Application of Herbicides

(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.