Integrated management of resistant weeds

The emergence of herbicide-resistant populations is a worldwide phenomenon, which began in the 70s and now affects more than 250 species of weeds, causing financial losses of billions of dollars, constituting a serious threat to sustainable food production in the world. world. Only the rational use of herbicides, in a system where products with different mechanisms of action combined with different control tools are used, can allow currently available technologies not to be lost and thus change this scenario in favor of producers.

The scene is becoming more and more common. On the one hand, a producer concerned and dissatisfied with the herbicide that showed inefficient control of one or more species of weeds present in his crop. On the other hand, professionals from the public and private sector, consultants and experts in weed management, sometimes perplexed when explaining to the farmer why that product, which was traditionally efficient on a certain species, is suddenly no longer able to exert its effectiveness. control.  

Resistance or tolerance? It is important to define what herbicide resistance is, and how it differs from another important concept known as herbicide "tolerance." Resistance is defined as the ability of a plant, biotype or weed population to survive and reproduce after the application of a herbicide at a dose that is lethal to susceptible plants of the same species. More recently, it was defined that resistant biotypes must survive the herbicide, when used in full doses (100% of the recommended dose according to the leaflet), to eliminate supposedly resistant plants. Resistance refers to plants of a given species that are capable of behaving differently from the species, in this case surviving the application of doses that would be lethal to the species. The concept of tolerance to herbicides is related to the species' ability to survive the application of the herbicide, therefore being the response inherent to the species and not exclusive to some populations or biotypes. Populations of three species of horseweed (Conyza canadensis, C. sumatrensis, and C. bonariensis) resistant to the herbicide glyphosate, the active ingredient present in products such as Roundup, were found in crops in several Brazilian states. Horseweed control with this herbicide was efficient in these states, due to the predominance of susceptible populations. This example is therefore characterized as a case of resistance. On the other hand, there are weeds, such as trapoeraba (Commelina benghalensis) and morning glory (Ipomoea spp.), which, due to morphological and physiological factors, are difficult to control with the use of glyphosate. These species require high doses of herbicide and are characterized as species in which plant escape and lack of control are common. In this case, tolerance to glyphosate is something innate to the species, and is not a particularity of some biotypes or populations, as is the case with horseweed. 

Emergence and mechanisms of resistance

Unlike crops, weed populations have great genetic variability between them. In the same geographic area, the greater the genetic variability and the greater the density of the infestation, the greater the chances of resistance to a given herbicide or mechanisms of action emerging. The mechanism of action of a herbicide is the way in which the herbicide affects the plant at the histological or cellular level. As an example, there is the active ingredient glyphosate (present in a range of formulated products), which acts by inhibiting the enzyme EPSPs (enol-pyruvyl-shikimate-phosphate synthase). This enzyme is present in the chloroplast and acts on the shikimic acid route, with glyphosate being the only active ingredient that has this mechanism of action. Resistance to a mechanism of action, in a population or biotype, occurs in different ways. It can occur when the mutation occurs directly at the site of action of the herbicide (such as mutations in a target enzyme or overproduction of it), or when it acts on mechanisms outside the site of action. Mechanisms outside the site of action are varied and include important resistance mechanisms, such as accelerated metabolization of the herbicide in resistant populations (thus preventing the herbicide from accumulating at the site of action and reaching phytotoxic levels), and reduced translocation and absorption of the herbicide, through the leaf cuticle, among others. Elucidating the resistance mechanism in a given weed population is extremely important so that management measures can be defined. Management measures must take into account the physiology of the plant, as some resistant biotypes may present more than one resistance mechanism in the same plant, making it difficult to use other herbicides with the same mechanism of action, or even other mechanisms of action. .

Resistance is, therefore, the loss of effectiveness of a herbicide that occurs over time, due to the intense selection pressure exerted by the herbicide, through its repetitive use in the field. Herbicides are highly effective tools for weed control, generally providing control above 80%-90%. Thus, year after year, resistant plants manage to survive and reproduce, undergoing selection until the problem of resistance to herbicides in the crop becomes noticeable and is established. However, herbicide resistance is not the only problem that can lead to control failures after herbicide application. In reality, before the presence of resistant populations is suspected, all factors involved in application failures must be eliminated (Figure 1).

Control failures

How to distinguish between resistance and other factors

It is possible to divide the factors related to inefficient weed control by a herbicide into two categories: (i) problems with the application of the product, such as the use of clogged application tips, poorly calibrated equipment, spraying in inappropriate climatic situations (very high temperatures , low relative humidity, presence of strong winds, rain after application), or use of expired products; (ii) aspects related to the target - application carried out at an inappropriate weed growth stage (for example, application on a plant at a more advanced stage than that indicated for the product), or on plants under conditions of environmental stress. It must also be ensured that possible tank mixtures, which, although not permitted, are sometimes used, are not responsible for reducing the effectiveness of products, as cases of antagonism between products are common. The distribution characteristics of weeds are generally associated with the development of resistance to herbicides in a given crop (Table 1).

Current scenario in Brazil and the rest of the world

 To date, more than 450 cases of herbicide resistance have been reported worldwide, encompassing more than 250 invasive species in 86 countries, and affecting a total of 66 different crops (Table 2). The problem becomes clearer when analyzing the number of mechanisms of action (M.D.A) affected by resistance, 23 out of 26 existing. Even though new herbicides are constantly being launched on the market, the last mechanism of action was launched more than 20 years ago, indicating that the rate of launch of molecules with new mechanisms has not kept up with the rapid growth of cases of resistance. In Brazil, 37 cases of resistance have been reported, affecting 27 species in 8 different crops. Despite the favorable outlook, it should be mentioned that cases of resistance are not always reported by Brazilian researchers to the global database in the field (website www.weedscience.org) and these numbers may not represent a faithful portrayal of herbicide resistance problems in national agriculture.

In Brazil, some herbicide-resistant weeds have received a lot of attention due to the extreme difficulty in managing resistant populations by producers. This list includes the three species of fleabane mentioned above, which are aggressive competitors in soybean, corn and wheat crops. It is worth noting that resistance to glyphosate was recently developed, in addition to cases of resistance to multiple herbicides in C. sumatrensis. In addition to these, the species bidens pilosa e B. subalternans, commonly called picão-preto, and aggressive invaders in soybeans, have also developed resistance to herbicides that inhibit the enzyme acetolactate synthase (ALS), such as those active in the group of sulfonylureas and imidazolinones. As a result of the emergence of resistant biotypes, there is often an increase in the complexity of the weed management program, accompanied by increased costs, financial losses caused by weed competition from resistant biotypes, among other deleterious effects. 

The case of the giant caruru

As if there weren't already enough problems to deal with when it comes to weed management in Brazil, in 2015, the giant pigweed weed (Amaranthus palmeri). Plants of the species, native to North America, were recently found in cotton fields in Mato Grosso - its presence had never been reported in Brazil. It is a highly aggressive species, with accelerated growth and tolerant to conditions of high temperatures and low water availability, capable of producing more than 1 million seeds per plant. The presence of the species, in itself, in national territory, would be enough to cause an uproar, as the climate favors its spread. However, a few months ago, it was confirmed that the populations found are resistant to the herbicide glyphosate, which raised concerns and put the containment measures developed by the competent bodies, and brought to the public through normative Instruction number 47 of 2015, under the spotlight. .

Integrated weed management 

Given the severity of the problem and the importance of herbicides for modern agriculture, it is extremely important to know how to use the various existing tools for managing weeds, rationally and in conjunction with other techniques, in a system, in which, integrated weed management is put into practice. Therefore, it is recommended that the use of herbicides to control weeds be combined with a diversified management system, aiming to make it difficult for weeds to adapt to the system, reducing the pressure to select resistant plants. Several measures can be included as essential parts of a robust weed management system, such as the use of mechanical management (such as mowing or cultivating to control seedlings), and cultural management (such as crop rotation, variations in spacing, the use of direct planting, since the presence of straw constitutes a physical barrier, hindering the germination of weed seeds that require light and the emergence of seedlings). The mechanisms of action of herbicides must also be alternated, not just the herbicides or active ingredients, so that species that may have survived applications in previous harvests are efficiently controlled, before they spread and become dominant. From this perspective, crop rotation is extremely important, as it favors the use of herbicides with different mechanisms of action. It is also recommended to use herbicides of different types and modes of use, such as pre-emergent or post-emergent, especially when there is the possibility of adding a mechanism of action in pre-emergence that is not available in powders. Finally, it is also important to control escapees or weeds that are present in the crop through the use of desiccant herbicides or mechanical control, even close to harvest, to avoid an increase in the area's seed bank and greater difficulties for the following harvest, especially in the case of resistant species. Modern agriculture demands producer proactivity and extensive field monitoring. The earlier the problem was identified, the lower the costs to overcome it and the lower the losses. It is therefore necessary to invest in weed control prior to the onset of the problem, so that there are no more expenses when the problem has gotten out of control and acquired proportions that make its resolution difficult. 

Rafael M. Pedroso, Carla Siqueira, Phytus Institute

 

Article published in issue 208 of Cultivar Grandes Culturas.