Application of Herbicides...things to consider
(This article was originally printed in the Wisconsin
Woodland Owners Association magazine. It is reprinted
with permission. The author is Pat Schrader, a
retired biochemist with a long-standing interest in
forestry)
Though herbicides are useful tools, many people wonder
about the effect these chemicals may have on health.
Because the long-term effects of herbicide
exposure are largely unknown, there is general agreement
that applicators should strive to minimize contact.
Minimizing health risk by minimizing exposure hasn't
always been stressed as strongly as it is today. This
seems a little strange, since herbicides are intended to
kill plants, which are distant cousins of ours. Let's
look at an herbicide history and the development of
these powerful chemical combinations.
Herbicides - a look back
Early attempts at chemical control of weeds involved the
use of corrosive chemicals such as calcium cyanide,
copper sulfate, and sulfuric acid. Since
these have always been known as toxins, it's clear that
humans are willing to take health risks to control
weeds. The first organic herbicide, 4.6-dinitrocresol,
was used in France in 1932. This chemical, more than an
herbicide, was also a poison and it killed both plant
and animal. Auxin, indole-3-acetic acid was identified
as a plant growth hormone in 1932 and this became the
foundation for the modern herbicide industry.
The idea
that synthetic compounds structurally similar to auxin
could be used to kill plants was conceived independently
in England and in the USA in 1945. Actually, these
synthetics were developed for possible use as biological
warfare agents. Hostilities came to an end shortly
thereafter, so they were never used. Soon, these
synthetic chemicals became commercially attractive. They
were inexpensive to manufacture, relatively nontoxic to
mammals, and they effectively killed broad-leafed weeds
while sparing the corn crop. This quickly yielded a huge
profit that ultimately launched the herbicide industry.
Ever since herbicides became a profit source, there has
been an intense hunt for new weed killers. To date,
there are three approaches used to search for and
identify new herbicides. These three are random
screening, lead compound use, and rational design.
Random screening
In general, new herbicide classes are discovered by
random screening. In the lab, whole plants or plant cell
cultures are exposed to compounds that might have
phytotoxic properties. When a compound shows promise,
tests such as identifying animal toxicity on rats are
conducted. A recent industry development yields greater
understanding of just how the herbicide works by
allowing the investigation of action sites at the
molecular level of the compound.
This investigation is
new; the industry is getting smarter. In fact, some of
the early enthusiasm for the safety of herbicides was
based more on wishful thinking than on an actual
understanding of the chemicals. A neighbor of mine
recounted an incident from the 1960s in which he watched
a herbicide company representative drink a solution of atrazine to show how "harmless" it was.
Lead (rhymes with need) compound use Herbicides,
identified by random screening, are often used as lead
compounds. This means that chemicals with the same basic
structures, but with some modifications, are tested.
Again, the emphasis in the early days was on finding
chemicals that killed plants, but not on understanding
how they work.
Times have changed. The lead compound
approach sometimes results in identification of
substances that are more potent or
have a better range of specificity than the original
compound. In at least one instance, a lead compound
called glyphosphate (the active ingredient in
Round-Up), turned out to be the most effective
ingredient. Even small changes in a compound's structure
can result in a significant gain or loss of activity.
Rational design
The most interesting strategy, rational design of
chemicals, is in its infancy. This concept requires the
identification of a target molecule, most likely a
protein, and the development of a compound that
specifically binds to it, thus affecting its function.
This requires the scientist to have a complete
understanding of the target plant's molecular structure.
Rational chemicals are not yet available in agriculture.
However, results of the use of similar
agents in biomedicine, in the fight against AIDS, for
example, suggest the approach is valid. Plant
biochemistry is well behind medical research.
I believe that when rational herbicides become
available, they will still include warning labels. Even
the best of drugs can have side effects. Selectivity -
the Holy Grail of herbicide research Modern herbicides
work on the theory of selective killing. In this
respect, herbicides are much like antibiotics or the
inhibitor of the AIDS virus. The idea is to target
processes that do not occur in mammals.
Stimulation of
plant growth by auxin is a good example of a
plant-specific process. Humans don't have auxin
receptors so, ideally, the synthetic auxins used as
herbicides should have no effect on us. Photosynthesis
is another example. Many herbicides, such as simazine,
kill plants by inactivating a protein involved in the
process of capturing energy from sunlight. Other targets
are the synthesis of essential amino acids. These are
the amino acids we don't make ourselves, but instead get
by eating plant material. RoundUp works on one of these
pathways. If plants can't make a complete set of amino
acids, a number of important secondary products,
including proteins, can't be synthesized.
How dangerous is an herbicide?
This commonly asked question is so vague, it is not easy
to answer. First, each herbicide and its active
ingredient has to be considered separately.
Herbicides can't be lumped together. You have to ask the
question about each herbicide, or at the least, about
its chemical family. Fortunately, the acute effects of
herbicides are well known. If you're going to use one,
read the label carefully. They are written for the user
and generally spell out short-term risks. Also, ask the
seller for a Material Safety Data Sheet for the product.
It contains many details on toxicity and safe handling.
Questions about the long-term effects of herbicides on
health are another matter.
In truth, there is too little
known about how the human body functions to determine
long-term effects. In general, though, manufacturers and
experienced applicators agree that exposure to
herbicides should be minimized. Exposure while handling
concentrates is one of the greatest dangers of dealing
with an herbicide. If contact results in a skin rash or
eye irritation, this is an indication of general
toxicity. And it's known that this type of exposure can
affect the tissues of the lung, liver, or kidney.
Do not
sacrifice your safety to save a few minutes or
dollars in handling, mixing, or applying herbicides.
Selectivity and dosage The idea of selectivity in the
action of herbicides often involves a question of
amount. The right application in the right concentration
will kill the broad leaves and spare the crop plant. If
you use too much of the herbicide, the crop could be
damaged. I do wonder if the manufacturers' tests are run
with cleaner preparations than those sold to us. Do not
exceed recommended dosages.
How do I feel about herbicides?
I will use them. However, I will use them only when
there is no alternative and I'll be careful. My days of
procreation are over, but do I want my sons to help
me apply herbicides? No. I'll do it myself.
Finally, I have these recommendations:
1. minimize your exposure,
2. some suppliers provide training tapes, these could
be useful to you,
3. get some training in handling chemicals, and
4. ask the extension service if they provide
herbicide application training. |