**5. Glyphosate-resistant weeds in Brazil**

*Multifunctionality and Impacts of Organic and Conventional Agriculture*

solids, which have high solubility in water (12 g L<sup>−</sup><sup>1</sup>

and the amount of material that exists initially.

a dissociation (▬PO2H▬) and a protonation (▬NH2

**Properties Values**

Common name Glyphosate CAS number 1071-83-6 Molecular formula C3H8NO5P Molecular weight 169 g mol<sup>−</sup><sup>1</sup> Class Herbicide Group Replaced glycine

Melting point 189.5°C

Degradation point 200°C Vapor pressure (PV) 1.31 × 10<sup>−</sup><sup>5</sup>

Solubility in water (Sw) 10.5 g L<sup>−</sup><sup>1</sup>

Acid partition coefficient (pKa) 2.34 (at 25°C) Octanol-water coefficient (Kow) 6.31 × 10<sup>−</sup><sup>4</sup>

Sorption coefficient (Kd) 209.4 mg L<sup>−</sup><sup>1</sup> Half-life time degradation in soil (DT50) 15 days

profile [27].

Under environmental conditions, both glyphosate and its salts are crystalline

are practically insoluble in common organic solvents such as acetone and ethanol.

significant water solubility in the presence of light, including at temperatures above

As shown in **Table 1**, glyphosate has high Sw and pKa values with acid character and a low Kow value, indicating that glyphosate has a great hydrophilic tendency, which decreases soil sorption. However, glyphosate quickly binds to positive soil charges (mainly in clays) such as in soils abundant in iron and aluminum oxides. This indicates high values of sorption coefficient normalized by soil organic carbon (OC) (Koc) than other herbicides, limiting their leaching in the soil

The pKa values found in the literature for glyphosate are pKa1, 0.8; pKa2, 2.16; pKa3, 5.46; and pKa4, 10.14. These dissociation constants indicate the degree of dissociation of the herbicide as a function of pH [24]. This shows the relationship between the amount of matter that exists after a certain reagent has been consumed

At pH values below 0.8, most glyphosate is found in a protonation on the amine

the compound will have dissociation despite maintaining protonation in the amine group. Among the pH values of 2.2 and 5.4, the predominant form of the herbicide

+

site. In a pH of 0.8, being the value of the first constant, 50% of the molecules present this protonation and the other 50% with a dissociation in the phosphate group. From this value up to pH of 2.2, the molecular formula is predominant, with

Chemical name (IUPAC) *N*-(phosphonomethyl) glycine

Boiling point Decomposes before boiling

Henry's law constant to 25°C (H) 2.10 × 10<sup>−</sup>07 (Pa m3

Glyphosate melts at 189.5°C and has an apparent density of 0.5 g cm<sup>−</sup><sup>3</sup>

60°C [24]. **Table 1** shows the physicochemical properties of glyphosate.

to 25°C, for glyphosate) and

▬), and in a pH of 2.2, 50% of

Pa (25°C, acid)

(pH 7, 20°C)

(20°C)

 mol<sup>−</sup><sup>1</sup> )

and presents

**12**

**Table 1.**

*Source: Adapted from PPDB [26].*

*Physicochemical properties of glyphosate.*

The occurrence of weed resistance to herbicides is a natural and inheritable capacity of certain biotypes within a given population to develop and reproduce after being exposed to herbicide doses that would be lethal to a normal population of the same species. This resistance is from an evolutionary process, occurring naturally at low frequency, and the selection pressure exerted by repetitive application of some herbicide or different types of herbicides that have the same mechanism of action increases the number of resistant individuals in the population. Herbicide resistance is identified when, generally, 30% of the plants are resistant [29].

Several weed species are inherently more resistant to glyphosate than others. A biotype of glyphosate resistance that occurred naturally was the *Convolvulus arvensis* without reporting the use of glyphosate [20]. A biotype of *Lotus corniculatus* resistant to glyphosate doses was identified by Boerboom et al. [30]. The natural resistance to this herbicide, these and other species, was not a problem until the emergence of glyphosate-resistant crops. With the adoption of these crops, many species became problematic because they occupied places where other weed species did not inhabit and glyphosate-resistant crops were cultivated [20].

Biotypes that have glyphosate resistance have been selected in crops such as corn, soybeans, and various orchards. In Brazil, glyphosate-resistant biotypes of *Conyza bonariensis* [31], *Conyza canadensis* [32, 33], *Conyza sumatrensis* [34], *Lolium multiflorum* [35, 36], *Digitaria insularis* [9, 37], *Chloris elata* [38], *Eleusine indica* [39], *Amaranthus palmeri* [40], and more recently *Amaranthus hybridus* [41] and *Euphorbia heterophylla* [42] were identified.

Due to the increase in the adoption of glyphosate-resistant crops, there was a great difficulty in selecting the herbicide to be used in weed populations in the past decade. The alternating herbicides that have different modes of action or herbicides mixed in tanks are recommended in resistance management programs; however, this is often ignored by farmers, because the cost to control weeds only with glyphosate is much cheaper.

It is recommended that a rotation be made between cultivating glyphosateresistant crops and nonresistant crops, so that the development of glyphosate resistance in weeds is delayed. However, the correct use of glyphosate with other herbicides, a survey of the weed population, extension of the area, and economy of producers are important factors in the management of weeds in glyphosateresistant crops [20].

**Figure 12** shows all the species of glyphosate-resistant weeds worldwide. The first case of reported resistance was in 1996 of the species *Lolium rigidum* in Victoria, Australia. In Brazil, the first case of glyphosate-resistant weed identified was in 2003, which was the species *Lolium perenne* ssp. *multiflorum*. In 2019, four

#### **Figure 12.** *Glyphosate-resistant weeds worldwide. Source: Heap [10].*

cases of new glyphosate-resistant weeds were reported: in Australia, the species *Avena sterilis* ssp. *ludoviciana*; in Argentina, the species *Carduus acanthoides* which presented multiple resistance to 2,4-D and glyphosate; in Colombia, the species *Chloris radiata* which showed simple resistance to glyphosate; and also in Argentina, resistance of the species *Echinochloa crus-galli* var. *crus-galli* which was reported [10].
