“Many farmers are unaware of their seed bank. In fact, many of them do not even assess the effectiveness of the control measures…”
Arthur Arrobas is a professor at the Federal University of Paraná, has a degree in agronomic engineering, an M.Sc. from Esalq/USP and a Ph.D. from Unesp in Jaboticabal.
AgriBrasilis – What is the connection between the soil seed bank and weed control?
Arthur Arrobas – There are several sources of weed infestations. One of them is the introduction of seeds or plant propagules through the contamination of equipment, such as harvesters or seeders. Another example is the presence of seeds already in the soil of a given area.
Agricultural areas often have soils with a high quantity of seeds and other plant reproductive structures. The larger and more diverse this seed bank is, the greater the challenge of controlling a weed community.
Some species in the seed bank will have a short lifespan, while others will have a long lifespan. Some seeds germinate throughout the entire soil profile, while others germinate only in the upper layers. Certain species may increase their germination rates when the soil is disturbed, and so on. These are some examples that highlight why farmers need to choose weed control methods carefully.
“The larger and more diverse this seed bank is, the greater the challenge of controlling a weed community…”
AgriBrasilis – Is it possible to eliminate all the plants present in a seed bank?
Arthur Arrobas – Complete elimination in large areas is impractical. Eradication can be done, for example, in small volumes, such as substrates used in vegetable gardens.
What can and should be done is to work on reducing the seed bank in an area and prevent the introduction of new species. It is also important to control weeds before they reproduce, preventing the work of reducing the seed bank, carried out over several years, from being lost.
AgriBrasilis – Why do not most farmers adopt seed bank evaluation?
Arthur Arrobas – Many farmers are unaware of their seed bank. In fact, many of them do not even assess the effectiveness of the control measures used over one or more seasons. They may know the weed species in their area, but they do not know the intensity of their presence. This is because evaluating a seed bank is a labor-intensive task.
Seeds are typically dispersed unevenly across the area, at varying soil depths. Identifying seeds in a soil sample is a complex process, and only well-trained individuals can perform this work. I believe this should be addressed of integrated weed management programs, where, with a soil auger, the farmer can collect samples from different points in the field, form composite samples, and place the soil to germinate in a tray or pot inside a greenhouse or near the farm headquarters.
Based on the soil samples, the farmer can identify the emerging plants, as well as when and at what intensity they emerge. Over time, this information can guide the choice of products, dosages, and adoption of other control methods. I believe that in the near future, this could also assist in the selection of biological products and production systems.
AgriBrasilis – What practices should be adopted to reduce the pressure of the seed bank and minimize the emergence of resistant plants?
Arthur Arrobas – All available practices should be adopted. Perhaps the most important is cultural control, which provides the best conditions for a crop to reach its maximum yield potential. An example of integrated crop-livestock systems is the intercropping of corn or soybeans with forage species. In pastures, it is important to use the appropriate grazing intensity, such as adopting rotational grazing.
Another important practice is the proper selection of cultivars or hybrids to be used. Other factors include planting at the right time and, most importantly, adopting no-till farming, where the crop residue from the previous harvest will form soil cover that, through physical, chemical, and biological processes, will help reduce the presence of weeds.
We should not forget other control methods, such as chemical control, which is the most commonly used. Even with resistant plants, we can minimize the emergence of resistant species. One example is the use of pre-emergence herbicides, particularly mixtures of different modes of action and molecules, reducing the likelihood of selecting resistant weeds and increasing the chances of successfully controlling existing resistant populations.
Regarding chemical control, it is essential to control weeds that have “escaped” prior to their reproduction. Often, post-emergence control becomes complicated due to factors such as crop selectivity or the size of the plants. Different herbicide-resistant crop technologies can be adopted, giving farmers more herbicide options. In the case of plants that are well beyond the control stage, some areas have resorted to manual weeding, for example.
When we talk about manual weeding as a control method, we highlight that it can also be done before planting a crop, either in a planting area to prevent plant reproduction or before applying herbicides, increasing control effectiveness, as in the case of sourgrass and the use of grass herbicides.
Another trend, which we have studied at the Federal University of Paraná – UFPR in Curitiba along with the Weed Research Network, is the development of more resilient systems to the presence of weeds and their impact on the selection of resistant species. Apparently, the more complex the system, the greater the diversity of microorganisms in the soil, which may contribute to faster degradation of the seeds present, functioning as a type of biological pre-emergence control of weeds. But I emphasize that this is something new, and different environments tend to alter these responses. We will have more data and information in the coming years.
AgriBrasilis – Why are ALS inhibitors so prone to the development of resistance?
Arthur Arrobas – ALS inhibitors are excellent herbicides that work effectively on plants even at low doses and have low toxicity to animals. One of the major issues is that the enzyme they target easily reduces its affinity for the herbicides through mutations in the base pairs that make up its gene.
Compared to other herbicides, such as glyphosate, the ALS enzyme has far more mutations that allow the plant to survive and become insensitive to the herbicides, compared to the EPSPS enzyme. Another factor is that the inheritance of resistance to ALS inhibitors is nuclear, meaning it can be transmitted through pollen or seeds, with partial dominance being enough to generate resistant individuals. Other herbicide mechanisms of action have fewer cases of resistance, such as tubulin synthesis inhibitors, which exhibit recessive inheritance.
In addition to changes at the herbicide’s site of action, there can also be modifications that reduce the absorption and/or translocation of herbicides. One of the worst examples is the metabolism of herbicides into less toxic compounds in weeds. Finally, the increased use of these herbicides has resulted in high selection pressure in species such as black nightshade, horseweed, milkweed, and wild mustard.
AgriBrasilis – You mentioned that it is necessary to choose herbicides with lower chances of developing resistance. What are these products?
Arthur Arrobas – We can use a database that records cases of resistance in Brazil and worldwide as a reference: https://www.weedscience.org/Home.aspx.
By examining the data frequency and number of cases, we can observe that the herbicide mechanisms with the fewest cases of resistance in Brazil are those inhibiting cellulose synthesis, phytoene desaturase, glutamine synthetase, carotenoid synthesis, very long-chain fatty acid synthesis, and microtubule synthesis inhibitors. It is interesting to note that several of these mechanisms involve herbicides registered for pre-emergence application, as mentioned in the previous section.
It is important to remember that this information is highly dynamic and depends on the evolution of weeds and the intensity of use of certain molecules. In five years, this information could be outdated.
AgriBrasilis – How to distinguish whether the control failure is caused by weed resistance or improper management?
Arthur Arrobas – This is a task that requires knowledge from the farmer or field technicians. First, other species can be observed after the application. Were these species controlled? If the product failed from one year to the next, for multiple species, in a single application, it is very likely there was an application failure.
Another point to consider is: what are the symptoms of phytotoxicity present in the weeds? Do these symptoms match what should be observed for the applied herbicide? For example, if we applied an auxinic herbicide, did the leaves or stems curl? Was there an epinasty?
Building on this observation, the analysis can be done within a single species. Do the plants of this species show symptoms across the entire population, or are some plants showing symptoms while others do not? Did these symptomatic plants receive the herbicide under the same climate conditions, dosage, and developmental stage? If so, it’s common for such variation to be present in cases of resistance that are in the process of selection. We should not confuse this with an ALS inhibitor application, for example, that fails to control all horseweed in an area, where selection pressure has been continuous over the years.
There are several other clues: is the control failure following a pattern, such as a clogged sprayer nozzle? Or is there a concentration of failures along the edges of the field? Are the failures distributed in patches or scattered across the production area? In the latter case, the chance of resistance is higher.
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