82
name cultivation., it is believed that., under such conditions of heavy annual
grass :in:f'estations, the plants which escape cultivation during the period from
cotton emergence to flame cultivation necessitate the use of some additional
weed control practice., whether it be oiling., hoeing., or otherwise$
Conclusion
Some general conclusions drawn from observations in
ful to growers are listed below:
19.54 which may be help-
1 0 Existing recommended aids for weed control in California cotton are not
being used to fullest advantage. Among these are the line diagram which assists
in the proper adjustment of equipioont., and :markets for uniformity of row-spacing.
2., Flruoo cultivation is another recommended practice which is not fully
utilized 0 The data show that flame alone or following other supplemental weed
control practices effectively controlled most annual weeds, and appeared promising
for nutgrasso
3 0 Under many conditions., there is a definite need for, a more positive
weed control practice from the time cotton emerges until it will tolerate flaneo
Selective post-eioorgence oils offer one solution to the problem., Other cultural
practices or even one hoeing, preceding the use of flame, may be economically
used in some cases o
4., Under very weedy conditions, the use of oil and flruJE, or flama alone,
has resulted in increased yields, grades,,and picker efficiencies.
5o Several spray materials, including dalapon and weed oils, used as spot
treatments in cotton have provided more effective control of Johnson grass and
have required less time for treatment than hand-hoeing. Among the materials
used, dalapon has appeared most promising because of its ability to translocate
and destroy rhizomes .. Because of economic limitations 7 this practice would appear to be most applicable to scattered infestationso
CMU AND DCMU FOR WEED CONTROL IN CITRUS
Boysie E. Day
University of California, Riverside
Non-cultivation management of citrus, using oil sprays instead of tillage to , ·
control orchard;'rweeds affords savings by eliminating cultivating costs., reducing
hand labor in connecting furrows to stands, reducing handling of orchard heaters,
eliminating weed competition for water and soil nutrients 3 reducing gopher
damage, and allowing greater .fie:a;:Lbility in pest control, picking, and other
grove operatiohso Costs for weed control by oil spraying are usually higher
than cultivation and furrowing costs during the first years of non-tillage
management; however, depletion of the weed seed population in the soil surface
is accomplished after two to four years of oil spraying resulting in a long term
economic advantagee
Our studies at the Citrus Experiment. Station have aimed at finding methods
of reducing the high initial cost of non..;,tillage management. This could be ac-
83
complished by the development of cheaper contact spray's or by selectively sterilizing the soil beneath the trees. Certainly many growers have been reducing
costs by using better weed oil in less volume and by spraying the weeds when
they are younger; however., increasing costs of petroleum.pro~ucts largely nullifies such economies. Three years ago we began an exaru..nation of a number of
herbicides in search of selective soil sterilants :for orchard use.
We tested IPC and CIPC and other phenylcarbamates as adjuvants to weed oil
with the objective of extending the effectiveness of oil sprays by a period of
temporary soil sterility. Results of these experilll.ents have been variable and
generally not promising.
In January 1953 we made soil applications of dinitro at rates up to 18
pounds per acre., Alanap at rates in the 2 to 8 pounds range., SES at 2 to 6
pound rates., and CMU at rates of 1., 2., and 3 pounds per acre. In subsequent
tests we have applied PCP to the soil under citrus trees at rates of lo., 20.,
4o1 and 80 pounds per acre., Alanap at several retes up to 32 pounds per acre.,
2.,4-D amine at rates from 2 to 8 pounds per acre., and CMU and DCMU to a large
number of plots at rates of l to 40 pounds of active chemical per acre.
Although Alanap., PCP., and dinitro have provided excellent weed control for
periods of three to four months without apparent tree injury., CMtJ and DCMU have
performed better than any of the other materials listed. In our first tests.,
plots under orange trees treated with l to 3 pounds per acre in January., 1952.,
were free of all annual weed growth until midsummer. There was no apparent
injury to trees from these treatments.
Two years ago at the Southern California Conference Si Dudley reported the
results of tests in which CMU wa£ applied at rates of 10 to 50 pounds per acre.
These treatments were followed by the normal disking and furrowing practices.
Although incomplete weed control resulted at the lower rates of treatment., it
is significant that no injury to the trees was observed. Reduced weed control
would be expected under such conditions because of distribution of the herbicide in depth by cultivation; however., tillage of the CMtJ into the root z.'One
of the trees certainly should have provided.a more severe test of tree tolerance
than surface applications.
$i
. -·
1953., CMU was applied under Valencia orange and grapefruit
trees at the Citrus Experiment Station under non-tillage conditions at rates of
1., 2., 4., 8., 16., and 32 pounds of active material per acre. No weeds were present
at the time of spraying. These plots received approximately ten inches of rainfall before the summer dry season of 1954.
In November,
"
Rates of one pound per acre controlled all annual weeds for 4-1/2 months.
The 2 pound rates controlled all annuals for 5-6 months •. The 4 pound rate was
effective for 6 months., and the higher rates were still free of weeds at last
observation., 13 months after treatment. Early in June., eight months after
treatment., typical chlorosis symptoms appeared on both the Valencia orange and
grapefruit trees treated with 32 pounds per acre. During the months of June
and July chlorosis increased and many chlorotic leaves and all fruit dropped.
Defoliation was about 75 percent on the orange and 40 to 50 percent on grapefruit.·
New foliage began to develop on grapefruit in August and by December the only
remaining symptom was absence of fruit. The orange had not fully recovered by
December. Trees in plots treated with rates lower than 32 pounds per acre were
not visibly affected.
Bio-assays of soil taken from the treated plots five months after treatment
BL~
showed that the soil were sterile to the growth of oat seedlings to a depth o:t two
feet (greatest depth tested) in the 16 and 32 pound plots and to a depth of 18
inches in the 8 pound plots o Sub-lethal a.mounts of CMU were present at lower
depths in plots treated with 1., 2 and 4 pounds of CMU. Since most of the .feeder
roots of citrus occur in the surface two feet of soil (estimated at 80% in these
plots) the selectivity of CMU to citrus could not be based entirely on lack of
penetration of the herbicide into the root zone of the treese
During the period from December., 1953., to }fay., 1954, seven additional series
of CMU test plots were put out in citrus orchards. Rates applied varied from 1
to 8 pounds per acreo Plots were replicated from two to five times in each test.
It was found that 2 pounds of CMU per acre and higher rates applied in late
winter or spring controlled annual weed growth until the beginning of autUJJlil.
rains except in the irrigation furrows. With 2 to 4 pounds per acre new growth
started in the furrows during the period from July to September o Six to 9
pound rates were effective in the furrows from 1 to 3 months longero
A test was run at the Citrus Experiment Station to determine the tolerance
of citrus to heavy rates of CMU and DCHU leached into the soil by floodingo The
trees used were orange on rough lemon rootstocko Shoots from the rootstock
had in past years grown through the tops of the orange scions producing trees
which in some cases were predominantly rough lemon.
Basins 28 feet square were laid up around the trees. CMU and DCMU was applied uniformly to the soil of eight trees at rates of 10., 20, 30 and 40 pounds
per acre (active ingredient basis). Treatments were applied on July 19., 19540
Immediately after treatment the basins were flooded with 18 inches of water
measured-in from a tank truck. Two control trees were basined and watered., but
received no treatment of herbicideo
Moderate chlorosis., uniformly distributed throughout the foliage first appeared on the trees treated with 30 pounds and 40 pounds of CMU two weeks after
treatment; By August 26., five weeks after treatment., symptoms were well developed on these trees and mild chlorosis had appeared on the tree treated with
20 pounds per acre of CMU. Two or three months after treatment very slight
symptoms first appeared on the tree treated with 40 pounds of DCMU per acre 0
Observations are given in the following table:
CMU
and DCMU Heavy Leaching Studies-Injury to Trees Five Months after Treatment
%leaf
Treatment
DCMU
DCMU
DGMU
DCMU
lbs./Ao
20 lbs./A.
30 lbs./A.
40 lbs .. /A.
10
10 lbs./A.
CMU 20 lbs.,/A.,
CMU 30 lbs /A.
CMU
CMU
40
injury
0
0
0
25
0
Trace
0
40
lbs.,/A.,
50
Remarks
No symptoms
No symptoms
No symptoms
Typical symptoms - leaf discoloration and
some defoliation.,
No symptoms
Slight chlorosis., no apparent defoliation.,
Leaf symptoms and moderate defoliation.
Leaf symptoms and heavy defoliation, some
new growtho
85
Tree injury by CMU as evidenced by chlorosis and defoliation reached its
peak early in September and new leaf growth begano This experiment gives evidence of a relatively high resistance of citrus to urea herbicideso
It is known that application rates of CM.U are sensitive to soil type, particularly to variation in certain adsorptive componentso We ma.de a toxicity survey of soils in the hope that we would be able to use soil maps to predict the
amount of C:MU required to obtain practical results in the several citrus districts.
Typical citrus soils were collected throughought Southern California along
with several additional soils typifying particular characterists such as high
organic matter.and heavy texture. These soils were tested in the greenhouse to
determine the amount of CMU and DCMU required to produce sterility with respect
to the growth of oat seedlings o
Wherever possible undisturbed soils were collected adjacent to citrus orchards. Top soil was scooped up, screened., and sun-dried to uniform weight.
Tests were made by methods originally developed by Dr. Crafts for evaluating
soil factors affecting other herbicides.
In the tests, 400-gram samples of the soils were weighed into waterproof
waxpaper cupso Increasing amounts of CMU and DCMU were added to a series of
samples of each of the soils and sufficient distilled water was added to bring
the soil to field capacity. 14 certified Kanota oats were planted in each culture and the cultures were watered to weight dailyo
A logarithmic series of quantities of herbicides was used. The concentrations in parts per million followed the series log2 1/3, 2/3, 1, 4/3,!o••••••
The widest range of concentrations used extended from log2 -4 to log2 +2o The
concentration of herbicide was thus about 26% greater in a culture than the concentration in the soil of the preceding member of each series o Tests have shown
that such a series approaches the limit of resolution practical for this type
of bioassay without extensive replicationo
Ten days after planting the oat seedlings were thinned to ten per culture
and 21 days after planting the cultures were harvested and the fresh weight of
the tops was determinedo A value was thus obtained for the minimum lethal dose
of CMU and DCMU for each of the soils. These values are given in the following table along with the field capacity and place where each soil was collected
The values for all soils may be compared with values obtained from a culture
containing refined quartz sand watered initially with Hoagland' solution. Data
are flu.mmarized in the following table.
0
It is apparent from the table that the dosage requirements of urea herbicides was not closely related to soil classification as expressed by soil maps.
The lethal dose of CMU varied ten-fold from the most sensitive to the most resistant soil. DCMU had a maximum variation of at least fii'ty-f old The quartz
sand culture indicates that were adsorption is not a major factor DCMU is more
toxic than CMU., but generally in soils more DCMU is required than CMU to secure
· equal results. As was illustrated by leaching experiments, DCMU is more strongly
adsorbed than CMU and is thus made unavailable to plants
0
0
Experiments were run to determine the extent to which CMU and DCMU were
leached in soils. The soils selected were ones which had characteristics that
might permit the evaluation of certain soil factors affecting applications in
orchards. Cylinders., 8 cm. in diameter and 60 cm. high were made from plasticcoated screen wire and packed with soils to approximately uniform density 0
86
..
Lethal Doses in Soils
Field
capacity
Soil
Quartz Sand
Ramona Sandy Loam
Mountain Soil
Diablo Clay
Imperial Clay
Vista Sandy Loam
Coachella Fine Sand
Placentia Loam
Chino Silt Loam
Hanford Fine Sand
Hanford Loa.IJ'.\V Sand
Hanford Fine Sandy Loam
Hanford Fine Sandy Loam
Visalia Sandy Loam
Visalia Loam
San Joaquin Loam
San Joaquin Loam
Ducor Clay
Exeter Sandy Loam
Ojai Loam
Yolo LOa.IJ'.\V Fine Sand
Yolo Fine Sandy Loam
Yolo Loam
Yolo Silty Clam Loam
Yolo Silty Clay
Fallbrook Loa.II\Y Sand
Escondido Sandy Loam
Source
Quarl:Y
Riverside
Big Bear Lake
Oceanside
Calipatra
Fallbrook
Thousand Palms
Redlands
Chino
Ontario
Mentone
Loma Linda
Hemet
Lemon Cove
Lemon Covl:3
Woodlake
Exeter
Lemon Cove
Exeter
Ojai
Oxnard
Oxnard
Santa Paula
Ventura
Ventura
Escondido
Escondido
%
30
25
47
45
47
25
20
32
37
25
30
27
25
17
32
27
CMU
Lethal dose
(ppm)
o.4o
0.79
3ol8
1.59
0.31
1.26
o.4o
0.79
3.18
o.5o
o.5o
1.0
0.19
2.00
1.26
o.5o
DCMU
Lethal dose
(ppm)
Oo06
0.79
3.18
1.59
0.79
1.59
0.06
2.0
3.J.B'
0.20
1.0
1.0
1.26
3.18
1.26
o.5o
25
35
25
35
30
25
35
35
0.79
3.18
1.26
2.0
2.52
3.18
0.79
1.26
2.52
2.0
J.18
1.26
2.0
22
1.26
1.26
o.5o
35
22
o.5o
o.5o
)ol8
2.0
)ol8
2.0
The weight of soil required to fill the cylinders was determined. Based upon
previous tests sufficient herbicide was applied to the top of each column to
sterilize all of the soil in the column with respect to the growth of oats.
The amount of chemical added was the Iidnµmlm required to sterilize the weight
of soil used in each case. The columns were leached with sufficient distilled
water to wet the column to the bottom. This amount of water for each soil, was
called one volume.
Test soils were leached with one volume, two volumes and three volumes of
water. Both CMU and DCMU were used. The .soils tested were Mountain Soil (high
organic matter), Diablo Clay, Imperial Clay,Vista Sandy Loam, Coachella Fine
Sand, and refined quartz sand.
After the chemical had been added to the surface of the soil and leached,
the screen cylinder was opened and the moist column of soil was cut into 8
sections each 7i cm. long. The sections of soil were placed in No. 2 tin cans
and seeded with 14 certified Kanota oat seeds. The cans were watered dai:cy by
~eight. Ten days after planting the oats were thinned to 10 plants per can.
After twenty-one days the oats were harvested and weighed. Untreated controls
for each soil served as a basis of comparison and results were calculated as
percent of growth of the controls. 100 ml. of Hoagland' solution was added to
~
87
the quartz sand cultures after planting.
In the quartz sand cultures CMU apparently moved with the initial front of
J.eaching water. When leached with one voll.lllB some CMU remained in all portions
of the column but the highest concentration was in the lower portion. When
leached with two volumes of water the upper one-third of the colunm. contained no
neasurable amount of CMU, however., the lower two-thirds retained increasing
amounts at deeper levels. Three volumes of water removed essentially all CMU
from the column.
The retention of CMU by Coachella fine sand was similar to that of quartz·
sand 0 After leaching with one volume, CMU was present in the bottom of the
column., in highest concentration but with detectable amounts throughout the
whole column 0 Two volumes of water removed the CMU from the upper portion of
the column and three volumes of water leached it from the remainder of the
column.
Leaching of Vista Sandy Loam with one volume sterilized the upper two-thirds
of the column without reducing plant growth at the bottom level. Leaching with
greater a.mounts of water washed increasing amounts from the surface levels and
extended the depth of sterilization correspondingly.
Of the three heavy soils CMU was readily leached to lower levels in Imperial
Clay, leached somewhat less readily in Diablo Clay and was strongly held in the
surface of the mountain soil. Successive leachings of Imperial Clay caused
relatively uniform distribution of C11tJ throughout all levels. Successive leachings of Diablo Clay moved the zone of highest concentration from the upper half
of the column to .the middle zone with the second and third volumes of water
partially freeing the surface layer of herbicide. Leaehing of the mountain
soil extended the effect in depth only slightly with increasing amounts of water.
These data indicate that CMU is leached in soils at a rate that is roughly
in proportion to the dose of CMU required to be effective.
The DCMU was less readily leached than CMUo DCMU is less soluble than CMU
and presumably more strongly adsorbed. Since the rate of leaching of this type
of compound is presumed to be dependent upon the ratio of adsorbed chemical to
the amount in solution., as in a typical chromatogram., the a:iuilibrium of adsorbed
to dissolved material is shifted in the direction of higher adsorption and the
rate of leaching is reduced.
One volune of water distributed DCMU throughout the entire column of quartz
sand. Successive leachings removed only part of the chemical. The entire
column of Coachella fine sand was sterilized by DC1'1U carried down by one volume
of water. DCMU was resistant to leaching in the other soils in the increasing
order of Imperial Clay, Vista sandy loam, Diablo Clay and mountain soil. In
each of these cases typical chromatogram types of distribution resulted.
,t.
Adsorption appears to be the governing factor in the amounts of urea herbicides required to secure a given result and in determining the amount of leaching that treated soil will withstand before the herbicide is removed from the
.surface soil to lower levelso Soils requiring light applications rapidly lose
the toxicity by leaching. Soils that require heavy dosages retain the active
herbicide · in the surface even when fairly large amounts of water have leached
through the soil. These two factors tend to counteract one another under leaching conditions in the field. The amounts required for field application should
not vary as widely as greenhouse dosage experiments indicate 0 Following are
...
88
conditions under which CMU would be expected to deliver the better performance in
. the field:
lo
20
3 o·
Soils having high adsorptive capacity.
Conditions of low rainfall.
To kill relatively deep-rooted weedf3 •
Use of DCMU is indicated under the following conditions:
Soils of low adsorptive capacity.
Under conditions of relatively high rainfall, sprinkler irrigation
or frequent flood irrigation+
3. Where damage to deep-rooted species in the treated area is not desired.
1.
2.
We conclude that a practical margin of selectivity exists for weed control
under orange and grapefruit trees. High-rate tests have not been run on lemons
nor have all of the various scion and rootstock varieties been tested. Appreciable variation in rates may be expected on different soils and these differences are not readily predictable from existing soil survey maps. Evidence
indicates that rate variation in the field will not be as great as laboratory
tests under non-leaching conditions indicate.
Use of urea herbicides in commercial citrus production is not recommended
by the Citrus Experiment Station. Further testing will be necessary before industry-wide use would be justified. We now have 2.5 sets of replicated field
plots under study in the several citrus counties; in addition we have about 3.5
acres of citrus on non-cultivation using CMU and DCMU f9r weed control exclusively.
These experiments along with additional laboratory work should tell us whether
or not urea herbicides have a place in citrus culture.
RIGHT OF WAY CLEARANCE WITH CHEMICALS
Robert H. Beatty
Di.rector of Research
Agricultural Chemicals Division
American Chemical Paint Company
Ambler, Pa.
The introduction of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,.5-trichJ.orophenoxyacetic acid (2,4,5-T) in 1944 has changed the approach to the
problem of brush control in this country. Bef o?"e 1944 most brush control was
mechanical; although there was limited ~e · of such cremicals as the chlorates,
dinitrocresol compounds, soluble arsenicals, copper sulfate and ammonium sulfa.mate (.Ammate), these compounds have been replaced by 2,4-D arid 2,4,5-T in
chemical brush cont~ol with the exception of Amma.te.
.
It is estimated that in the Um,ted States near:Ly 5,000,000 pounds of
2,4,5-T were produced last year, a.11 · for.use in controlling woody plants. As
most brush killer formulatiqns contain at least 50% 2 1 4.;.D acid equivalent it
is evident that brush killing in this CoUlltry isconsuming many million po~
of these pheno:xy acids. The supply has been adequate to meet the demands.