**2.2 Evolution of glyphosate resistance in** *Chloris virgata*

On national ranking basis in Australia, *Chloris virgata*, as an herbicide-resistant weed, ranks ninth, resulting in herbicide-resistant weed cost of \$2.6 million [78]. In the northern region of Australia, it is the top fourth herbicide-resistant weed after ryegrass (*Lolium rigidum*), wild turnip (*Brassica rapa*), and barnyard grass (*Echinochloa crus-galli*) [78].

Minimum tillage due to its benefits like reduced soil erosion and improvement in moisture conservation has resulted in the reduction of soil disturbance in grain cropping fields. The factors that aided the adoption of minimum tillage systems in Australian cropping systems include machinery modifications that allow greater flexibility in the cropping systems, precision agriculture and refinement of controlled traffic farming, improved crop resistance or tolerance to plant diseases associated with stubble retention, availability of more crop options and rotations, development of a broader spectrum of effective herbicides, and the use of genetic modification technologies to breed herbicideresistant crops [79].

Minimum tillage has increased the use of herbicides and consequently increased the rapid appearance of herbicide resistance in weeds [75]. Another reason for evolution is the introduction of glyphosate-resistant crops in the mid-1990s that has resulted in a sharp increase in the populations of *Chloris virgata* [80].

Glyphosate resistance was first reported in broadleaf *Conyza* (horseweed) species. The mechanism suggested for resistance was an altered subcellular distribution resulting in sequestration of the glyphosate molecule away from the enzyme target site in the chloroplast [81]. Weeds receiving repeated exposure to a single mode of action of herbicide are the most likely candidates to develop resistance [82].

From the evolution point of view, minimum tillage along with reliance on glyphosate has contributed the most towards glyphosate resistance in *Chloris virgata*. The evolution of the glyphosate resistance in *Chloris virgata* highlights the need for diversity in weed management strategies for successful control of *Chloris virgata* and other *Chloris* species [82].

#### **2.3 Crop competition as a strategy to control** *Chloris virgata*

Crop competition can be used as an effective strategy against *Chloris virgata*, especially when herbicides like glyphosate fail or underperform [83]. Crop competition to control weeds has proven to be one of the most effective cultural strategies in Australian cropping systems, aiming at suppression of weed biomass and fecundity resulting in crop yield gains [84]. Three major weed variables that affect crop-weed competition are:

• Time of emergence of the weed relative to the crop and weeds that emerge later than the crop are much less competitive than the weeds that emerge before the crop.

**113**

**3. Summary**

*Glyphosate Resistance of* Chloris virgata *Weed in Australia and Glyphosate Mobility…*

ment, high-density root systems, and plant heights [85].

a competitive advantage in a nutrient-limited system [85].

ability between crop and weed plants [85].

the reduction of light available to weeds [92].

leaf appearance, and root and shoot biomass [87, 88].

• Weed seedling density is the second most important factor influencing

• Differences in the competitive ability of weeds due to rapid leaf area develop-

Crop and weed plants compete for limited resources like water, nutrients, and light. Competition for nutrient uptake is dependent on intrinsic nutrient requirements and uptake efficiencies. Uptake efficiencies are further dependent on root length densities and nutrient membrane transporters. Species with a low nutrient requirement, extensive root systems, and effective membrane transporters will have

Crop and weed plants compete for water, as water is required for plant growth.

In the absence of water, a reduction in photosynthesis, wilting, and nutrient deficiencies can occur. The length, magnitude, and timing of the drought periods as well as soil attributes (water holding capacity, texture, structure, and hydraulic conductivity), plant traits (root structure and density, drought tolerance, and water use efficiency) are the major factors that influence the competition for water avail-

Light as a third major factor affects the growth of crop and weed plants [86]. Different phenophases of both crop and weed plants are affected by light. Morphological changes in both crop and weed plants due to competition for light include an increase in stem elongation and reduction in stem diameter, the rate of

Crop competition studies under field conditions are mainly influenced by the environment, soil type, plant density, spatial arrangement, the proportion of each species, and design of experiment [89]. The design of the crop competition experiment depends on the objective, as different objectives require different techniques [90].

Crop species may outcompete weed species depending on factors such as crop density, crop planting pattern, crop vigor, and weed vigor. Crop density or the number of plants per unit of area is important for competition studies considering the relationship among plant yield and the number of individuals and resources present in the area [91]. The competitiveness of a crop can be enhanced using competitive cultivars, higher plant densities, narrow row spacings, and different row orientation [92]. Weed growth can be substantially reduced by shading weeds in the inter-row space by physical orientation of the crop rows [92]. Competitive ability of the crops can also be increased by increasing plant density [84]. The significant interaction between sorghum cultivars and planting densities in suppressing weed biomass has been observed [93]. A high-density crop can limit water and nutrients available to weeds more effectively than a low-density crop, and high-density crops can result in

In summary, the review paper covered two major problems associated with single reliance on glyphosate application for controlling weeds. The first one is glyphosate mobility via soil systems, a potential risk for aquatic environments, and there is no information on the fate of glyphosate on Australian soils from the last 22 years apart from a single study in Western Australia. This research gap prompted an investigation into glyphosate sorption behaviour in Australian soils of the different mineral composition due to increased usage of glyphosate as a single strategy to

*DOI: http://dx.doi.org/10.5772/intechopen.90323*

weed-crop competition.

*Glyphosate Resistance of* Chloris virgata *Weed in Australia and Glyphosate Mobility… DOI: http://dx.doi.org/10.5772/intechopen.90323*


Crop and weed plants compete for limited resources like water, nutrients, and light. Competition for nutrient uptake is dependent on intrinsic nutrient requirements and uptake efficiencies. Uptake efficiencies are further dependent on root length densities and nutrient membrane transporters. Species with a low nutrient requirement, extensive root systems, and effective membrane transporters will have a competitive advantage in a nutrient-limited system [85].

Crop and weed plants compete for water, as water is required for plant growth. In the absence of water, a reduction in photosynthesis, wilting, and nutrient deficiencies can occur. The length, magnitude, and timing of the drought periods as well as soil attributes (water holding capacity, texture, structure, and hydraulic conductivity), plant traits (root structure and density, drought tolerance, and water use efficiency) are the major factors that influence the competition for water availability between crop and weed plants [85].

Light as a third major factor affects the growth of crop and weed plants [86]. Different phenophases of both crop and weed plants are affected by light. Morphological changes in both crop and weed plants due to competition for light include an increase in stem elongation and reduction in stem diameter, the rate of leaf appearance, and root and shoot biomass [87, 88].

Crop competition studies under field conditions are mainly influenced by the environment, soil type, plant density, spatial arrangement, the proportion of each species, and design of experiment [89]. The design of the crop competition experiment depends on the objective, as different objectives require different techniques [90].

Crop species may outcompete weed species depending on factors such as crop density, crop planting pattern, crop vigor, and weed vigor. Crop density or the number of plants per unit of area is important for competition studies considering the relationship among plant yield and the number of individuals and resources present in the area [91]. The competitiveness of a crop can be enhanced using competitive cultivars, higher plant densities, narrow row spacings, and different row orientation [92].

Weed growth can be substantially reduced by shading weeds in the inter-row space by physical orientation of the crop rows [92]. Competitive ability of the crops can also be increased by increasing plant density [84]. The significant interaction between sorghum cultivars and planting densities in suppressing weed biomass has been observed [93]. A high-density crop can limit water and nutrients available to weeds more effectively than a low-density crop, and high-density crops can result in the reduction of light available to weeds [92].

### **3. Summary**

*Sorption in 2020s*

dry biomass m<sup>−</sup><sup>2</sup>

*truncata*) (146 g m<sup>−</sup><sup>2</sup>

(*Echinochloa crus-galli*) [78].

resistant crops [79].

*virgata* and other *Chloris* species [82].

) [75].

in spring, summer, and autumn in South Australia [75].

**2.2 Evolution of glyphosate resistance in** *Chloris virgata*

compared to irrigated conditions [75]. Under irrigated conditions, 619 to 730 g of

this value was much higher than one of its related species, windmill grass (*Chloris* 

*Chloris virgata* has several characteristics like rapid germination and low base temperature (2.1 to 3.0°C) for seed germination enabling it to survive rainfall events

On national ranking basis in Australia, *Chloris virgata*, as an herbicide-resistant weed, ranks ninth, resulting in herbicide-resistant weed cost of \$2.6 million [78]. In the northern region of Australia, it is the top fourth herbicide-resistant weed after ryegrass (*Lolium rigidum*), wild turnip (*Brassica rapa*), and barnyard grass

Minimum tillage due to its benefits like reduced soil erosion and improvement in moisture conservation has resulted in the reduction of soil disturbance in grain cropping fields. The factors that aided the adoption of minimum tillage systems in Australian cropping systems include machinery modifications that allow greater flexibility in the cropping systems, precision agriculture and refinement of controlled traffic farming, improved crop resistance or tolerance to plant diseases associated with stubble retention, availability of more crop options and rotations, development of a broader spectrum of effective herbicides, and the use of genetic modification technologies to breed herbicide-

Minimum tillage has increased the use of herbicides and consequently increased

Glyphosate resistance was first reported in broadleaf *Conyza* (horseweed) species. The mechanism suggested for resistance was an altered subcellular distribution resulting in sequestration of the glyphosate molecule away from the enzyme target site in the chloroplast [81]. Weeds receiving repeated exposure to a single mode of action of herbicide are the most likely candidates to develop resistance [82]. From the evolution point of view, minimum tillage along with reliance on glyphosate has contributed the most towards glyphosate resistance in *Chloris virgata*. The evolution of the glyphosate resistance in *Chloris virgata* highlights the need for diversity in weed management strategies for successful control of *Chloris* 

Crop competition can be used as an effective strategy against *Chloris virgata*, especially when herbicides like glyphosate fail or underperform [83]. Crop competition to control weeds has proven to be one of the most effective cultural strategies in Australian cropping systems, aiming at suppression of weed biomass and fecundity resulting in crop yield gains [84]. Three major weed variables that affect crop-weed

• Time of emergence of the weed relative to the crop and weeds that emerge later than the crop are much less competitive than the weeds that emerge before the

the rapid appearance of herbicide resistance in weeds [75]. Another reason for evolution is the introduction of glyphosate-resistant crops in the mid-1990s that has

resulted in a sharp increase in the populations of *Chloris virgata* [80].

**2.3 Crop competition as a strategy to control** *Chloris virgata*

of *Chloris virgata* (89 days after sowing) was observed; however,

**112**

competition are:

crop.

In summary, the review paper covered two major problems associated with single reliance on glyphosate application for controlling weeds. The first one is glyphosate mobility via soil systems, a potential risk for aquatic environments, and there is no information on the fate of glyphosate on Australian soils from the last 22 years apart from a single study in Western Australia. This research gap prompted an investigation into glyphosate sorption behaviour in Australian soils of the different mineral composition due to increased usage of glyphosate as a single strategy to

control weeds. The second major problem is the evolution of glyphosate-resistant weeds like *Chloris virgata* in New South Wales and Queensland, Australia, a major threat to sustainable weed control strategies, and due to paucity of information on the management of *Chloris virgata*, we hypothesized that cultural methods like crop competition can be used as a strategy to control glyphosate-resistant *Chloris virgata*.
