Herbicides: Scary and Not So Much

What are herbicides and why are they used at all?

Agriculture, like any other kind of business activity, has indicators that speak about its effectiveness. One of these indicators is the yield per unit area. The yield depends on many factors, including the efficiency of plants in absorbing nutrients and water from the soil. And here they have to compete with weeds, which must be eliminated if we want to obtain a real harvest, not just a meadow for grazing. No matter how good the bred, high-yield varieties of crops are, their genetic potential cannot be realized without effective weed control.

Essentially, herbicides are substances that kill grass that is not a crop, or, in other words, weeds (from the Latin words < > — grass, < > — to kill). The practice of applying these agents spans more than 70 years. The chemical method of weed control up to now relied on achievements of organic chemistry. The true revolution in the development of the chemical method of weed control began with the discovery of herbicidal activity in the first synthetic auxins (analogues of plant hormones). Today there are many of them, and they are fairly difficult to classify. Most often herbicides are classified by mechanism of action (MOA) (according to the Weed Science Society of America, 2003, this covers more than 27 classes of active substances).

How do herbicides act on plants?

The idea behind using herbicides is to very selectively interrupt a crucial step in metabolism that would lead to the death of the weed plant. Most often this is the inhibition of enzymes that are only present in plants. And therefore it makes their use safe for us. Among the most popular and well-studied are herbicides inhibitors of the enzyme acetolactate synthase (ALS), which is the first enzyme in the pathway that synthesizes in plants the branched-chain amino acids — valine, leucine, and isoleucine. We need indispensable amino acids that we cannot synthesize and must obtain from food. Plants, on the other hand, can synthesize these amino acids. And these herbicides from the ALS-inhibitor group block the synthesis of enzymes in this pathway and thus the weed dies.

Other possible herbicidal mechanisms include interference with the photosynthesis process. And since we are not capable of photosynthesis either, such substances are also safe for us.

So where does the selectivity of herbicides come from? Why do weeds die and the crop remains unharmed?

Differences in sensitivity among plant species to herbicides are related to anatomic–morphological differences of plants, their different detoxification capabilities, or the presence of various sensitivity variants of the same enzyme in different plants. That is, the herbicide either does not enter the crop plant at all, or it is quickly detoxified, or it has a insensitive form of the enzyme and therefore the crop is not damaged. Additionally, there is a factor called the half-life of herbicides in soil. Today’s requirements for producers are quite high — to have this period as short as possible.

Which crops in Ukraine need weed protection the most?

The leading crops in Ukraine’s agriculture are grains: winter and spring wheat, rye, winter and spring barley, corn, oats, buckwheat, millet, and rice. Crops differ in their competitiveness with weeds. For example, crops grown in full stands (wheat) are more competitive than row crops (corn). Modern herbicides are actually aimed at creating optimal conditions for the productive process of the crop. Due to the emergence of resistance (weed resistance to herbicides), it is very important that there are several weed-control options for a given crop. I must not fail to mention that my colleagues have worked very productively in this direction and described in detail the effectiveness of the combined action of different herbicide mixtures in one of the monographs.

Are herbicides really safe?

Each certified herbicide undergoes thorough testing before firms are allowed to sell them. In Ukraine this process is controlled by the Ministry of Ecology and Natural Resources of Ukraine. There are rules and standards for applying herbicides codified in law. (Law of Ukraine < > Article 18. Rights and duties of enterprises, institutions, organizations and citizens in the field of plant protection)

Safety depends indirectly on both compliance with the law and the purchase of truly certified herbicides, applying them in appropriate amounts and within appropriate time frames.

Here the analogy with antibiotics comes to mind. In competent hands, it is a weapon against deaths and diseases, but with uncontrolled use — resistance and danger. In addition, herbicides are applied mainly at early stages of plant development, so by harvest time the active substances have had time to degrade. If we must talk about hazards, for example, other pesticides, particularly insecticides, have a greater potential danger to us, because their targets are organisms closer to us, and with lower selectivity and possible overdoses the toxicity risk is much higher.

At the beginning of summer, everyone was shaken by news of pesticides entering the Ros’ River as a result of an accident. Water analysis showed that at the border of Vinnytsia and Kyiv regions there were no recorded exceedances of the maximum permissible concentration of chemical substances. But we must always remember about potential danger. In this regard I would like to recommend the article by nutritionist-dietitian Oksana Skitalinska Nutrition in case of pesticide danger entering the body, which was published immediately after this incident. These safety measures will be relevant in case of potential danger from herbicides as well.

Can herbicides be carcinogenic?

There is currently no convincing scientific data to support this. Although the potential danger associated with long-term effects of herbicides cannot be ruled out. But this also concerns many other substances and factors we use and encounter.

How are new herbicidal products developed?

It’s a long and expensive process. Chemists synthesize hundreds, thousands of new compounds. Then the full spectrum of their activities is studied — fungicidal, insecticidal, herbicidal, etc. Now such screening can be conducted only by absolute corporate giants. And these are three to four companies worldwide. The cost of developing one effective and relatively safe active substance is about 250 million euros and 10–15 years of work by many researchers.

What alternatives to using herbicides exist today?

Inspiration and hoeing. And don’t forget to protect your lower back! :) But there is one nuance. Some weeds, especially creeping couch grass, actually reproduce very well when hoeing. Indeed, if fields are large, it is quite difficult to cope without herbicides. However, it is possible. For example, supporters of organic farming believe in this. There, physical weed suppression methods are used. However, switching to organic farming is multi-step. On a large export-oriented agricultural operation, this may be difficult and economically unprofitable.

If investments permit, modern robotized devices equipped with weed-recognition cameras can also be used, which weed out weeds quite effectively. A convenient worker with a steel back, but expensive.

A rather interesting method of weed control is the use of electricity. The principle of the electric method is that between the positive and negative electrodes a high-voltage electric field is created, using a voltage and frequency at which plasmalysis occurs in the weeds and they die, while crops are not harmed.

I personally really like the biological method of weed control. It is based on using allelopathically active compounds (allelopathy — mutual influence of one plant on another through substances of various nature), phytopathogens, or toxins that they themselves produce to destroy weeds. However, so far the effectiveness of these compounds cannot match the efficiency and economic viability of modern herbicides. Thus, several grams of some ALS inhibitors can be sufficient for effective weed control over 1 hectare. Meanwhile, the ecological and toxicological risks associated with using phytopathogens are too high. Therefore, < > cannot yet compete with chemical methods. On the other hand, studying the phenomenon of allelopathy, as well as the mechanisms of phytotoxic activity of toxins produced by rhizobial bacteria and fungi, can be a source for creating new herbicides.

One of the modern strategies for creating new herbicides is the study and cloning of genes whose mutations are lethal or semi-lethal for plants. By sequencing, the nucleotide sequences of mutated genes and, accordingly, the amino acid composition of the peptides encoded by these genes are determined. Then ligands capable of binding these peptides and blocking the activity of corresponding enzymes are sought. It is expected that the effect of these ligands on plants will be equivalent to the consequences of a gene mutation, i.e., will lead to the death of weed plants. The first results and near-term prospects are well laid out in a fresh article by American colleagues.

Are modern herbicides being developed in Ukraine? What questions are Ukrainian scientists studying?

As already noted, this is very costly. Therefore most herbicides are still foreign. However, very slowly, there are Ukrainian studies in this sector. In our Department of Herbicide Physiology we study the effectiveness of interactions of herbicides in mixtures. The need for a holistic approach to herbicide use was first raised by our colleagues worldwide even long before weed resistance to monoprays appeared. The holistic approach has proven to be truly effective in this regard. After all, the list of effective and safe in all respects herbicides is not that long, and with long-term use of monoprays or antibiotics, resistance can develop over time.

The Main State Plant Protection Inspectorate of Ukraine annually issues a brochure-forecast, in which there is a table of the most effective mixtures recommended for the year, based on the Institute’s experiments. It is incredibly popular at agricultural exhibitions!

Beyond applied questions, we also study fundamental aspects. At this stage, for example, this is the study of the relationship between programmed cell death (PCD) and the phytotoxic action of herbicides. This process has long been studied in animal cells, which was the basis for the Nobel Prize in Physiology or Medicine in 2002.

What recommendations can you give for herbicide use?

First, buy only certified products from reliable suppliers and manufacturers, monitor expiration dates, and the storage and transport conditions. Consumers should also not be lazy about asking suppliers for the origin of products and accompanying documentation. Study the instructions on the packaging of each herbicide and choose the product that best solves the task on a given plot. Follow the recommended application rates and do not exceed them, because at higher doses the products can be toxic and have a harmful effect on the ecosystem as a whole. Also — ensure even application of products into the soil and carry this out in exactly recommended periods. Use personal protective equipment, as safety is discussed after all those half-lives, and contact and inhalation of these substances will not benefit health. Before treating objects located within or near settlements, farmers should install warning signs and inform people about the upcoming work and the periods during which staying in the treated area is prohibited.

Wishing you good yields and strong health! :)

Photo by Sweet Ice Cream Photography on Unsplash