Tải bản đầy đủ - 0trang
VII. Emulsions and Emulsion Stabilizers
polar groups in the surface layer. At an oil-water interface the polar
group is in the water, the nonpolar in the oil. I n this way they overcome
the inherent incompatibility of these interfaces so that when the oil is
broken up into minute droplets there is little or no tendency for the
droplets to run together and form a Pepurat,e phase. I n other words, the
emulsion is stable.
Even in such stabilized emulsions the stability is a relative state; if
the oil particles are large enough, the force of gravity will cause them
to accumulate and eventually to coalesce. I n the case of light oils the
droplets cream out on top and form a layer over the water; in the case
of oils of high density the layer may form beneath the water. Two
factors are concerned in the long-time stability of such emulsions: (1)
the density of the oil, if i t is close to unity the tendency to separate is
less; (2) the droplet size is also concerned, if the droplets are so small
that they are violently agitated by thermal agitation they have less
tendency t o coalesce and form a separate layer. Homogenization of the
emulsion therefore is often of an advantage by reducing droplet size of
the oil below a certain threshold value so that they stay permanently
dispersed in a uniform concentrRtion throughout the mass of the dispersion medium.
The various mechanisms of herbicidal selectivity have been considered in detail (Crafts, 194613). It suffices here t o enumerate the types
of selective herbicidal treatments and illustrate them with examples from
Selective herbicidal action may result from :
(1) Differential wetting, (2) morphological selectivity, (3) biochemical selectivity, (4) selective spray placement, ( 5 ) selective soil sterilization, (6) differential life span of plants.
Differential wetting is illustrated by Sinox application to kill mustards
and fiddleneck in flax. Leaves of the weeds are broad and readily
wetted; those of flax are small and difficult to wet. Peas are another
crop that illustrates this phenomenon.
Morphological selectivity results from differences in the location of
the vital tissues in plants. For exanple, in cereals, onions, and lilies the
growing points of the plants are located a t the base near or below the
ground level; they are also protected by the surrounding leaves. In
contrast, most broad-leaved weeds have exposed growing points which
can readily be wet by the spray. The selective action of herbicides containing iron or copper salts, sulfuric arid, or Sinox is due to a combination of these two mechanisms.
A. S. CRAFTS AND W. A. HARVEY
Biochemical selectivity depends upon inherent differences in the
makeup of plants with respect to their tolerance of certain poisonous
chemicals. When weeds in crops of the carrot family are sprayed with
Stoddard Solvent, all the plants are thoroughly wet; only the weeds die.
When mustard in a wheat crop is dusted with 2,4-D or given a low
volume spray treatment with an ester formulation of 2,4-D in oil, the
selectivity is biochemical; all plants are exposed to the chemical, only
the weeds die.
When a robust crop plant such as corn, milo, or sugar cane attains
a height of 12 to 18 inches, it has proved feasible to spray small weeds
around the base with a general-contact herbicide, the growing tissues of
the crop being protected from the spray by the bases of the older leaves.
B y using such sprays i t has been possible to bring a crop to maturity,
completely eliminating the frequent tillage commonly used (Crafts,
1948b). Under Home conditions this may be less expensive than the
common tillage practice; under some it is preferable because it minimizes
erosion and prevents the formation of cultivator sole. This latter is
important in the semi-arid west where such sole interferes with irrigation
and where impervious layers in t.he soil are not broken up by freezing
of the soil during the winter.
The same biochemical selectivity shown by the tops of certain crops
and weeds is reflected in their roots. For instance, the salts of 2,4-D in
the soil will kill mustards at concentrations t,hat cause little or no injury
to cereal crops. For this reason 2,4-D salts may be used either as preemergence or post-emergence treatments in small grains and corn, providing soil moisture conditions are right and dosage is carefully
Certain perennial crops may be treated during their dormant period
or between cuttings. This is exemplified by alfalfa that may be sprayed
with a general contact herbicide to rid it of weeds during the winter or
early spring, or immediately after a cutting during the summer. This
is becoming common practice in the western states (Harvey and Riddle,
1946; Raynor, 1947).
The foregoing discussion presents a number of the types of herbicidal
action that are now being widely used in the field of chemical weed
control. The remainder of this paper will be devoted t o description of
the various chemicals and review of current literature concerning them.
IX. THE2,4-D HERBICIDES
The advent of 2,4-D has so rapidly and so profoundly changed the
practices of chemical weed control that the terms 2,4-D and herbicide
are considered synonymous by many. While a proper balance in em-
phasis should be maintained with respect to the different herbicides
available, a material that has reached a production of around 20 million
lbs. by the fourth year of its use in the field deserves recognition. Particularly is this true when one realizes that in many cases one pound or
less of this chemical is used per acre. For the first time, chemical weed
control may be recommended as an economical substitute for tillage or
cropping practices. Selective weed control has assumed a major position
among pest-control practices.
Apparently 2,4-D was discovered independently in England and in
the United States during the war (Blackman, 1945; Kraus and Mitchell,
1947; Nutman et al., 1945; Slade et at., 1945). After its announcement
(Hamner and Tukey, 1944; Mitchell and Hamner, 1944), a wave of
interest spread across the land and thousands of field tests were made
by investigators and private individual farmers before adequate experience in its use had been gained.
Early experiments with 2,4-D and similar chemicals are reported by
the British workers, Slade et al., (1945), Nutman et al., (1945), and
Blackman (1945). Hamner and Tukey (1944) were the first to publish
experimental work in this country; Kraus and Mitchell (1947) later
described many experiments performed during the war and carried out
under restriction. These firmly established the herbicidal properties of
2,4-D and similar growth regulators. Subsequent work has been carefully reviewed by Akamine (1948). Readers are referred to this paper
where scientific references as well as government publications, popular
articles, and commercial reports are listed. Popular aspects are al3o
treated by Mitchell and Marth (1947) a n d by Avery et al. (1947).
2,4-Dichlorophenoxyacetic acid is a white crystalline substance
formed by reacting dichlorophenol with monochloroacetic acid (Wellman,
1948). It is only slightly soluble in water but may be readily dissolved
in alcohol and acetone (Anonymous, 1948j).
Early 2,4-D formulations involved the use of Carbowax (polyethyleneglycol) as an intermediary but it was soon realized that the
sodium and ammonium salts were soluble in water and convenient to use.
More recent formulations involve the amine salts, the principal advantage of these being that they are liquid a t high concentrations, which
makes for easy mixing by dilution. Some of these formulations contain
a quantity of the salt equivalent to four pounds or more of acid per
The esters of 2,4-D are somewhat more effective than the salts since
they are less polar in nature. As these compounds lose in polarity,
however, they lose correspondingly in water solubility and most of the
formulations involve the ester or t.he ester in solution in oil, plus a large
A. S. CRAFTS AND W. A. HARVEY
amount of emulsion stabilizer. Thus the material may be dissolved in
oil and applied as a very fine spray, or emulsified in water with application possible over a wide range of volume dosages.
Though appreciably more toxic on an acid equivalent basis (up to
twice), some esters have the drawback of being volatile and problems
of injury to adjoining sensitive crops have been more serious with ester
formulations than with the salts (Brown et al., 1948; Dunlap, 1948;
Because 2,4-D is available in such a variety of forms, workers have
agreed to recommend and use it on an acid equivalent basis. In order
to compute formulations, niolecular weights of the various 2,4-D compounds are necessary. Table I from Weeders Readers of November 1,
Concentrations and Weights of 2,4-D Materials
2,4-D Ammonium Salt
2,4-D Sodium Salt (anhydrous)
2,4-D Sodium salt (monohydrate)
2,4-D Diethanolamine salt
2,4-D Triethanolamine salt
2,4-D Methyl ester
2,4-D Ethyl ester
2,4-D Isopropyl esters (2, same
10. 2,4-D Butyl ester (4, same weight)
11. 2,4-D Amy1 esters (about 15, all
with same weight)
12. 2,4-D Isopropanolamine salt
Units required to
contain 100 units
of 2,4-D acid
1947, gives these values. Structural formulas are given by Freed (1946).
From what has been said, it is apparent that 2,4-D is unique among
herbicides. Having growth-regulating properties (Synerholm and Zimmerman, 1945, 1947; Zimmerman and Hitchcock, 1942), it i8 effective
at dosage rates that are phenomenally low. Only the nit.ro- and chlorophenols approach 2,4-D in toxicity and while they have definite uses in
the field they do not approach 2,4-D in multiplicity of herbicidal characteristics.
Because 2,4-D has hormone properties, it differs from the older
herbicides in its physiological action on plants. Upon coming in contact
with leaves or roots, it rapidly migrates to other parts of the plant; this
is the action that makes possible low-volume and dust treatments as
contrasted with most other herbicides that are required in much greater
volume t o effect sufficient cover. In spite of this remarkable quality,
2,4-D may be used in essentially the same way as the older contact
herbicides such as sodium arsenite, chlorate, and dinitro compounds.
Therefore, in what follows, where 2,4-D is being used as a contact herbicide it is so labelled even though it may move within the plant after its
1. General Contact Spray
While not widely used as such, 2,4-D has found valuable use as a
general-contact killer. I n California it has been used to kill full grown
pigweeds, radish, and Chinese lettuce in ripe grain to facilitate harvest
by combining. It is being widely used in the Imperial Valley to kill
weeds on ditch banks. And, odd as it may seem, it has proved effective
in killing the leaves and stems of cotton preparatory to harvest with
2. #elective Contact Spray
The outstanding application of 2,4-D has been as a selective spray
in cereal crops. Here the relatively low susceptibility of t8hegrass plants
is made use of in the control largely of broad-leaved plants. I n 1948,
many million acres of cereal crops have been treated (Anonymous,
194%). Application has been by ground sprayer and by airplane. Ester
and amine salt formulations have been predominant mainly because of
convenience; sodium and ammonium salts have been used in lesser quantities.
I n addition to the cereal crops, wheat, barley, and oats, corn has been
sprayed to the extent of over one million acres (Lee, 1948a, b; Stahler,
1948; Willard, 1948), rice t o several hundred thousand acres (Anonymous,
1948f; Brown, 1947a; Higgins, 1947; Tullis, 1948), sugar cane t o a like
extent (Brown, 1947b; Hance, 1948a, b; Hanson, 1948; Nolla, 1948; Shaw
et al., 1947; Van Overbeek and Velez, 1946a, b ; White and Mangual,
1948; White and Villafane, 1946), milo to many thousands, and flax has
been treated on a considarable acreage. Grass seed crops have been
profitably sprayed (Anonymous, 1947a, d ; Marth and Mitchell, 1946a ;
Marth et al., 1947), and many thousands of acres of pastures and range
A. S. CRAFTS AND
have been treated (Kephart and Evans, 1948; North Central Weed Control Conf., 1947; Savage and Costello, 1948; Savage et al., 1948). Lawns
and fairways have been successfully weeded (Ahlgren and Cox, 1947;
Marth and Mitchell, 1946a, b, 1947; Mitchell and Marth, 1947). Experimental work has been done on potatoes (Ennis et al., 1946; Smith e t
al., 1947), asparagus, strawberries (Carlson, 1947a), and a number of
3. Translocated Spray
Probably the most remarkable characteristic of 2,4-D is the fact that
it is readily translocated in plants. Being relatively nonpolar, i t is
absorbed through the cuticle of leaves; the acid and ester forms are most
readily taken in. Arriving in the leaf, this chemical is apparently subject
to the same mechanisms of transport as auxin, namely polar transport
in parenchyma cells and rapid movement in the assimilate stream along
with organic foods. Literature on such transport is reviewed by Crafts
(1938, 1939a) and Crafts e t al. (1949).
Polar 2,4-D movement shows a positive geotropic pattern; taking
place through living parenchyma cells, it probably accounts for the complete destruction of the leaves and stems of annual weeds when application is by dust to localized areas or by low-volume application to a
relatively small portion of the total surface of the plant.
Transport of 2,4-D with the assimilate stream accounts for the movement of this toxicant deep into the root systems of perennial weeds. The
widespread and generally successful control of wild morning-glory, poison
oak, sage brush, and a host of other perennial weeds depends upon this
movement from the tops of the treated plants into the root systems
underground. Killing of the roots of wild morning-glory to depths of
15 or 20 feet has been noted on deep alluvial soils of California and other
states. The observation that the vertical tap root is often more completely killed than are lateral roots is probably a manifestation of polar
movement; geotropic movement in parenchyma cells would result in
accumulation in the cortex of the first few centimeters of a lateral and
the remainder of the root would not be killed.
Experiments by Mitchell and Brown (1946), Weaver and DeRose
(1946), and Rice (1948) have shown that 2,4-D transport takes place
most rapidly and effectively in plants in which photosynthesis and food
movement are going on. Evidently 2,4-D is carried along with foods
in the phloem of plants and attains a similar distribution in the roots.
It has been a matter of some confusion to plant physiologists that
2,4-D gives the most satisfactory root kills on wild morning-glory, hoary
cress, chicory, artichoke thistle, St. Johnswort, and similar perennial
weeds if applied just before or during the early stages of blossoming,
whereas maximum translocation and storage of food is known to occur
somewhat later. The answer lies in the &ate of the roots at this time.
Van Overbeek (1947) has pointed out that 2,4-D accumulates in and
effectively kills meristematic tissues. During the earlier growth stages
the roots of perennials are still meristemat,ic and are using assimilated
foods in the production‘of new tissue. Later, the bulk of the assimilates
is accumulated in mature storage parenchyma, cells that can absorb
2,4-D (Mitchell e t al., 1947) but t,hat do not respond by rapid death as
does meristem. Hence, during the early blossoming period 2,4-D absorbed by leaves is rapidly taken to the meristems of roots and shoots,
later it is taken t o storage parenchyma where it has less killing action.
Although t.he translocation of 2,4-D into the roots is essential to the
killing of perennial weeds, a similar transport in annuals probably explains the extreme effectiveness of this chemical as a selective spray.
The control of many aquatic weeds such as Alisrna and Sagittaria species
in rice, nutgrass, t.ules, and cattails in wet lands and ditches, water
hyacinth, yellow primrose, and alligator weed in waterways, and a number of others illustrates further the wide herbicidal usefulness of 2,4-D.
These instances all involve translocation, though to a lesser extent than
in wild morning-glory.
The successful control of brushy and woody plants on pastures and
in right-of-way maintainance is of great credit t o 2,4-D (Anonymous,
1946; Ashbaugh, 1948; Barrons and Coulter, 1948; Hamner and Tukey,
1946; Savage and Costello, 1948; Savage et al., 1948). Successful treatment of such species during active growth undoubtedly results from
transport of the 2,4-D with the assimilate st,ream. The recent finding
that many such plants also respond to treatment with the esters of 2,4-D
in oil during the dormant period (Barrons and Coulter, 1948) indicates
some other mechanism of movement, possibly slow cell-to-cell transport
in living parenchyma.
4. Temporary Soil Sterilization
Although 2,4-D is outstanding in the uses noted above, probably its
unique role is that of a temporary soil “sterilant.” Such compounds do
not sterilize the soil in the sense that all biological life is killed. The
word “sterilant” is employed by weed men for compounds that prevent
germination of seeds or establishment of seedlings. I n contrast to arsenic,
borax, chlorate, and others of the older sterilants that are required in
amounts of hundreds of pounds per acre (Crafts, 1935, 1936; 1939c;
Crafts et al., 1941), 2,4-D performs many useful functions when applied
at rates of 1to 3 or 5 Ibs. per acre. This depends on two facts: (1) 2,4-D
A. 8. CRAFTS AND W. A.' HARVEY
is not strongly fixed nor rapidly altered by contact with the soil (DeRose,
1946; Hanks, 1946; Kries, 1947; Taylor, 1947) ; (2) it is from 10 to 100
times as toxic to roots as to the tops of the plants. For these reasons it
will kill weed seedlings in concentrations of a few parts per million in the
soil. The same relatively high tolerance shown by the tops of grasa
plants is also present in the roots (Mitchell et al., 1947). Methoxone
(4 chloro-2 methyl phenoxyacetic acid) and 2,4-D have been tested by
the British workers (Nutman et al., 1945; Slade et al., 1945) and shown
to be effective when applied through the soil (see also DeRose, 1946).
I n contrast with resu1t.s in the eastern states and Britain, California
farmers have experienced deleterious effects from 2,4-D residues in the
soil (Crafts, 1946a, 1949). Studies have shown that decomposition of
2,4-D takes place most rapidly in warm, moist soils of acid reaction.
Seldom are all of these factors favorable for breakdown in California
soils a t any one time (DeRose, 1946; DeRose and Newman, 1947; Kries,
1947; Mitchell, 1948; Taylor, 1947).
Because 2,4-D breaks down too rapidly in soils to serve as a means
for controlling deep-rooted perennials by soil applications, its most logical
role is that of a selective herbicide. There are 3 general methods for
using 2,4-D as a selective temporary soil sterilant: (1) as a preplanting
treatment to provide a weed-free seedbed; (2) as a preemergence treatment to provide a weed-free soil medium, and (3) as a postemergence
treatment to eliminate weeds that have come up with the crop (Norman,
Where soil and climatic conditions favor rapid 2,4-D breakdown it
is possible that this chemical might be used as a general preplanting
treatment for all crops, including 2,4-D susceptible ones; however, since
a period of 4 to 6 weeks is usually required to free the soil of 2,4-D, the
most logical use would be as a selective treatment in tolerant crops. One
obvious advantage of the soil treatment method is the possibility of controlling grass seedlings that would not succumb to contact spray treatment (Anderson and Ahlgren, 1947; Mitchell and Brown, 1946).
Considering the use of 2,4-D as a soil treatment, there are 4 rather
distinct climatological conditions that might determine its effectiveness
(Crafts, 1948b): (1) no rainfall, that would result in failure because the
chemical would not be washed into.the soil around the roots of the weeds;
(2) very light rainfall or foggy drizzly weather, that might cause serious
injury because the chemical would be localized a t the soil surface in high
concentration; (3) moderate rainfall, that should be most favorable for
selective action, and (4) heavy rainfall or flood t.hat would leach the
chemical excessively and allow the weeds to survive. Results of trials
have shown that the soluble salts of 2,4-D are most effective during
periods of light rainfall; 2,4-D acid may be used when heavy rainfall i s
anticipated (Anderson, 1948; Crafts, 1948b).
Preemergence treatment with 2,4-D has tremendous possibilities for
reducing the labor of tillage so commonly practiced in row crop agriculture. Not only will it lower costs and reduce drudgery, it will minimize
cultural operations that tend to break down soil colloids and render soils
impervious to water and air. Under some conditions it will appreciably
increase yields. It may also be used to control weeds in a mulch-covered
soil so that wind and water erosion are minimized.
6. Permanent Soil Sterilization
Because of its tendency to leach and to break down in soils, 2,4-D
offers little promise as a permanent soil sterilant. Possibly in desert
regions it might be used around power poles or on fences and around
structures. In agricultural areas the hazards from leaching into cropped
areas or of being blown on dust are too great. Permanent soil sterilization is apparently the least useful category into which 2,4-D may fall;
arsenic, chlorate, and borax would seem to be more logical materials to
use. However, 2,4-D may be useful to kill deep-rooted perennial weeds
growing in sterilized soil that are not killed by the shallow chemical
treatment of permanent soil sterilants.
XI. NITRO-AND CHLORO-SUBSTITUTED
Sodium dinitro-ortho cresylate (Sinox) was introduced into the
United States in 1935 and soon became the most widely used selective
spray (Westgate and Raynor, 1940). Iron sulfate and sulfuric acid
sprays were displaced and selective weed killing was introduced in a
number of new crops such as flax, peas, onions, corn, alfalfa, lawns, etc.
Within a short period the use of acid salts to increase toxicity (activation) was introduced and thousands of acres of crops were sprayed
annually. Instead of the injury so often experienced with sulfuric acid,
Sinox proved beneficial to many crops and though occasional burning
of leaf tips occurred, in most instances the crops were actually stimulated.
This was particularly true of small grains the yields of which were often
significantly increased over and above those resulting from elimination
of weeds (Westgate and Raynor, 1940).
Following chemical studies the mechanism of Sinox activation was
described (Crafts and Reiber, 1945) and the suggestion was made that
the ammonium salt be used instead of the sodium salt. Further studies
along this line proved that the parent phenols in oils could be used as
general conttact sprays, that emulsions of such fortified oils were efficient
and inexpensive herbicides both for pre- and postemergence treatment.,
A. S. CRAFTS AND W. A. HARVEY
and that pentachlorophenol and its salts could be used as direct substitutes for the dinitrocresols (Barrons, 1947; Crafts, 1945b, 1947b, 1948a,
b; Crafts and Reiber, 1945).
Because dinitrocresol proved more toxic than dinitropheno1, screening
tests were made on the ethyl, propyl, butyl, and amyl substituted phenols.
These proved that toxicity increased up to the butyl substitution, t,hat
the ortho compounds were more toxic than the meta or para compounds,
and that dinitro substitution was more effective than mononitro (Crafts,
1945b). With chloro substitutions on the phenol ring, toxicity increased
regularly to the penta compound.
At present the ammonium salt of dinitro secondary butyl phenol is
being used extensively as a selective herbicide (Dow Selective, Sinox W) .
Dinitro-secondary butyl phenol (Dow General) and amyl phenol (Sinox
General) are widely used to fortify oil sprays and to prepare fortified
oil emulsion sprays for general contact weed control, for potato top killing (Anonymous, 1947b) and as a selective spray in sugar cane (Crafts,
1948a, b; Crafts and Emanuelli, 1948; White and Mangual, 1948), corn,
milo, and similar crops (Crafts, 1948b) by selective placement of the
spray. Pentachlorophenol has recently been introduced as an oil fortifier
(Anonymous, 1948e; Crafts, 1947b, 1948a, b; Crafts and Emanuelli, 1948;
Crafts and Reiber, 1945, 1948; Hance, 1948a). Sodium pentachlorophenate is used in large quantities as a preemergence treatment and as a
selective soil sterilant in pineapples in Hawaii (Anonymous, 1948m).
There are still many opportunities for introduction of nitro-, chloro-,
and nitro-chloro substituted phenols as selective and general contact
herbicides as relatively few of such compounds have been tested as to
their special selectivities.
Combinations of nitro- and chlorophenol contact herbicides with 2,4-D
have proved very effective in certain situations where weeds susceptible
to both types of toxicants are present. Work on such combinations is
presented by White and Villafane (1946), Mangual (1948), White and
Mangual (1948), Crafts (1948a, b), Nolla (1948), and Hance (1948).
Petroleum fractions have long been used as weed killers. At first
waste products such as acid sludge, waste engine oil, and similar materials
were used; later smudge pot oil, Edeleanu extract, and diesel fuel were
adopted (Robbins et al., 1942). More recently stove oil has been used
as a selective weed killer in crops of the carrot family (Crafts, 1947a;
Crafts and Reiber, 1944, Grigsby, 1946; Lachman, 1945; Raynor, 1943;
Sweet, 1945; Sweet e t al., 1944, 1945; Warren, 1946; and Warren and
Hanning, 19461, and since actual research on the herbicidal properties