**2. Glyphosate-resistant weeds**

The second major aspect in this review paper is the evolution of glyphosate resistance in weeds due to heavy reliance on glyphosate. Glyphosate toxicity and glyphosate resistance are not different but connected problems, as glyphosate is applied to control weeds and its application results in movement of glyphosate to water bodies via soil systems affecting human health. When glyphosate-contaminated drinking water is used for human consumption, it may potentially result in diseases like cancer or chronic kidney disease; however, frequent application of glyphosate not only results in its downward movement via soil systems but also results in the development of glyphosate resistance in weeds. Hence these problems are interconnected.

While assessing the weeds at risk of evolving glyphosate resistance in Australian subtropical glyphosate-resistant cotton systems, species with the highest risk to glyphosate resistance were *Brachiaria eruciformis*, *Conyza bonariensis*, *Urochloa panicoides*, *Chloris virgata*, *Sonchus oleraceus*, and *Echinochloa colona* [51]. Thirtyeight weeds in total distributed over 37 countries have shown resistance to glyphosate [52]. These weeds represent the greatest threat to sustainable weed control practices [52]. Weed surveys in the cotton-growing areas of New South Wales (NSW) and Queensland, Australia, indicated the dominance of *Conyza bonariensis*, *Echinochloa colona*, and *Chloris virgata* [53].

*Chloris virgata* is a high-risk species to glyphosate resistance in summer fallow [51]. Glyphosate resistance in *Chloris virgata* populations in Australia has emerged due to transformation in Australian cropping systems, particularly unirrigated cotton systems, from regular tillage and use of residual herbicides to minimum or no-tillage systems with a heavy reliance on glyphosate [54]. This lack of tillage is the major reason for the emergence of weeds like *Chloris virgata* that are small-seeded and emerge at or close to the surface [54]. A weed management system depending on only one tactic, for example, application of glyphosate, is the main driver for this species shift. With repeated use of glyphosate, *Chloris virgata* populations have become less susceptible to glyphosate formulations, especially after the early tillering stage [54].

Mechanisms involved in providing resistance to glyphosate in weeds include (i) target-site alterations (target site mutation, target site gene amplification) [55, 56] and (ii) non-target site mechanisms involving reduced glyphosate uptake and/ or reduced translocation of glyphosate [57–59]. The alterations inhibit glyphosate binding or increase the effective dose needed for enzyme inhibition. Target site EPSPS mutations are the primary mechanism conferring glyphosate resistance in populations of *Chloris virgata* [55].

**111**

be 4.2–28.2 plants m<sup>−</sup><sup>2</sup>

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

*Chloris virgata* as a glyphosate-resistant weed [51] has also been identified as a host for barley yellow dwarf and cereal yellow dwarf viruses [60]. As *Chloris virgata* can tolerate high-salinity and high-alkalinity soil environments, *Chloris virgata* can form a dominant community in these environments [61, 62]. *Chloris virgata* is tolerant to drought stress [63]. Many studies on *Chloris virgata* seed biology have been completed in China, India, Qatar, and Honduras [63], while very few studies have

*Chloris virgata* grass seed biology includes the study on dormancy, germination conditions, seed bank dynamics, growth, and development [66]. Dormancy mechanisms enable the seed to sense the optimum environmental conditions for the establishment of seedlings and hence play a pivotal role in control strategies for weedy grasses [67]. There are two types of seed dormancy mechanisms, those based in the tissues surrounding the embryo (seed coat based) or those found within the embryo [67]. The role of smoke in breaking the dormancy of plump windmill grass (*Chloris ventricosa*), a related species to *Chloris virgata* grass [68], has been reported; but no study related to dormancy breakdown of *Chloris virgata* grass by smoke has been reported. The seeds of *Chloris virgata* are triangular in shape and light in weight and hence shed easily from the heads making them good wind (anemochory) and water

Seed germination is a key event in the growth of annual plants like *Chloris virgata* grass which is regulated by several environmental factors such as temperature and water potential [69–71]. High rainfall has been associated with *Chloris virgata* population outbreaks [72], suggesting that water plays an important role in the germination process. *Chloris virgata* grass possesses the C4 photosynthesis mechanism and has better water use efficiency than grasses having the C3 photosynthesis mechanism. Among all the potential factors for *Chloris virgata* germination; light, salinity, and osmotic potential are the most critical factors [64]. A light requirement for germination has been observed among many small-seeded species and warm-season grasses [67, 73]. In a study related to germination responses of *Chloris virgata* to temperature and reduced water potential, maximum germination percentages of *Chloris virgata* seeds were found at 15–25°C [74]. Germination of *Chloris virgata* seeds is affected by several factors; however, temperature and light play a significant role in the germination of *Chloris virgata* seeds. More studies on factors affecting *Chloris virgata* growth are needed due to the paucity of information. In a study related to growth, development, and seed biology of *Chloris virgata* in South Australia, *Chloris virgata* seedlings emerging after summer rainfall events under field conditions needed 1200 growing degree days from emergence to mature seed production [65]. Harvested seeds of *Chloris virgata* were dormant for a period of about 2 months and took 5 months of after-ripening to reach 50% germination [75]. Seedling emergence of *Chloris virgata* was highest (76%) for seeds present on the soil surface and seedling emergence was significantly reduced by burial at 1 (57%), 2 (49%), and 5 cm (9%) soil depth. Furthermore, *Chloris virgata* seeds

buried in the soil persisted longer than those left on the soil surface [75].

and 8.3–146.1 g dry biomass m<sup>−</sup><sup>2</sup>

Water stress due to extremely low rainfall over the summer months was the reason for the delayed growth of *Chloris virgata* under rained conditions when

The thermal time to panicle emergence of *Chloris virgata* is similar to shattercane (*Sorghum bicolor*) [76]. A related species of *Chloris virgata*, windmill grass (*Chloris truncata*) under irrigated field requires 21–23, 43–45, and 74–75 days from seedling emergence to reach tillering, panicle emergence, and mature seed stage [75]. Maximum plant density and biomass in case of windmill grass have been found to

depending on location [77].

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

been conducted in Australia [64, 65].

(hydrochory) dispersers [64].

**2.1** *Chloris virgata* **(feathertop Rhodes grass)**
