of days from
treatment to observation is given in Table 3, along with rain-
fall and temperature data recorded during each experiment.
This information aids considerably in explaining the observed
responses to the chemical treatments. Visual ratings from all
experiments involving each of the 25 herbicides have been sum-
marized and are given in the Appendix. Data were recorded
twice from Experiments 56-1 and 56-3. Since the weed control
and soybean tolerance ratings for both dates corresponded
closely, only the means are given in the result tables.

1956 TO 1958.

Rainfall After Mean from
Experiment Soil Moisture Treatment Ter- Treat-
No. on Treatment -pera- ment to
Date 1st 1st ture* Obser-
week 3 weeks vation

56-1 dry 1.70 3.33 81.0 23**
56-2 moderately dry 0.20 0.65 82.5 39
56-3 moderate 0.16 8.98 76.4 20**
57-1 moderate 1.84 4.29 78.2 19
58-1 moderate 1.59 1.64 78.9 28

Mean temperatures are averages of mean daily minimums and mean daily maximums
for first three weeks after treatments were applied.
"** Days from treatment to observation given for Experiments 56-1 and 56-3 are averages
for two observation dates, in each case approximately two and four weeks after treatment

Five different annual weeds were common to all of these
screening trials, since all were conducted during the warm sea-
son on the same organic soil farm of the Central Florida Station.
The weed species are detailed later along with the discussion of
results of the secondary evaluation trials. For these primary
trials, the weeds were classed in two general categories, broad-
leaf weeds and grasses. Though sedges were present in some of

12 Florida Agricultural Experiment Stations

the plots, the populations were too small and variable to per-
mit rating.
Chemical rates were varied from season to season in an effort
to learn which ones were most desirable from the standpoint of
both weed control and crop tolerance. Since other crops in ad-
dition to soybeans were planted in some of these trials, these
rates were not always adjusted in favor of this one crop. Some
chemical treatments were judged to be unsatisfactory early and
were excluded from the later tests. Treatment eliminations
were based on the following factors:
1. Inadequate weed control;
2. Insufficient tolerance of crop to chemical;
3. Too little latitude between the threshold chemical level of
good weed control and the highest rate tolerated by the
crop without injury;
4. Wide variability in performance in different tests, reveal-
ing extreme sensitivity to environmental factors;
5. Poor chemical formulation, mechanically unsuited to com-
mercial use;
6. Required herbicide rates too high for convenient or eco-
nomical application;
7. Danger to succeeding crops from persistent residues; and
8. Lack of interest by herbicide manufacturers in product
development and government registration. In several
cases, satisfactory herbicide treatments have been found
for certain crops, but where there are not adequate pat-
ent controls or where the potential sales volume for the
herbicide is small, companies can not justify further de-
velopment cost.

A brief discussion of each of the herbicides included in these
screening trials, with an analysis of the responses observed, is
given in the Appendix with the data for each chemical. Com-
monly known characteristics of chemicals beyond those displayed
in these tests are occasionally given, since they are often im-
portant in determining the future disposition of a new weed
killer. The herbicides are treated using the same common names
or designations and alphabetical order as listed in Table 2.
Of the 25 chemicals screened in these trials, 21 were elim-
inated from further consideration, mainly because of poor weed
control or poor crop tolerance. In some cases, adequate weed

Evaluation of Herbicides for Soybeans 13

control without crop damage was possible at very high rates,
but such treatments were neither mechanically or economically
practical. Selected for further study were four chemicals. These
were CDAA, CDEC, EPTC, and PCP. CDEC was not included
in the first yield trial in 1957, because at that time its activity
did not appear noticeably different from that of CDAA, which
was already registered for use on seed crops of soybeans. It
was found desirable to include CDEC in 1958, however, because
of its slightly superior broadleaf weed control.

Secondary Evaluation Trials

Experiment 57-2.-Due in part to the rainfall and excellent
moisture conditions prevailing (Table 4), weed control was good
in all treatment plots in this experiment. In contrast, weeds in
the adjacent unsprayed and uncultivated border rows flourished.


Rainfall on Exp. 57-2 Rainfall on Exp. 58-2
After Daily records Daily records
Treatment Total -Total
Date Inches for week Date Inches for week

First Jul. 4 0.42 0.65 Jul. 18 0.03 0.50
8 .23 21 .23
22 .24
Second Jul. 16 1.25 1.25 Jul. 25 0.35 0.73
28 .38
Third Jul. 18 0.10 1.01 Aug. 4 0.27 1.34
19 .40 6 .58
22 .51 7 .49
Fourth Jul. 29 0.41 1.35 Aug. 11 0.79 0.79
30 .70
31 .24
Fifth Aug. 1 0.31 3.32 Aug. 18 0.56 1.13
5 2.04 19 .54
6 .03 20 .03
7 .94
Sixth Aug. 8 0.45 1.20 Aug. 25 0.79 0.79
12 .18
14 .57

Six Weeks' Total: 8.78 5.28

Weekly Average: 1.46 0.88

14 Florida Agricultural Experiment Stations

In these rows, they completely covered and were smothering the
soybeans by lay-by time. Check plots required frequent culti-
vations and hand hoeings to keep them clean. Weed species
populations, ascertained from several counts in the border areas,
were approximately as follows:

Careless weed (Acnida cuspidata) ..----.....-.--...- 7%
Goosegrass (Eleusine indica) -.......-....--....---- 35%
Ground cherry (Physalis angulata) ..-......--... 5%
Purslane (Portulaca oleracea) ........---......---..... 5%
Spiny amaranth (Amaranthus spinosus)..... 30%
Yellow sedge (Cyperus odoratus)...................... 18%

All of these, including the sedge, were growing as annuals from
Weed control and soybean crop tolerance ratings were re-
corded on July 22, approximately three weeks after planting.
In Table 5, all broadleaf weed species were considered as a group
and rated separately from the goosegrass. At this time, the
yellow sedge was not prominent. Eleven days later, on August
2, the plots were all rated again, with the five leading weed spe-


Treatment Weed Control Ratings* Crop
Chemical lb/A Broadleaf Grass Ratings*

CDAA 6 8.8 8.6 10.0
9 9.0 9.2 9.2
12 9.2 9.2 9.7
EPTC 5 7.9 8.6 10.0
10 8.5 9.5 10.0
15 8.9 9.6 10.0
PCP 10 8.9 7.2 10.0
15 9.5 7.8 9.8
20 9.7 8.2 10.0
Check cultivated 9.0 9.0 10.0

L.S.D. (5% level) 1.0 0.8 N.S.

Weed control and crop tolerance ratings were based on a 0 to 10 scale, with high
values indicating favorable results. Data are means of 12 observations taken from three
variety blocks and four replications.

Evaluation of Herbicides for Soybeans 15

cies being considered separately. Individual weed control ratings
were made for careless weed, goosegrass, purslane, spiny ama-
ranth, and yellow sedge. These data, along with the ratings for
crop tolerance to the nine herbicide treatments and the check,
are given in Table 6.
The data show that all chemical treatments resulted in ex-
cellent weed control, and few of the responses differed signifi-
cantly from others. The only treatment resulting in broadleaf
weed control inferior to others was EPTC at 5 pounds. This
was entirely due to the high tolerance of spiny amaranth to this
chemical (Fig. 2). At the first observation date, CDAA gave
nearly complete control of all weed species, missing only an oc-
casional broadleaf weed at the lower 6 pound rate (Fig. 3).
After 11 more days and 2.17 inches of additional rainfall, the
CDAA treatments had weakened slightly, permitting some de-
velopment of several species, including the yellow sedge. While
PCP persisted as a good broadleaf weed killer, it was weaker
than the other two chemicals on grass at both observation dates
(Fig. 4). In most of these PCP plots, surviving grass appeared
in streaks, indicating an uneven distribution of the chemical at
the time of application.


Weed Control Ratings*
Treatment Spiny Crop
Careless Goose- Purs- ama- Yellow tolerance
Chemical lb/A weed grass lane ranth sedge ratings*

CDAA 6 8.9 7.3 8.9 8.1 6.8 10.0
9 9.0 8.2 8.9 7.8 7.2 9.6
12 9.1 7.4 8.7 7.8 7.0 9.8
EPTC 5 9.3 7.8 8.8 7.5 8.2 10.0
10 9.4 9.0 9.2 8.3 9.4 10.0
15 9.4 9.2 8.6 8.6 9.6 10.0
PCP 10 8.6 6.9 8.7 9.1 8.1 10.0
15 9.8 7.3 9.2 9.3 8.2 10.0
20 9.8 7.7 9.2 9.4 8.8 10.0
Check cultivated 9.0 9.0 9.0 9.0 9.0 10.0

L.S.D. (5% level) 1.1 1.0 N.S. 0.8 1.2 N.S.

Weed control and crop tolerance ratings were based on a 0 to 10 scale, with high
values indicating favorable results. Data are means of 12 observations taken from three
variety blocks and four replications.

16 Florida Agricultural Experiment Stations

' ,/. ?, .a.. ..

Fig. 2.-Weed control in plot which received 10 pounds per acre of
EPTC three weeks after pre-emergence spraying. Note occasional resistant
spiny amaranth plants. Unsprayed check rows are at right.

Fig. 3.-Weed control in plot which received 6 pounds per acre of CDAA
three weeks after pre-emergence spraying. One purslane and two or three
spiny amaranth plants appear in the plot. Unsprayed check rows are at

Evaluation of Herbicides for Soybeans 17


Fig. 4.-Weed control in plot which received 15 pounds per acre of PCP
three weeks after pre-emergence spraying. Note scattered resistant goose-
grass plants. Unsprayed check rows are at right.

There was little noticeable variation in either size or vigor
of the soybean plants growing in any of the plots. Recorded
tolerance ratings revealed no significant differences at either
date. Due to a lack of consistency, the slight injury in certain
high rate CDAA plots was questionable. The weed pattern in-
dicated that this was probably due to uneven spray distribution
by partially clogged nozzles.
Mean yields of cleaned and dried soybeans from these plots
are summarized in Table 7. Variance analysis reveals that there
was no difference in the production of soybeans between any of
the treatments or between treatments and cultivated checks.
Average ratings from the July 22 observation date and yield
data are further consolidated in Table 8. None of the effects
appearing in this table show statistically significant differences,
except the yield response of the three varieties. Jackson yielded
considerably less than either Lee or CNS-4, a response not in-
fluenced by the herbicide treatments, since the treatment x
variety interaction was not significant. A comparison of the
mean of all check plots with the total of all chemical treatments
shows a yield difference of 2.4 bushels in favor of the check.
This trend is supported by mean yields for treatment rates.
Although approaching significance, these differences were in-

18 Florida Agricultural Experiment Stations


Treatment Bushels per Acre*
Chemical lb/A CNS-4 Jackson Lee Average

CDAA 6 32.7 24.1 34.4 30.4
9 28.1 21.6 37.8 29.2
12 34.0 23.8 30.0 29.3
EPTC 5 35.0 23.1 31.6 29.9
10 32.0 26.4 33.0 30.4
15 33.9 24.4 33.0 30.4
PCP 10 33.7 23.3 35.7 30.8
15 29.4 22.4 34.3 28.7
20 30.7 21.7 32.7 28.4
Check cultivated 34.6 23.6 38.1 32.1

Data are means from four replications. There were no significant differences, except
between varieties.


Weed Crop
Control Tolerance Yield
Factors Ratings* Ratings* (bushels/A)

Chemicals: CDAA 9.0 9.6 29.6
EPTC 8.8 10.0 30.2
PCP 8.5 10.0 29.3
L.S.D. (5% level) N.S.** N.S. N.S.

Treatment rates: None (Check) 9.0 10.0 32.1
Low 8.5 10.0 30.4
Medium 9.0 9.7 29.4
High 9.3 9.9 29.3
L.S.D. (5% level) N.S. N.S. N.S.

Check vs. chemicals: Check 9.0 10.0 32.1
All chemicals 8.9 9.9 29.7
L.S.D. (5% level) N.S. N.S. N.S.

Varieties: Lee 8.9 10.0 34.1
Jackson 8.7 9.9 23.4
CNS-4 8.8 9.7 32.4
L.S.D. (5% level) N.S. N.S. 2.7

Weed control and crop tolerance ratings were based on a 0 to 10 scale, with high
values indicating favorable results.
** Differences not significant.

Evaluation of Herbicides for Soybeans 19

sufficient to prove a reduction in yield as a result of the use of
Experiment 58-2.-Although weed control was slightly less
perfect in this 1958 trial, it was again commercially adequate
with all chemicals and rates tested. The natural weed popula-
tion, as counted in the check plots before cultivation, was limited
primarily to three annual species, as follows:

Goosegrass (Eleusine indica) .....-................. 80%
Purslane (Portulaca oleracea) .......---.......--.... 9%
Spiny amaranth (Amaranthus spinosus) ..-..-. 10%
Miscellaneous species ...---.....---.......----...--- 1%

A summary of the weed control and crop tolerance observa-
tions is given in Table 9. PCP was inferior to the other three
chemicals in this trial, especially with regard to control of goose-
grass. CDEC, not included in the 1957 experiment, performed
very well, giving slightly better broadleaf weed control than
CDAA (Fig. 5). Response to EPTC, notably erratic following
surface spray applications, was again very satisfactory in this
test. This was possibly due to the fact that the soil was fairly
dry at the time of application, permitting rapid absorption of
the EPTC by the soil particles. Subsequent rainfall during the
next few days assisted the downward chemical movement into


Treatment Weed Control Ratings* Crop
Chemical lb/A Broadleaf Grass Ratings*

CDAA 4 8.2 9.2 10.0
6 8.4 9.3 10.0
CDEC 4 9.5 9.0 10.0
6 9.8 9.1 10.0
EPTC 8 9.1 9.4 10.0
12 9.5 9.8 10.0
PCP 8 7.0 6.4 10.0
12 8.0 7.2 10.0
Check cultivated 8.5 8.5 10.0

L.S.D. (5% level) 1.1 0.9 N.S.

Weed control and crop tolerance ratings were based on a 0 to 10 scale, with high values
indicating favorable results. Data are means of 12 observations from two varieties, three
replications, and two record dates. August 4 and 12.

20 Florida Agricultural Experiment Stations

the surface soil, thus preventing its loss by vaporization. None
of the chemical treatments produced visible crop injury.

Fig. 5.-Weed control three weeks after pre-emergence spraying with
6 pounds per acre of CDEC. Upper: Broadcast-note complete absence of
weeds in contrast with check area at right. Lower: Banded-12 inch bands
were applied over two center rows. Adjacent rows were left untreated.

Yields of soybeans were taken as in 1957. Mean yield data
for all nine treatment plots and the two varieties are given in
Table 10. Again, there were no differences in yields between
any of the chemical treatments or between the treatments and
the checks, which were hoed and cultivated. To make compari-
sons easier, these data have been condensed to isolate individual
factors in Table 11. Here it may be seen that the weakness of

Evaluation of Herbicides for Soybeans 21


Treatment Bushels per Acre*
Chemical lb/A CNS-4 Lee Average

CDAA 4 33.8 36.2 35.0
6 30.6 40.5 35.6
CDEC 4 30.4 38.3 34.4
6 34.8 37.9 36.4
EPTC 8 30.9 38.7 34.8
12 34.4 37.8 36.1
PCP 8 32.1 32.8 32.5
12 35.4 31.1 33.2
Check cultivated 34.1 33.4 33.8

Data are means of three replications. There were no significant differences, except
between varieties.


Weed Crop
Control Tolerance Yield
Factors Ratings* Ratings* (bushels/A)

Chemicals: CDAA 8.8 10.0 35.3
CDEC 9.4 10.0 35.4
EPTC 9.5 10.0 35.4
PCP 7.2 10.0 32.8
L.S.D. (5% level) 1.1 N.S.** N.S.

Treatment rates: None (Check) 8.5 10.0 33.8
Low 8.5 10.0 34.2
High 8.9 10.0 35.3
L.S.D. (5% level) N.S. N.S. N.S.

Check vs. chemicals: Check 8.5 10.0 33.8
All chemicals 8.7 10.0 34.7
L.S.D. (5% level) N.S. N.S. N.S.

Varieties: CNS-4 8.6 10.0 32.9
Lee 8.1 10.0 36.3
L.S.D. (5% level) N.S. N.S. 3.0

Weed control and crop tolerance ratings were based on a 0 to 10 scale, with high
values indicating favorable results.
** Differences not significant.

22 Florida Agricultural Experiment Stations

PCP was the only significant effect revealed by the weed control
ratings. Mean crop tolerance ratings of 10.0 for all factors show
the lack of any visible crop toxicity. In the case of yields, the
only measurable difference was between varieties. In contrast
to the 1957 results, in this experiment Lee slightly outyielded
CNS-4. In line with the poorer weed control observed early
during the growing season, the PCP treated plots yielded slightly
fewer soybeans than did either the check plots or those treated
by any of the other chemicals. These differences, however, were
not statistically significant.

During the years 1956, 1957, and 1958, five screening trials
were conducted on the organic soil at Zellwood to evaluate 25
different selective herbicides, applied pre-emergence for weed
control in soybeans. Out of these trials, four chemicals were
selected for more detailed studies in advanced replicated trials.
These were CDAA, CDEC, EPTC, and PCP. All of these gave
excellent weed control without crop injury at rates considered
satisfactory for the development of a practical treatment.
The discarded chemicals were eliminated for reasons such
as poor weed control, crop injury, too great variability in ac-
tivity, or combinations of these factors. The chemical rates re-
quired to give adequate weed control on this organic soil often
were found to be higher than would be economically convenient
to use or practical. In others, the useful zone between the thresh-
old chemical level just giving satisfactory weed control and the
highest rate tolerated by the crop was too narrow to give any
factor of safety to the grower.
The advanced herbicide evaluation program included two
replicated yield trials. The first was conducted during 1957
using three rates of each of three herbicides and three varieties.
CDAA was applied at 6, 9, and 12 pounds per acre; EPTC at 5,
10, and 15 pounds per acre; and PCP at 10, 15, and 20 pounds
per acre. All treatments performed very well, giving excellent
weed control without producing any significant injury to the
crop. This was further corroborated by the yield data, which
proved that none of the treatments injured the soybeans suffi-
ciently to cause a reduction in the bushels of beans produced.
A similar trial in the summer of 1958 tested CDEC in addi-
tion to the chemicals used during 1957. Since the high rates

Evaluation of Herbicides for Soybeans 23

used in 1957 proved to be unnecessary, two low rates of each
chemical were selected for this second trial. CDAA and CDEC
at 4 and 6 pounds per acre and EPTC at 8 and 12 pounds all gave
excellent weed control, in some cases surpassing that secured by
hoeing and cultivating. With both 8 and 12 pounds of PCP,
however, observed weed control, especially of grasses, was poorer
than that obtained in the other treatment plots or the cultivated
checks. The soybean plants tolerated all herbicide treatments
used in this experiment without showing visible symptoms of
chemical injury. Again, as in the first trial, yields in 1958 did
not reflect any effects attributable to herbicide treatments.
Further studies with PCP are not warranted at the present
time, although it performed well in several tests. Since it is not
superior to CDAA and CDEC, which are already registered and
available to growers, there is little justification for additional
development costs for toxicity tests and needed formulation im-
provement. Also, there is little commercial interest in this
herbicide, due to a lack of adequate patent controls.
EPTC applied as a surface spray gave excellent weed con-
trol without harming soybeans. Considering present costs for
the rates required on organic soil, this chemical does not com-
pete favorably with CDEC or CDAA. Also, difficulty has ap-
peared at other locations where the use of EPTC has resulted in
weed control failures or crop injury. Soil moisture at the time
of application appears to be an influencing factor. Further
studies involving lower rates of EPTC incorporated into the
surface soil are planned. At present, EPTC is not registered
with the U. S. Department of Agriculture for use on soybeans.
This leaves two herbicides of proven value selected among
the 25 evaluated for pre-emergence control of annual weeds in
soybeans on central Florida organic soils. These chemicals,
CDAA and CDEC, have given excellent weed control at rates
from 4 to 6 pounds per acre and were used safely on soybeans at
8 or more pounds. At the lower rates, CDAA was found to be
slightly more effective against grasses than CDEC. The latter,
on the other hand, gave slightly superior control of broadleaf
weeds than did CDAA. Both have been approved by the Food
and Drug Administration and are registered by the U. S. De-
partment of Agriculture for grower use on soybeans. Each is
available in both emulsifiable concentrate and granular formula-
tions, under the proprietary names "Randox" and "Vegadex"
for CDAA and CDEC, respectively. Confirming the experimen-

24 Florida Agricultural Experiment Stations

tal results, these herbicides were used with two successful grow-
er plantings involving over 400 acres of soybeans during the
1958 and 1959 crop seasons. Both CDAA and CDEC at 4 and 6
pounds per acre were found to be commercially practical in these
large-scale trials.


1. Antognini, J. et al. Soil incorporation of selective herbicides. Proc.
Noeast. Weed Control Conf. 13: 421-422. 1959.
2. Burt, E. O. Herbicides in soybeans. Proc. So. Weed Conf. 9: 61-64.
3. Burt, E. O. Herbicidal control of weeds in corn and soybeans. Fla.
Agr. Exp. Sta. Ann. Rpt., p. 48. 1957.
4. Burt, E. 0. Chemical weed control in soybeans. Univ. of Fla. Agron.
Mimeo 59-4. 1958.
5. Chappell, W. E. Screening tests on peanuts, soybeans, and castor
beans in 1955. Proc. So. Weed Conf. 9: 57-60. 1956.
6. Indyk, H. W. Pre-emergence weed control in soybeans. Weeds 5:
362-370. 1957.
7. Peters, E. J., D. L. Klingman, and R. E. Larson. Rotary hoeing in
combination with herbicides and other cultivations for weed con-
trol in soybeans. Weeds 7: 449-458. 1959.
8. Scarborough, E. F. et al. Florida soybeans for beans, p. 139. In
Annual agricultural statistical summary, 1958-59 season. Fla. State
Mkt. Bur. 42nd Ann. Rpt. 1959.
9. Shaw, W. C. et al. Report of the Terminology Committee, Weed So-
ciety of America. Weeds 8: 487-521. 1960.
10. Staniforth, D. W., and C. R. Weber. Residual pre-emergence herbicides
in soybean production in Iowa. Weeds 6:115-125. 1958.

Evaluation of Herbicides for Soybeans 25


Following are data and notes dealing with the primary evalu-
ation trials. Weed control and crop tolerance ratings were based
on a 0 to 10 scale, with high values indicating favorable results.
Amitrole.-Amitrole was a good broadleaf weed killer, but
was less effective against grasses. Although the 6 pound per
acre rate was insufficient for good grass control, it was slightly
toxic to soybeans, causing chlorosis and dwarfing of the plants.
This ruled out higher rates necessary for better annual grass

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

3 56-3 6.7 6.7 10.0
58-1 10.0 3.3 10.0
6 56-3 8.9 7.8 8.9
58-1 10.0 6.7 10.0

Atrazine.-In the single trial where it was observed, atrazine
performed like other triazine compounds, producing toxicity
symptoms on soybeans. It was an excellent weed killer, but
the 4-pound rate yellowed and stunted the crop. Although 2
pounds performed well here, 3 pounds have usually been neces-
sary to give dependable weed control on this soil. This leaves
too narrow a margin for safety below the level toxic to the crop.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

2 58-1 10.0 8.9 10.0
4 58-1 10.0 9.4 7.8

CDAA.-With the exception of one experiment, number 56-2,
use of this chemical at 3 or more pounds per acre resulted in
excellent weed control. Crop tolerance was good at all rates up
to 8 pounds per acre. CDAA was selected for further testing in
advanced trials.

26 Florida Agricultural Experiment Stations

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

2 57-1 7.8 7.8 10.0
3 56-2 1.1 5.6 10.0
56-3 9.4 9.4 10.0
58-1 10.0 9.4 10.0
4 56-2 1.1 7.8 10.0
57-1 8.9 8.9 10.0
6 56-1 9.2 9.6 8.3
56-2 3.3 8.9 8.9
56-3 10.0 10.0 9.4
57-1 10.0 8.9 10.0
58-1 10.0 10.0 10.0
8 56-2 5.6 5.6 10.0
12 56-1 9.6 9.6 7.0

CDAA + TCBC.-This mixture of CDAA and trichlorobenzyl
chloride was developed to give an improved kill of broadleaf
weeds in corn over that given by CDAA alone. In the single
1958 test reported here, its performance was comparable to that
of CDAA. However, its known toxicity to broadleaved plants,
including soybeans, as reported by other workers, makes it in-
ferior to CDAA for use with this crop.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

214+33 58-1 10.0 9.4 10.0
4% +7z 58-1 10.0 10.0 10.0

CDEC.-This chemical, sold under the trade name "Vegadex",
gave consistently favorable results when used at rates of 4 to 8
pounds per acre. It is much more effective on organic soil than
on Florida light sands, where it frequently fails to give good
weed control. In these tests, serious crop injury was observed
only in Experiment 56-1, where 12 pounds per acre were applied.
Only one-third of that amount was usually necessary to give
adequate weed control on this Zellwood soil. Because of this
wide safety margin above the herbicidally effective rates, CDEC
was selected for further evaluation in the secondary trials.

Evaluation of Herbicides for Soybeans 27

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

2 57-1 6.7 6.7 10.0
3 56-2 7.8 6.7 10.0
56-3 9.4 9.4 10.0
58-1 10.0 8.9 10.0
4 56-2 10.0 8.9 10.0
57-1 7.8 7.8 10.0
6 56-1 9.6 9.6 8.3
56-2 10.0 10.0 10.0
56-3 10.0 10.0 10.0
57-1 8.9 8.9 10.0
58-1 10.0 9.4 10.0
8 56-2 10.0 10.0 10.0
12 56-1 10.0 10.0 5.9

Chlorazine.-This triazine-the first to reach the commercial
developmental stage-has been tested extensively for use on
soybeans by several workers. Although the crop appeared rel-
atively tolerant in these organic soil screening trials, weed con-
trol, even at 16 pounds per acre, proved to be insufficient to
justify continued appraisal of chlorazine after 1956.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

6 56-3 9.4 5.0 10.0
8 56-1 9.2 6.7 9.7
56-2 7.8 7.8 10.0
12 56-2 7.8 7.8 10.0
56-3 10.0 7.2 9.4
16 56-1 9.6 8.9 9.7
56-2 7.8 7.8 10.0

CIPC.-The performance of CIPC when applied during warm
weather was very erratic. Under hot dry conditions, such as
those prevailing during Experiment 56-2, this volatile chemical
was almost completely lost. On organic soil, soybeans displayed
tolerance to extremely high rates, ranging to 20 pounds per acre,
but kill of broadleaf weeds and annual grasses was not depend-

28 Florida Agricultural Experiment Stations

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

5 56-3 6.7 6.1 10.0
57-1 4.4 4.4 10.0
71/2 58-1 6.7 7.8 8.9
8 56-1 9.2 3.6 10.0
10 56-3 8.3 8.9 10.0
57-1 6.7 6.7 10.0
15 58-1 8.9 8.9 7.8
16 56-1 10.0 6.7 10.0
56-2 0 0 10.0
20 56-2 2.2 0 10.0

Dinoben.-This experimental herbicide, 2,5-dichloro-3-nitro-
benzoic acid, at 4 pounds per acre performed well when first
tested on soybeans in 1958. Phytotoxicity was severe at 8
pounds. Although further work with this chemical was planned,
its production was discontinued by the manufacturer, making
it unavailable for testing after 1958. According to recent re-
ports from others, a reduced form of Dinoben, 3-amino-2,5-di-
chlorobenzoic acid, with the coined name amiben, has consist-
ently surpassed Dinoben in performance. Amiben will be in-
cluded in future trials as part of this herbicide program for

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

4 58-1 8.9 9.4 8.9
8 58-1 9.4 10.0 4.4

Diuron.-Diuron failed to give dependable weed control on
peat soil at 1 and 2 pounds per acre. Even though soybeans ap-
peared to be tolerant to higher rates, including 4 pounds, the
possibility of crop injury and the danger to succeeding crops
make such treatments impractical. Diuron is an efficient broad-
leaf herbicide and is generally non-selective with regard to such
plants. Its low water solubility and consequent persistence in
soils has led to its widespread use as a durable soil sterilant where
total eradication of plants is desired.

Evaluation of Herbicides for Soybeans 29

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating
1 56-3 8.9 5.6 10.0
57-1 7.8 5.6 10.0
58-1 6.7 3.3 10.0
2 56-1 9.6 4.7 10.0
56-2 8.9 8.9 10.0
56-3 10.0 8.9 9.4
57-1 8.9 6.7 10.0
58-1 7.8 6.7 10.0
4 56-1 9.6 8.5 9.7
56-2 10.0 10.0 10.0

DNBP.-The high tolerance of soybeans to the alkanolamine
salts of DNBP has led to widespread testing with this herbicide.
At Zellwood, however, weed control was not sufficiently depend-
able to merit inclusion of this chemical in the replicated trials.

Weed Control Ratings Crop
Rate Experiment Tolerance
(Ib/A) No. Broadleaf Grass Rating
3 56-2 0 0 10.0
56-3 6.1 6.7 10.0
57-1 2.2 2.2 10.0
4/2 56-2 0 0 10.0
58-1 7.8 8.9 10.0
6 56-1 9.6 8.5 10.0
56-2 2.2 0 10.0
56-3 8.3 8.3 10.0
57-1 6.7 5.6 8.9
9 57-1 7.8 6.7 7.8
58-1 9.4 9.4 10.0
12 56-1 10.0 10.0 10.0
56-2 4.4 0 10.0

EPTC.-Applied as a pre-emergence spray and left on the
soil surface, EPTC produced little or no phytotoxic effect on soy-
beans at rates up to 18 pounds per acre of active ingredient.
Although its control of broadleaf weeds in these primary evalu-
ation trials was only fair, EPTC was selected for inclusion in
the advanced trials because of its excellent grass control and
because of favorable results reported by Burt (3). Further
screening tests with this chemical were planned using lower
rates incorporated into the soil to reduce volatilization losses.
Several workers, including Antognini, et al. (1), have reported
that the activity of EPTC is enhanced when it is used in this

30 Florida Agricultural Experiment Stations

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

4 58-1 6.7 10.0 10.0
5 57-1 7.8 8.9 10.0
6 56-1 2.9 9.6 9.4
56-3 6.7 8.9 10.0
8 58-1 7.8 10.0 10.0
10 57-1 7.8 8.9 10.0
12 56-1 4.0 10.0 8.9
56-2 4.4 10.0 10.0
56-3 8.3 10.0 10.0
58-1 8.9 10.0 10.0
15 57-1 8.9 10.0 10.0
18 56-2 8.9 10.0 10.0

EXD.-This chemical failed to give weed control when tested
on organic soil at 15 and 30 pounds per acre.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

15 57-1 0 1.1 10.0
30 57-1 0 3.3 10.0

Fenuron TCA.-Soybeans were severely damaged by this
chemical preparation even at 11/2 pounds per acre, which was
insufficient for good weed control. At 2 to 4 pound rates, fenu-
ron TCA gave complete control of all broadleaf weed species,
but was weaker on grasses.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

11/2 58-1 7.8 5.6 5.6
2 57-1 10.0 6.7 8.9
3 58-1 10.0 7.8 2.2
4 57-1 10.0 7.8 7.8

Evaluation of Herbicides for Soybeans 31

Ipazine.-In the 1958 trial, ipazine gave complete broadleaf
weed control but resulted in very poor grass control and severe
crop damage. Its failure to produce any effect on either the
weeds or soybeans in the 1957 trial was attributed to a poor
chemical formulation wherein the active ingredient had pre-
cipitated prior to use.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

1 57-1 0 0 10.0
2 57-1 0 0 10.0
58-1 10.0 1.1 3.3
4 58-1 10.0 4.4 0

Monuron.-Two pounds of monuron, like diuron, gave good
broadleaf weed control but only fair grass control. In the 1957
trial, the weed control was the same for both of these substituted
urea compounds, but monuron was more toxic to soybeans. This
precluded its use at higher rates.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

1 57-1 6.7 5.6 10.0
58-1 7.8 5.6 10.0
2 57-1 8.9 6.7 7.8
58-1 8.9 7.8 10.0

NPA.-Although it was not toxic to soybeans, trials with
NPA on organic soil were discontinued after 1956 because it
was ineffective in controlling weeds, especially grasses. Under
the hot dry soil conditions of Experiment 56-2, even 20 pounds
per acre gave no grass control. High rates such as this are
economically impractical, since 10 gallons of commercial formu-
lation would be required to supply this much technical chemical.

32 Florida Agricultural Experiment Stations

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

8 56-1 8.1 3.6 10.0
56-3 8.3 5.6 10.0
16 56-1 7.8 5.9 10.0
56-2 5.6 0 10.0
56-3 8.9 7.2 10.0
20 56-2 6.7 0 10.0

PCP.-Weed control with this sodium salt of pentachloro-
phenol at rates ranging from 6 to 15 pounds per acre was good
in all trials except in Experiment 56-2. Failure in this test was
attributed to moisture deficiency in addition to faulty applica-
tion resulting from clogged spray nozzle screens and tips. The
industrial formulation of PCP used contained a considerable
quantity of gums or other insoluble ingredients. Since this
chemical did not produce crop injury in any of the tests, it was
selected for inclusion in the advanced trials.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

6 56-2 0 0 10.0
56-3 7.2 7.2 10.0
7 56-1 9.6 7.8 10.0
58-1 8.9 7.8 10.0
9 56-2 1.1 0 10.0
12 56-2 3.3 0 10.0
56-3 9.4 8.3 10.0
15 56-1 10.0 9.2 10.0
56-2 5.6 0 10.0
58-1 10.0 9.4 10.0

Propazine.-Although a good pre-emergent weed killer, pro-
pazine at 4 pounds per acre resulted in severe crop injury when
used in 1958. Soybeans tolerated 2 pounds, but some annual
grass survived, indicating no margin of safety.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

2 58-1 10.0 7.8 10.0
4 58-1 10.0 8.9 1.1

Evaluation of Herbicides for Soybeans 33

Sesone.-According to these tests, 5 to 10 pounds of sesone
were required to give adequate control of both broadleaf weeds
and grasses on organic soil. These rates were toxic to soybeans
all 3 years.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

3 56-3 5.6 7.8 9.4
4 58-1 5.6 7.8 10.0
5 57-1 8.9 8.9 7.8
6 56-3 7.2 8.9 8.9
8 58-1 8.9 8.9 5.6
10 57-1 10.0 8.9 5.6

Simazine.-Most extensively tested of the five triazine com-
pounds included in these trials, simazine appeared to be an ex-
cellent weed killer at all rates from 1 to 6 pounds per acre. At
levels of 2 or more pounds per acre, it frequently was toxic to
soybeans. This eliminated it from further consideration as a
selective herbicide for use with this crop.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

1 57-1 10.0 7.8 10.0
11/ 56-3 10.0 8.9 10.0
2 56-2 8.9 8.9 10.0
57-1 10.0 8.9 10.0
58-1 8.9 7.8 6.7
3 56-1 9.6 9.6 8.1
56-2 8.9 8.9 10.0
56-3 10.0 10.0 8.9
4 56-2 10.0 8.9 10.0
58-1 10.0 8.9 3.3
6 56-1 10.0 10.0 4.7

TCA.-The effectiveness of sodium trichloroacetate, the form
of TCA used here, on both crop and weeds was extremely vari-
able. It is a water soluble herbicide, and at times high rates ap-
parently dissipated completely without any effect on the weeds.
At other times, low rates produced severe crop injury.

34 Florida Agricultural Experiment Stations

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

9 56-1 2.9 4.4 10.0
56-3 4.4 7.8 6.7
10 57-1 6.7 7.8 5.6
58-1 6.7 8.9 3.3
18 56-1 4.7 8.5 10.0
56-2 0 0 10.0
56-3 7.2 9.4 5.0
20 57-1 7.8 7.8 4.4
58-1 7.8 10.0 1.1
221/2 56-2 0 0 10.0

Trietazine.-Use of this chemical at 4 pounds per acre re-
sulted in excellent broadleaf weed control, but only fair grass
control and some crop injury, sufficient to eliminate it from fu-
ture consideration for soybeans.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

2 58-1 8.9 5.6 10.0
4 58-1 10.0 7.8 7.8

2,3,6-TBA.-This trichlorobenzoate, a sodium salt consisting
of a mixture of six isomers with the 2,3,6- isomer predominat-
ing, was found to be exceedingly toxic to soybeans. It has not
been included in the tests since 1956. Its persistent residual
activity in peat soil would be a further deterrent to its use with
any crop in this area. Residues in plots where it had been used
in 1956 produced severe injury to succeeding crops in 1957.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

3 56-1 8.5 6.7 5.2
56-3 9.4 6.7 4.9
6 56-1 8.5 8.1 3.3
56-2 7.8 10.0 2.2
56-3 10.0 8.3 2.2
9 56-2 8.9 10.0 1.1

Evaluation of Herbicides for Soybeans 35

2,4-D.-In the 1956 tests, soybeans appeared to be completely
tolerant to pre-emergence applications of the amine salt of 2,4-D
up to 4 pounds per acre. During 1957 and 1958, however, even
1 pound produced severe stand reductions and plant stunting.
Weed control was not dependable at rates below 3 pounds.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

1 56-1 5.9 4.7 10.0
56-3 4.9 6.1 10.0
57-1 8.9 7.8 6.7
58-1 7.8 8.9 6.7
2 56-1 8.9 6.7 10.0
56-2 6.7 7.8 10.0
56-3 6.7 7.8 10.0
57-1 10.0 8.9 4.4
58-1 8.9 9.4 3.3
3 56-2 7.8 10.0 10.0
4 56-2 8.9 10.0 10.0

2,4-DEP.-Even though soybeans displayed considerable tol-
erance to 2,4-DEP, excessively high quantities were required to
give adequate broadleaf weed control. It proved to be a good
grass killer on organic soil, frequently effective 6 or 7 weeks
after application.

Weed Control Ratings Crop
Rate Experiment Tolerance
(lb/A) No. Broadleaf Grass Rating

6 56-1 5.1 8.5 10.0
56-3 5.6 7.8 8.9
57-1 6.7 7.8 10.0
58-1 9.4 8.9 10.0
12 56-1 6.3 9.6 9.1
56-2 7.8 10.0 8.9
56-3 6.7 8.9 7.8
57-1 8.9 8.9 7.8
58-1 10.0 10.0 10.0
18 56-2 8.9 10.0 8.9

36 Florida Agricultural Experiment Stations


The author is indebted to Dr. Kuell Hinson of the Agronomy Depart-
ment, Florida Agricultural Experiment Station working cooperatively with
the Soybean Investigations Sections, Agricultural Research Service, USDA.
In addition to supplying the seed used for these trials, he provided facili-
ties and assistance in harvesting, drying, and weighing the grain and aided
with the analysis of the data.
Appreciation is also expressed to the following herbicide manufacturers
who gratuitously supplied the chemicals used in the experiments:
Amchem Products, Inc., Ambler, Pa.
American Cyanamid Co., New York, N. Y.
Dow Chemical Co., Midland, Mich.
E. I. duPont de Nemours & Co., Wilmington, Del.
Geigy Agricultural Chemicals, Geigy Chemical Corp., Yonkers, N. Y.
General Chemical Division, Allied Chemical Corp., New York, N. Y.
Monsanto Chemical Co., St. Louis, Mo.
Naugatuck Chemical Division, U. S. Rubber Co., Naugatuck Conn.
Niagara Chemical Division, Food Machinery & Chemical Corp., Mid-
dleport, N. Y.
Roberts Chemicals, Inc., Nitro, W. Va.
Stauffer Chemical Co., New York, N. Y.
Union Carbide Chemicals Division, Union Carbide Corp., New York,
N. Y.

o 0

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