Correct horseweed management

In addition to losses due to competition for resources such as light, water and nutrients, horseweed plants serve as hosts for insects such as stink bugs and caterpillars, which worsens losses in productivity. With a dizzying capacity for multiplication and a history of resistance to herbicides, this weed needs to be correctly managed, using cultural and chemical control practices together.

22.03.2022 | 14:36 (UTC -3)

In addition to the losses due to competition for resources such as light, water and nutrients, horseweed serve as hosts for insects such as bedbugs and caterpillars, which worsens losses in productivity. With dizzying capacity to multiplication and history of resistance to herbicides, this weed needs to be correctly managed, using a set of practices cultural and chemical control.

Currently the The need to manage weeds in agricultural areas is a practical recurring and extremely important for the farmer. Some invasive species are commonly found in crop fields due to characteristics linked to their ecological adaptability, the ability to adapt to the management system employed and the evolution of resistance to herbicides. The presence of such species cause losses in crop productivity due to competition for resources such as light, water and nutrients and, as they are alternative hosts for pests. Among the numerous weed species incident in agricultural areas, in especially those grown with soybeans in Brazil, horseweed is considered the most important due to its high competitive potential and impact on crop productivity. The reduction in productivity occurs as a function of the number of plants per m². Therefore, high infestations can reduce by up to 80% of soybean crop yield and, even if its occurrence is low, losses are significant.

To the plants popularly known as horseweed belong to the genus Conyza, and can be of three different species, Conyza canadensis, C. bonariensis and C. sumatrensis. In Brazil, C. bonariensis is the one with the greatest agricultural importance, present mainly in the central-south region. Horseweed has a high capacity of multiplication. A single plant W. Canadians e C. bonariensis he can produce more than 200 seeds 800 thousand seeds, respectively. These seeds are light and have “papus”, structure that allows it to be spread by wind over long distances. This feature enables its presence in the most diverse crops, such as corn, wheat, cotton, fruit trees and pastures. It is important to highlight that horseweed plants are often found in marginal areas, such as slopes, edges of roads, abandoned areas, among others. This allows your multiplication and future reinfestations in nearby cultivated areas.

Figure 1 - Horseweed inflorescence: Time of flowering (left). Ripe seeds (right)
Figure 1 - Horseweed inflorescence: Time of flowering (left). Ripe seeds (right)

the management of Conyza spp. been carried out until recently, especially in the culture of transgenic soybeans (RR), with the herbicide glyphosate. After many years using this molecule constantly, more than once in the culture cycle, the evolution of resistance occurred in a widespread manner, making the use of this herbicide in the management of Conyza spp. At the Brazil, the first cases of resistance to glyphosate were identified in 2007 for species C. bonariensis e C. canadensis. In mid-2010, the first case of glyphosate resistance was reported for C. sumatrensis. The resistance is result of continuous use of the same herbicide or different herbicides, with same mechanism of action. This practice selects naturally resistant plants, that survive and produce seeds, which culminates in an increase in the frequency of resistance in agricultural areas. From occurrence of resistance to glyphosate, other herbicides began to be used to control this invader, such as chlorimuron-ethyl, which is a product with a different mechanism of action than glyphosate. Again, continued use and isolated form of this herbicide provided the evolution of resistance, being found in 2011 biotypes of W. sumatrensis with multiple resistance to the herbicides glyphosate and chlorimuron-ethyl. Reports of cases with multiple resistance narrow the spectrum of herbicides available for the control of Conyza and that do not cause phytotoxicity in the crop of interest economic.

Figure 2 - Buca capitulums with ripe seeds being dispersed by the wind
Figure 2 - Buca capitulums with ripe seeds being dispersed by the wind

In different regions of the world, cases of resistance to Conyza spp. herbicides with mechanisms of action: photosystem II inhibitors (atrazine, simazine, diuron and linuron), photosystem I inhibitors (paraquat and diquat), ALS enzyme inhibitors (chlorsulfuron, imazapyr, cloransulam-methyl and chlorimuron-ethyl) and inhibitors of the EPSPs enzyme (glyphosate). Likewise, cases of multiple resistance: EPSPs enzyme inhibitors + photosystem inhibitors I, ALS enzyme inhibitors + photosystem II inhibitors, and EPSPs + ALS inhibitors. This scenario warns of the high variability Genetics of horseweed populations and the evolutionary potential of multiple resistance herbicides, as it encompasses practically all mechanisms of action used in Brazil for its control. In this context, the correct use of of these herbicides, respecting doses, successions and plant stages sprayed, in order to avoid the complete loss, in the near future, of the efficiency of these products.

O Horseweed control must be carried out in an integrated manner, using methods cultural at the same time as the chemical. The presence of dense layers of straw (more than 4 tons/ha), from winter crops or covers, provides drastic reduction in the germination and emergence capacity of this weed. That reduction occurs because horseweed presents positive photoblastic germination, or In other words, it needs light to germinate. The small seeds of Conyza under dense layers of straw receive the light stimulus for germination and, therefore, there is no establishment in the area. Seed viability still tends to decrease in depending on the time they remain on the ground, in addition to increasing the possibility of action of microorganisms that destroy them, reducing the seed bank in the soil and future reinfestations.

Figure 3 - Chapter of mouth, in detail structure of dispersion "papus" and seeds
Figure 3 - Chapter of mouth, in detail structure of dispersion "papus" and seeds

O physical effect of straw on the soil in isolation makes emergence difficult of seedlings, requiring a greater energy demand for them to reach the surface, begin the photosynthetic process and become established. During this period, many seeds with low vigor fail to emerge, reducing the incidence of horseweed in the area. In a complementary way, plants that can overcome the straw layer, present slow and less vigorous development, facilitating chemical control. Winter crops such as wheat and oats, in addition to being a source of income, provide a dense layer of straw that leads to a reduction of more than 90% in the number of weeds in the area. Vetch and forage turnip have also been shown to be excellent straw formers in reducing horseweed emergence.  

A Using chemical control in isolation is still an effective tool, however, it requires attention. Pre-emergent herbicides, such as diclosulan, are a promising alternative for reducing horseweed infestation. As for herbicides emerging powders, the use of products with different mechanisms of action associated or in succession, provide excellent results. Currently, no there are confirmed reports of horseweed resistance to auxin herbicides, making them an important tool in the management of this weed. However, It is important to highlight that the isolated use of these products may cause resistant genotypes, making this technology unfeasible.

Studies Scientists have reported that horseweed can behave as a host alternative for pests, such as the small green stink bug (Piezodorus guildinii), brown soybean bug (euschistus heros) and black-winged stink bug (Meditating Edessa), and also Lepidoptera, such as the soybean caterpillar (Anticarsia gemmatalis), false caterpillar (Chrysodeixis includens) and fall armyworm (Spodoptera frugiperda). In addition to these insects, producers and technicians in 2013 they found the caterpillar Helicoverpa armigera, in an advanced stage of development, feeding on plants of horseweed in the state of Mato Grosso do Sul, during the preparation phase for the soybean planting. Horseweed gains importance for these pests, especially in times when the food supply is reduced, that is, between harvests and periods close to harvest. Another species that can use fleabane as a host alternative is the mite Brevipalpus phoenicis, vector of citrus leprosis, which is a polyphagous species. Studies found the mite's preference B. phoenicis because it feeds on the base of the trichomes present on the surface of the stem of the plant, therefore serving as an alternative host, which can contribute to the spread of the mite and, consequently, leprosis in citrus orchards.

Figure 4 - Horseweed plant six days after emergence
Figure 4 - Horseweed plant six days after emergence

O Integrated horseweed management is the most effective tool for reducing losses caused by this weed. The use of practices together cultural and chemical control, seeking the complete elimination of this plant during the period of crop development, in the off-season and in orchards, is essential to avoid direct loss of income caused by competition, or indirect effects generated by pests opportunely lodged in horseweed. Per Finally, the rational use of herbicides is essential, especially those that are still perform satisfactory control over horseweed, in order to avoid the selection of resistant plants and prolong the use of this management tool.

Article published in issue 217 of Cultivar Grandes Culturas, June 2017. 

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