Brazilian agriculture faces a growing and complex challenge: the emergence of weeds that are resistant and tolerant to herbicides. This phenomenon is not only a result of lack of crop rotation and repetition of herbicide mechanisms of action, but also represents a significant obstacle to agricultural productivity and sustainability.

The resistance and tolerance of these weeds have profound implications, from increased production costs to environmental impact. This article explores the different types of resistance, the underlying mechanisms and effective strategies for controlling these weeds, with a special focus on horseweed, the most challenging plant to control in Brazil.

Ao entender a complexidade deste problema, os produtores podem adotar práticas mais eficazes e sustentáveis, minimizando o impacto econômico e ambiental da resistência de plantas daninhas. Acompanhe para descobrir as melhores estratégias de manejo e as alternativas disponíveis para enfrentar este desafio crescente na agricultura brasileira.

 

Difference Between Resistant and Tolerant Weeds

According to the Herbicide Resistance Action Committee (HRAC), resistant weeds are those that survive an herbicide when they should be controlled under normal conditions, that is, resistance is acquired from the moment selection pressure occurs.

Tolerance refers to the plant's natural ability to survive a given treatment. Thus, tolerant plants are those that have never been controlled by a certain active ingredient.

 

What are the types of weed resistance?

There are basically two types of resistance: Cross and multiple. The first refers to weeds that have developed only one resistance mechanism, being resistant to herbicides with the same mechanism of action (for example, resistance to glyphosate whose mechanism of action is the inhibition of EPSPS).

The second concerns plants that have developed more than one type of resistance mechanism and, therefore, are resistant to more than one type of herbicide action mechanism (for example, resistance to glyphosate + 2-4D, whose action mechanisms are EPSPS inhibitor and auxin mimic, respectively).

 

What makes a weed resistant to herbicides?

The main reason we have major problems in Brazil and the world with weed resistance to herbicides is the continuous use of products with the same mechanism of action, which causes selection pressure on the group of plants, causing the emergence of resistant plants.

Year after year, the number of resistant weeds increases (figure 1), and more productivity losses occur, in addition to the increase in the cost of controlling them.

 

Figure 1. Increase in the number of weed species resistant to different sites of herbicide action over time. Source: WeedScience.

The two groups that have the most cases of resistance are ALS inhibitors (systemic, selective and widely used to control broad and narrow leaf weeds in the initial post-emergence of soybean crops, such as Imazethapyr, Imazapyr and Chlorimuron) and those that act on photosystem II (have a wide range of products, but are mainly systemic and can be used both in pre-emergence and post-emergence of broad and narrow leaf weeds, such as Atrazine and Diuron, widely used in sugarcane -of sugar.

 

What is the most difficult plant to control in Brazil?

With the advent of RR technology, the herbicide glyphosate (EPSPs mechanism of action) began to be widely used to control weeds, however this triggered cases of weed resistance, mainly horseweed (C.bonariensis, C.canadensis e C.sumatrensis), the most difficult plant to control in Brazil in the cultivation of grains (soybeans and corn, mainly).

Estima-se que 90 % da área do Brasil cultivada com soja é resistente ao glifosato (Dionísio Luiz Pisa, eng. agrônomo da Embrapa Soja). Para se ter uma ideia do prejuízo ocasionado, uma planta de “buva” (Conyza bonariensis) per square meter can result in losses of 4 to 12% in soybean cultivation (Nortox).

 

What herbicides is horseweed resistant to?

Para contornar essa situação, herbicidas de outros mecanismos de ação passaram a ser utilizados, entretanto a buva tem se tornado resistente a outros produtos, além do glifosato (Heap, 2020),  e a explicação para isso pode estar no fato de não haver rotação de mecanismos de ação no curto espaço de tempo e a buva ser uma planta com grande taxa de reprodução, produzindo mais de 200.000 sementes por planta (BHOWMIK & BEKECH, 1993), sendo estas facilmente transportadas pelo vento, podendo levar populações resistentes de uma região para outra. Abaixo são listados alguns exemplos de detecção de resistência da buva a diferentes princípios ativos de herbicidas ao longo do tempo:

• Inibidores de ALS (clorimuron-etil) em 2011;
• clorimuron-etil e glifosato em 2011 (casos de resistência múltipla);
• Inibidores de fotossistema I (Paraquat) em 2016;
• Inibidores de protox (Saflufenacil) em 2017;
• Inibidores do fotossistema II (diuron) e mimetizadores de auxina (2,4-D) em 2017.

How to carry out horseweed control?

Many may then wonder how this weed can be controlled, as it has shown resistance against a series of herbicides with different mechanisms of action.

The rotation of action mechanisms continues to be a highly recommended practice for horseweed control, but it is worth highlighting that good soil coverage greatly reduces the incidence of horseweed, as it is a positive photoblastic plant, that is, it requires solar radiation to germinate. Therefore, keeping the soil covered throughout the year is essential for good horseweed control.

Another alternative for control is to use pre-emergent herbicides, such as diclosulam and flumioxazin, thus reducing the weed seed bank in the soil.

It is recommended that horseweed control with post-emergent herbicides should be carried out in the early stages of development. However, a study carried out by Professor Alfredo Albrecht, from the Federal University of Paraná, showed that glyphosate and paraquat (their use is currently prohibited) may not cause any symptoms in horseweed plants, even in the early stages.

According to the study, the most effective control of horseweed with up to 8 leaves and smaller than 15 centimeters is the use of Saflufenacil, ammonium glufosinate and a mixture of glyphosate with these products, which has a synergy effect. For the control of larger plants, with a single application in mind, the mixture of Glyphosate + 2,4-D + Saflufenacil or Glyphosate + Dicamba + Saflufenacil provides very efficient control. Thinking about sequential application, the management that appears efficient is to apply glyphosate + auxin (2,4-D or Dicamba) and approximately 7 to 10 days later using a contact product (ammonium glufosinate).

 

What is the economic impact of weed resistance?

A study carried out by EMBRAPA-SOJA in 2017 sought to estimate the cost of controlling weeds in soybeans when there is resistance to glyphosate and when there is not, as shown in the table below:

 

Estimation of the cost of controlling isolated or mixed populations in soybean crops with resistance to glyphosate.

The table shows that the increase in costs can be very large, and this is because different herbicides are needed, which are more expensive than glyphosate, in addition to the greater number of applications. The average cost of R$120.00 in the case of no resistance refers to one application of glyphosate during desiccation and two applications post-emergence.

The most significant cost occurs in two different scenarios: glyphosate-resistant bittergrass and glyphosate-resistant bittergrass + horseweed, which can increase costs by up to 290% and 403%, respectively.

When analyzing the entire soybean area in Brazil infested with weeds, the result was obtained that the increase in control costs due to weed resistance is between R$ 3,796,540,000.00 to R$ 6,046,500,000.00, with an average increase of R$ 4,918,820,000.00 per year.

 

 

The SaveFarm Solution: A Breakthrough in Weed Management

 

Faced with the growing challenges presented by resistant and tolerant weeds, technology emerges as a powerful ally for rural producers. The SaveFarm selective spray system is an innovation that directly addresses these issues, offering an efficient and sustainable solution.

Using high-precision sensors and artificial intelligence, SaveFarm identifies weeds in real time and applies pesticides only where necessary. This not only saves inputs, but also minimizes environmental impact, contributing to more sustainable agriculture.

Para saber mais sobre como o SaveFarm pode ajudar você a enfrentar os desafios das plantas daninhas resistentes e tolerantes, fale agora com um de nossos representantes.

 

 

References

ADEGAS, F.S. Economic impact of weed resistance to herbicides in Brazil. Embrapa Londrina, PR, August, 2017. Available at: https://ainfo.cnptia.embrapa.br/digital/bitstream/item/162704/1/CT132-OL.pdf;

Internetional herbicide-resistant weed database. Available in: http://www.weedscience.org/Home.aspx;

CropLife Latin America. Available in: https://www.croplifela.org/pt/pragas/lista-do-pragas/plantas-daninhas;