Resistance Management - UPL OpenAg

Weeds Resistance

The problem

Weed resistance is one of the main problems global agriculture and needs to be treated with responsibility. The consequences to farmers and growers are severe when not controlled properly. Here we explain what makes weed resistant, how this problem became an issue, management importance, and the best practice.

What it is and how it appears

Plants, different from animals, are not able to move to a site that offers better conditions for its survival and reproduction. However, they have the ability to adapt to different environments due to the genetic variability among species and among individuals of the same species.

When subjected to some stress, this variation allows some individuals to survive and produce offspring. If this stress occurs repeatedly, the number of adapted individuals increases, while the sensitive ones disappear from the environment as generations go by.

Weed resistance occurs for these reasons and herbicides play the role of selection agents, so they cause stress. Naturally existing in nature plants are capable to survive the action of certain herbicides. If the application of the product is performed several times in a row, without alternating with another herbicides or control method, the amount of resistant plants tends to increase until it is the majority.

It is extremely important to make it clear that herbicides do not cause genetic mutations in plants; they only select between susceptible and resistant ones. Therefore, it is possible that there are herbicide resistant weeds that have not yet been discovered.

The occurrence of resistance weeds to herbicides and its dispersion speed depends on several factors. Among them are the genetic, bioecological and agronomic factors.

The genetic factors are related to the initial frequency of mutation that confers resistance, characteristics of the inheritance of resistance, type of fertilization and gene flow; the bioecological are related to the characteristics of the weed’s life cycle; and the agronomic factors are linked to the herbicide and management which can be conducted positively or negatively, depending on techniques used.

Some of the agronomic factors that favor the occurrence and spread of resistance are the exclusive use of the chemical method, lack of crop, herbicide rotation, escape of permanence, fallow periods, use of uncertified seed, machinery flow between crops.

Below you can see some more factors that affect the evolution of plant resistance.


  • Initial frequency of resistance;
  • Dominance of resistant alleles;
  • type of fertilization;
  • number of resistant alleles;
  • Ecological adaptation.


  • Specie;
  • Generations number per year and reproductive rate;
  • Seeds longevity in seed bank;
  • Specie density;
  • Herbicide weeds susceptibility.


  • Herbicide Characteristics;
  • Chemical Group;
  • Residual;
  • Control Efficiency;
  • Used Dose.
  • Cultural Practices
  • Exclusive use of herbicides in weed control
  • Repetitive use of the same herbicide or Mechanism of Action
  • Application Frequency
  • Cultivation System

(Adapted by HRAC-BR, 2008)


Weeds Resistance in Brazil and in the world

The chemical method is the most widely used for weed control, since it is fast, practical and efficient, allowing selective control. Its use began in the late 1940s, and success in weed control led in the following years to a high rate of herbicides adoption, arising just after cases of resistance.

The first resistance case was recorded in 1957, on wild carrot plants (Daucus carota) that resisted to herbicide 2, 4-D. Currently, there are 480 cases (species x site of action) of resistance recorded in the world, including 252 species, being 105 monocotyledons and 147 dicotyledons. These cases include weeds that infest 91 crops, including wheat, corn, rice, soybeans, barley, orchards, canola and cotton, distributed in 67 countries. In Brazil there are 44 cases of herbicide resistant plants.


Source: Adapted by Heap (2017)

Weed management importance

Weeds are one of the main limiting problems of crop productivity. The losses can reach 80%, besides the reduction of production quality. And the problem is aggravated when there is presence of herbicides resistant plants in the field, since it is the main control method used

The main problems involve glyphosate resistance, the most widely used weed-killer herbicide in the world. Precisely because of its high adhesion, glyphosate resistance was selected and rapidly scattered. In Brazil, there are eight species resistant to this active ingredients, including three species of hairy fleabane, horseweed, sumatran fleabane (Conyza bonariensis, C. canadians and C. sumatrensis), sourgrass (Digitaria insularis) and annual ryegrass (Lolium multiflorum).

Once the resistance is present in the crop, dynamics management should be changed to obtain a more effective control, with introduction of new herbicides, mechanisms of action and control methods. Such measures are considered reactive, because they are adopted after the problem appears. This is the main behavior of farmers and growers: Fight the resistance rather than prevent it.

Before the problem exists in the crop, farmers/growers must use resistance-weaken techniques, which are listed below.

Proactive Action Consequencies

Clean-up of non- cultivated areas (roadsides and fences lines)

Reduction of Seed crop introduction

Machinery cleaning

Prevents seed flow between crops

Use of Certified Seeds

Weed and seed crop prevention 

Young Plants Control

Improves  herbicides efficiency

Prevents weed seeds production

Post-harvest Control

Reduces seed bank feeding in off-season period

Crop rotation practices

Increases Soil Cover and Allelopathy

Provides Rotation and Control Methods

Allows Herbicides Rotation according to crop selectivity

Rotation and Control Methods 

Control by non-chemical methods

Scouting and escapes remove

Prevents Initial reproduction of resistant individuals

Active Ingredients Rotation and Mechanism of Action

Control of eventual offspring and survived hardy plants from previous application

Mixture use of Active Ingredients

Interaction in more than one site of plant action, reducing survival chances

Herbicides Sequential applications

Complements First Application Control, reduces escape remove possibilities

Appropriate  technology

Allows optimal dose to reach target, reducing survival chances

It is important to prevent and treat this problem with responsibility. Although all proactive actions described in the table above are fundamental, herbicide rotation and mechanisms of action are essential so the resistance do not establish in the crops.