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WHAT WE KNOW ABOUT
THE usE oF WETTING AGENTS WITH '.H:.itaBrcrpEs
Virgil H. Freed
Associate Chemist
Oregon State College
It is a privilege to be able to prepare and present for your consider.ation
the information in this paper. Through the years the California State Weed
Conference has come to be one of the outstanding meetings of th.is type, so
that it is a pleasure to be invited to participate in your program.
Today we are attempting to answer the question, "What do we.knowabou:t
the use of wetting agents with herbicides?"· Dr. Crafts and Dr. Currier have
some of the more recent findings of botanical nature concerning the role of
surfactants in absorption. Our interest in this particular paper will have to·
do with some of the chemical aspects of the problem.
In order to answer our question of "What do we know about the use of
wetting agents with herbicides, 11 let us find out what we mean by this questiorJ..
It seems to me that basically we are asking of what value are surfactants in
chemical weed control. Since our interest in the subject is of rather recent
vintage, we are unfortunately not in a position to give a very definite answer.
However, with the vigorous interest in absorption of herbicides by plants~ it
will not be long until we know a great deal more about the problem. Obviously our interest in studying absorption and the role of surfactants in this
process has a two-fold purpose. First, we are interested in the purely
scientific aspects of the problem - wanting to know just how a plant absorbs
and translocates chemicals. The second aspect of this problem has a little
more practical nature, £or it is here th.at we want to know more about this
process in order to turn it to our own advantage. In other words,. we want
to know about this process in order to improve formulations of our chemicals
to obtain the maximum benefit from their use. This desire means, of course,
that we will be looking for the best surfactant to go with any particular
herbicide.
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One of the basic assumptions we make in believing that surfactants increase the effectiveness of herbicides is that the surfactant increases the
amount of chemical absorbed. We believe that if an increased amount of the
chemical gets into the plant, that we will get better results. Let us attempt
to answer the question, then, of whether or not th.is is actually the case if
we use a surfactant with a herbicide. Does more of the chemical get into
the plant? Let us quickly look at three different slides of data obtained in
our studies trying to answer this question.
The first slide shows the amount of 2, 4-D absorbed with and without sur=
factant (see slide 1, p.47). The.second slide shows the effect of surfactant
on the absorption of amitrol, or ATA (see slide 2, p. 47), and the third
slide shows the amount of indoleacetic acid taken in over a six-day period
with and without surfactant { see slide 3, p. 4 7 ) •
Now, I want you to remember carefully what you saw in the last slide
concerning the absorption of indoleacetic acid. Note that on the first day
there was an increase of over 15% more absorption in the presence of surfactant, and that at the end of six days there was a difference of 49%, Keep
these figures in mind because we want to talk a little bit more about them
44
later.
Now, the next question to be answered is whether or not increased absorption is of any real value in helping to kill plants with chemicals. I
believe that we can answer this question in the affirmative on the basis of
the experience of a number of people who have added surfactants to herbicides,
and reported materially increased effectiveness in the control of plants.
Another link in the chain of evidence to support the belief that increased absorption also increases the kill is our experience with adding wetting agents
to dinitrophenols or to 2, 4-D. We know that if we add a wetting agent to
dinitros used as selective herbicides we lose the selectivity. In other words,
the crop is seriously damaged by this mixture, where it ,would not be
damaged if we used the dinitro salt without the surfactant. In spraying flax
in the midwest, it was found that broad-leafed weeds were controlled with the
sodium salt of 2, 4-D, but when a wetting agent was added severe damage to
the flax resulted. These facts all illustrate that the increased absorption of
the herbicide, results in more toxicity to the plant.
I am sure that at least some of you feel that we are belaboring a point
and that it should be perfectly obvious that if you get more chemical into a
plant that you are going to get a better kill. It simply follows from the increased degree of kill that we get by increasing the amount of chemical we
apply. However, remember believing something to be true and actually
proving it may be two different things. In this case, we want'.:to establish
proof in order that we can move on to answer other questions. Now as a
clincher to our argument as to whether the increased absorption results in
an increased kill, let us look at another slide showing the increase in effectiveness of sodium chlorate upon addition of a wetting agent ( see slide 4,
p. 48 ). This data would seem to establish quite reasonably that the increased absorption brought about by addition of a surfactant to a herbicide
does indeed increase the effectiveness of the treatment.
I think at this point, however, we must face the dilemma brought on by
the experience of many people of having added a surfactant to a herbicide
without finding a material increase in effectiveness. Unless a solution is
found to this particular dilemma we will still have skeptics insisting that
surfactants are of .little value with herbicides. Such an attitude could do
much harm to the securing of support for continued studies along this line.
The cases where surfactants have not given an appreciable increase in
kill that have come to my attention usually have been under favorable growing
conditions and a heavy application of the chemical. Both of these situations
would tend to nullify the effect or gain achieved through the addition of a surfactant. The addition of a surfactant to increase the effectiveness of a
herbicide will produce an advantage under one of three conditions. They are
(a) growing conditions or weather conditions are adverse following application, (b) the dosage applied is low, or (c) the plant itself is difficult to wet.
I think in many cases where surfactants have not given decided help in
killing plants, you will find one or more of those conditions violated. I
think, too, we ought to call attention to the fact that when you have a formulation already containing a lot of surfactant, as with 2, 4-D esters, the addition
of further surfactant is of little value. There is a limit in what these surfactants do in lowering the surface tension of water and increasing the
wettability. Let me illustrate this last remark by a slide that I have. This
slide is a graph of the fall of surface tension of water with increasing amounts
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of wetting agent ( see slide 5, p. 48). Actually, one of the things that a surfactant does when added to water is to lower the tension of the water and increase its wettability, so that the smaller the figure, the more effective has
been the surfactant. You will note from this slide that there is a minimum
point at a certain concentration of the surfactant. If you continue to add surfactant there is no continuing lowering of the surface tension, and indeed when
you get excessive amounts of surfactant in solution, the surface tension tends
to go up a little bit. This clearly proves that there is an optimum amount of
surfactant to be added and that there is no point in adding any further amount.
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At this point, we are ready to ask what are some of the ways the _increased
absorption brought about by the surfactant is helpful to us. First of all, we
see that surfactants increase the effectiveness of the herbicides as a result
of the increased absorption. This result was shown, as you will remember,
by the slide on sodium chlorate. Secondly, it helps by increasing the wetting
by the spray solution of hard-to-wet plants. Thus more chemical will remain on the plant, and there will be a bigger area' of the plant wetted, and
consequently a larger area of contact between the spray and the plant, resulting in absorption of more chemical. Third, when plants are growing
under adverse conditions they develop rather thick cuticles and may not.have
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their stoma open to absorb. The surfactant not only wets this cuticle, but
actually reacts with it to soften and open it up to the pen~tration of the
herbicide. Fourth, we find the conditions of the field or certain reactions of
the chemical may operate against us in absorption and killing the plant. For
example, the chemical may be broken down by the ultra violet rays of the sun
and speedy absorption is necessary. Again, the chemical may be volatile
and if it isn't absorbed within a few hours the chemical is lost from the plant
surface. Or' the chemical may be applied under very low humidity conditions,
and the water used as a carrier may evaporate, leaving a deposit of the dry
crystalline chemical. We know from past studies that a chemical is absorbed by the plant far more readily when it is present as a liquid than when
it is there as a crystalline product. This fact can be illustrated very simply
by the comparative effectiveness of a spray of 2, 4-D as contrasted to the
same chemical formulation applied as a dust. I don 1 t know how many of you
have ever made this comparison, but having made it myself, let me assure
you there is a world of difference, mostly in favor of the spray application.
Finally, we have the case of rain following a few hours after an application
and washing the chemical off. Now, these are all conditions where the in_
creased rate of absorption brought on by the presence of a surfactant coupled·
with the greater total amount absorbed in any given time definitely establishes the value of the use of a surfactant with herbicides. A wetting
agent or surfactant in effe~t is an insurance policy to guarantee maximum
effectiveness under these conditions.
Since we can agree then that the use of a surfactant is valuable to
herbicide applications, let us then turn our attention to the surfactants themselves. The first thing we note is that there are a large number of sur- _
factants. The question is naturally going to arise as to whether .there is
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enough difference between these surfactants to justify this number. In other
words, are all wetting agents about equally effective? The answer to this
question is a resounding NO. Although there have been very few studies
made comparing different surfactants in h~rbicide applications, it is already
apparent that there are tremendous differences between surfactants in .their
promotion of absorption, and that no single surfactant is going to be the right
one for all problems. To illustrate the difference in surfactants, let us look
at another slide, wherein there :i.s presented a comparison 0£ the surface
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tension of l /10th of l % of a solution of four different surfactants, and then the
time in seconds required for the solution to wet a skein of yarn ( see slide 6,
p. 48 ), This wetting of yarn, or the time required £or this skein of yarn to
sink the solution, is known as the Draves test. This is a test that is used extensively by the textile industries to measure the value of surfactants for·
their particular purposes. It is of interest at this point to note that a recent
study in England revealed that the Draves Test is probably one of the best
laboratory tests to correlate with the ability of the surfactant to increase the
wettability of spray solutions.
At this point, you may say, this is all very well and good. Yo~ 1ve been
ab1e to show the differences in surfactants on the basis of laboratory tests.
but not out on a plant. What is the situation there? Let us look at a few
slides of data showing the difference in surfactants on plants,
First, we have a slide where ATA was mixed with three different surfactants at . 05 of 1% (see slide 7, p. 48 ). The surface tension of these solutions was measured and the amount of chemical absorbed in 24 hours determined. You will have noticed in this slide that the surfactant X-?7 with the
lowes.t surface tension gave the greatest absorption, but the middle two surfactants having reasonably near the same surface tension - the ones with the
highest surface tension, actually gave the greatest absorption. This result
let us to conduct the next test where the same wetting agents were taken and
the concentration adjusted until we had very nearly the same surface tension
(see slide 8, p. 49 ). You. will note from this slide that even though surface
tension of ttle solution was very nearly the same, one surfactant was outstandingly more effective in promoting absorption than the others. At this
pofot, you may be thinking that this is all very fine. You have shown differences between surfactants in the case of one material, but how about the
case with other materials of different chemical composition. Take a look,
if you will, at a slide covering some data from one of our more recent investigations ( see slide 9 • p. 49). This work has been concerned with the
study of some of the chemical problems involved in role of surfactants in absorption, In this instance we are using indoleacetic acid as the absorbed substance. Many different surfactants were used. One was a non-ionic surfactant; one was an anionic surfactant; and one was a mixture of non-ionic
and anionic surfactants. You will note that the mixed surfactants gave the
lowest surface tension. But the best absorption was obtained from the surfactant of highest surface tension. Obviously, there is something more
subtle than mere reduction of surface tension involved in promotion of absorption by the surfactants. We have reason to believe from some of the
studies that we have carried out recently that not only does a surfactant increase the contact between the spray and the plant, but it softens the cuticle
and opens it up £or entry of the chemicals, and that it also conditions the
chemical physically to keep it in a state from which it will be absorbed most
readily by the plant.
In concluding the talk, perhaps we should review just what ground we have
covered in order to determine where we can go from here. On the basis of
the information available at present, we are able to make the following statements:
1. Surfactants definitely promote absorption of herbicides by the plant.
They do this not only by increasing the rate at which herbicides are absorbed,
but also by increasing the total amount absorbed.
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2. Increased absorption produced by a surfactant results in greater
effectiveness of an herbicide; if absorption is not limited, a surfactant insures greater uniformity of kill. The surfactant can serve as insurance for
almost every situation. It i~sures against_ loss due to chemical breakdown,
wind, or water.
3. We have established that surface tension alone is not the best criterion given us of a surfactant. The interaction of a surfactant with a herbicide is more subtle than that.
4. We have shown that not all surfactants are equally good with different materials. It definitely appears that a particular surfactant may be
good with a given chemical, but not with other materials.
We are prepared, therefore, to say with complete assurance that the use
of a surfactant is justified in herbicide applications.
Absorption of 2, 4-D in
Presence of Surfactant
Slide 1.
% Surfactant
% 2, 4-D Absorbed
o.o
o. 1
10. 1
36.5
38.8
1. 0
Slide 2.
Absorption of ATA in
Presence of Surfactant
Dose/
Plant
S. T.
.E!.1
% Absorbed
in 11 7 hrs.
ATA
250
69.6
5.87
55.4
ATA plus
O.lo/oX-77
250
33.3
5.82
87.3
Solution
Slide 3.
Influence of Surfactant on the
Absorption of Indole-3-Acetic (IAA)
l day
% Absorbed in
4 days
2 days
6 days
lAA
34
30
42
45
!AA plus O. 1%
Surfactant
49
60
87
94
48
Slide 4.
Effectiveness of Sodium Chlorate
at 40/lbs /Acre
Solution
S. T.
% Kill
Sodium Chlorate
70.6
10
Sodium Chlorate plus
0. 1 % Multifilm L
33.9
52. 5
Slide 5.
Surface Tension Lowering Limited
S. T.
dynes/Cm
. o/1
o
Slide 6.
·----···"""'""""""''"
.
by Wt. Surfactant
Variation of Wettability
Wetting
Agent
S. T.
#1
33.5
39.
47.
62.
2
3
4
Slide 7.
Time Required
To Wet·
Sec. ,·
20
30
15
35
Effect of Various Surfactants
on ATA Absorption
Solution
ATA
ATA plus . 05%
Multi. C
ATA plus • 05%.
Multi. L
ATA plus . 05%
X-77
% Absorbed
Dose/
Plant
S.T.
250
69.6
5.87
13.1
250
50.3
5.90
18.8
250
59.1
6.40
25.5
250
33.3
5.82
77.8
Ett
in 24 hrs.
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Slide 8.
Effect of Different Surfactants at the
Same .Surface Tension on Foliar
·
Absorption of Amitrol
Surface
Tension
(dynes/cm)
Solution
Amitrol
Amitrol plus
Mult. C (. 1%)
Amitrol plus
Mu.lt. L (. 5%)
Amitrol plus
X-77 ( .. 013%)
Slide .9.
Pct. Amitrol
absorbed
24 hrs.
69.6
12.2
49. 8
16. 7
47.5
37. 2 ·
50.9
51. 0
Comparison of T.hree Surfactants
on Absorption of IAA
% Absorbed in
Surface
Combination
Tension
4 days.·
2 days
1
69.2
IAA
42
dynes/.cm
39
IAA plus O. l %
58.5
Sur.£. A
60
· (nonionic)
87
dynes/cm
IAA plub O. 1%
Surf. B
34.8
(Mixed .non78
ionic anionic)
64
dynes/cm
IAA plus O. l %
57.8
· Surf. c.
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15
(anionic)
dynes/cm
SURFACTANTS
Alden S. Crafts
Botanist
University of California
When I view the herbicide picture today l feel like Pm spread too widely
trying to cover· it all. It is getting so broad that no one can cover the whole
field; each expert is going to have to select out his own little area and
specialize in that as is done in many other fields. It is nice to be. back to the
California Weed Conference again after a year spent in merry old· England
and I bring you greetings from that countl".y. Some of you fellows have ·undoubtedly been there. We enjoyed the year very much. It was .a year away
from the cares and obligations of a department job, away from teaching, ·
telephone, committees, and all .of those things which break in on your
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