**1. Introduction**

The successive and intensive cultivation of the same crop species in Brazil, with practically no crop rotation, is leading to an increase in the presence of weeds [1]. One should emphasize that

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the term "weed" had effectively no botanical meaning, since by the classical definitions, a plant can be a "weed" in a given situation while it may be desirable in another. Distinct plant species included into the same botanical order may be considered as weed or desirable. Furthermore, the same species may be considered as a "weed" into an arable field while it can be desirable in gardens, for instance, where they are usually considered as "beneficial weeds" [2].

The use of herbicides for weed control in Brazilian agriculture has increased significantly in the last years, due to a series of factors, such as the growing difficulty to find human labor for manual weeding and the excessive damage caused to plants when adopting incrop mechanical control [2]. Moreover, the chemical control represents an easy and efficient approach for weed control and therefore farmers are most prone to use this method despite of the other weed suppression strategies [1].

Weed species that have been indirectly selected for adverse conditions obtain their vital elements more efficiently by extracting water, nitrogen, phosphorus, and potassium, respectively, four, five, three, and six times more than crop plants [3]. Thereby, due to their ability to compete for environmental resources with cultivated plants, it is essential to eliminate them from cropping fields. Considering also that usually for every crop there is specific companion weeds [4], weed control based solely on herbicides tends to reduce quickly their efficiency due to plant selection which become resistant or tolerant to these compounds [1]. Compared to other pests, weeds have longer reproduction cycles [5] and produce propagules which survive in soil for several years [1]. These factors contribute to the relatively slow evolution of resistant weeds compared to other pests.

Weed resistance to herbicides is defined as the inherited ability of a plant to survive following application of the commercially used dose of the herbicide recommended for its control. This dose, in regular conditions, should be able to control that weed species [2]. There are several factors responsible for selecting resistant weed biotypes, as the selection pressure imposed by the herbicide [1]. Herbicides differ in the risk that they present to select a given resistant weed biotype, and this depends, among other aspects, on its specificity in terms of point of action into the plant; more specifically the local of action [6].

For instance, the herbicide 2,4-D, is a synthetic auxin used continuously since 1948 and the first case of resistance to this compound was reported only in 1957 [6]. Currently, 40 years later, resistance cases to synthetic auxins was documented only for 14 species in 11 countries [1]. Thus, herbicides from this mode of action and other inhibitors, such as Protox and EPSPs inhibitors, are examples of "low-risk" herbicides for resistance evolution [1, 7]. On the other hand, herbicides like the ones included in the acetolactate synthase (ALS)-inhibiting group are considered as "high risk" for herbicide resistance evolution. This classification for a new resistance case to appear is based on the location in which the herbicide acts into the plant and other aspects [7]; more specifically, its mechanism of action, as a single mutation into the plant, could turn it resistant to the herbicide [6]. The time required for the appearance of the first resistant biotype to commonly used herbicides worldwide from their introduction in the market are shown in **Table 1**.

The history and concepts about weed resistance have been widely explored in the literature including topics dealing with the mechanisms conferring resistance and herbicide traits which most easily select resistant biotypes. Although weed resistance is a well-known problem and is


the term "weed" had effectively no botanical meaning, since by the classical definitions, a plant can be a "weed" in a given situation while it may be desirable in another. Distinct plant species included into the same botanical order may be considered as weed or desirable. Furthermore, the same species may be considered as a "weed" into an arable field while it can be desirable

The use of herbicides for weed control in Brazilian agriculture has increased significantly in the last years, due to a series of factors, such as the growing difficulty to find human labor for manual weeding and the excessive damage caused to plants when adopting incrop mechanical control [2]. Moreover, the chemical control represents an easy and efficient approach for weed control and therefore farmers are most prone to use this method despite of

Weed species that have been indirectly selected for adverse conditions obtain their vital elements more efficiently by extracting water, nitrogen, phosphorus, and potassium, respectively, four, five, three, and six times more than crop plants [3]. Thereby, due to their ability to compete for environmental resources with cultivated plants, it is essential to eliminate them from cropping fields. Considering also that usually for every crop there is specific companion weeds [4], weed control based solely on herbicides tends to reduce quickly their efficiency due to plant selection which become resistant or tolerant to these compounds [1]. Compared to other pests, weeds have longer reproduction cycles [5] and produce propagules which survive in soil for several years [1]. These factors contribute to the relatively slow evolution of resistant weeds compared to other pests.

Weed resistance to herbicides is defined as the inherited ability of a plant to survive following application of the commercially used dose of the herbicide recommended for its control. This dose, in regular conditions, should be able to control that weed species [2]. There are several factors responsible for selecting resistant weed biotypes, as the selection pressure imposed by the herbicide [1]. Herbicides differ in the risk that they present to select a given resistant weed biotype, and this depends, among other aspects, on its specificity in terms of point of action

For instance, the herbicide 2,4-D, is a synthetic auxin used continuously since 1948 and the first case of resistance to this compound was reported only in 1957 [6]. Currently, 40 years later, resistance cases to synthetic auxins was documented only for 14 species in 11 countries [1]. Thus, herbicides from this mode of action and other inhibitors, such as Protox and EPSPs inhibitors, are examples of "low-risk" herbicides for resistance evolution [1, 7]. On the other hand, herbicides like the ones included in the acetolactate synthase (ALS)-inhibiting group are considered as "high risk" for herbicide resistance evolution. This classification for a new resistance case to appear is based on the location in which the herbicide acts into the plant and other aspects [7]; more specifically, its mechanism of action, as a single mutation into the plant, could turn it resistant to the herbicide [6]. The time required for the appearance of the first resistant biotype to commonly

used herbicides worldwide from their introduction in the market are shown in **Table 1**.

The history and concepts about weed resistance have been widely explored in the literature including topics dealing with the mechanisms conferring resistance and herbicide traits which most easily select resistant biotypes. Although weed resistance is a well-known problem and is

in gardens, for instance, where they are usually considered as "beneficial weeds" [2].

the other weed suppression strategies [1].

134 Herbicide Resistance in Weeds and Crops

into the plant; more specifically the local of action [6].

**Table 1.** Time required for appearance of the first resistant weed biotype following introduction of a new herbicide mechanism of action into the market.

relatively characterized, its occurrence is constantly increasing in a worldwide basis. To assist researchers to keep updated about herbicide resistance spread around the world, there is a website, www.weedscience.org, which is used as a platform for researchers to register the new cases of weed resistance [8]. This website is maintained by the Global Herbicide Resistance Action Committee and CropLife International, and it is an open access tool. The basic worldwide data about weed resistance used in the present study were obtained from that site, which were used with permission from the owners. Further data to botanically characterize the weed species listed on the WeedScience website were obtained from specialized literature.

Besides herbicide risk and frequency of application to the field, which are already well studied in the literature related to the weed science, other plant traits could turn them resistant to herbicides. Morphophysiological characteristics as dormancy behavior, number of seeds produced, annual distribution of emergence, and several others [1] can maximize the chance for the occurrence of weed species in the fields at the time of herbicide application, thus exposing them to the selection pressure imposed by the herbicide [7]. These traits can be studied as not being exclusive for the plant species but a characteristic innate to the botanical order, family, or genus of the weed species with resistant biotypes. Supposing this relationship exists, plants which are most closely related to resistant species could also be most prone to evolve resistance. The present study is based upon this hypothesis.

In order to have a wider comprehension about the path resistance takes into the botanical classification of weed species, from the appearance of the first resistant species to the current situation of resistance in Brazil and its most probable future, four reference countries were selected to serve as background for understanding the Brazilian context of weed resistance and new future resistance cases. The countries with higher number of resistant weed biotypes were first selected; in the second stage of selection were selected among these countries two that represented the American Continent, where Brazil is located; one which represented Europe, from where some weed species are known to be introduced in Brazil, and one to represent Oceania, where the climate is more alike to the observed in several regions of Brazil. In this context, the following countries were selected to be studied: Australia, Canada, France, and the United States.
