Changes in weed species since the introduction of glyphosate-resistant cotton

Abstract

Weed management practices in cotton systems that were based on frequent cultivation, residual herbicides, and some post-emergent herbicides have changed. The ability to use glyphosate as a knockdown before planting, in shielded sprayers, and now over-the-top in glyphosate-tolerant cotton has seen a significant reduction in the use of residual herbicides and cultivation. Glyphosate is now the dominant herbicide in both crop and fallow. This reliance increases the risk of shifts to glyphosate-tolerant species and the evolution of glyphosate-resistant weeds. Four surveys were undertaken in the 2008–09 and 2010–11 seasons. Surveys were conducted at the start of the summer cropping season (November–December) and at the end of the same season (March–April). Fifty fields previously surveyed in irrigated and non-irrigated cotton systems were re-surveyed. A major species shift towards Conyza bonariensis was observed. There was also a minor increase in the prevalence of Sonchus oleraceus. Several species were still present at the end of the season, indicating either poor control and/or late-season germinations. These included C. bonariensis, S. oleraceus, Hibiscus verdcourtii and Hibiscus tridactylites, Echinochloa colona, Convolvulus sp., Ipomea lonchophylla, Chamaesyce drummondii, Cullen sp., Amaranthus macrocarpus, and Chloris virgata. These species, with the exception of E. colona, H. verdcourtii, and H. tridactylites, have tolerance to glyphosate and therefore are likely candidates to either remain or increase in dominance in a glyphosate-based system.

EPSPS gene amplification conferring resistance to glyphosate in windmill grass (Chloris truncata) in Australia

“…Diffusion is the most likely mode of transport across the plant cuticle, which varies in composition and thickness in various plant species. These results support field observations of high glyphosate tolerance in C. truncata and studies reporting resistance to glyphosate in C. truncata populations …”

Target‐site mutations conferring resistance to glyphosate in feathertop Rhodes grass (Chloris virgata) populations in Australia

“…However, resistant populations required higher doses than the susceptible population for control (Table ). These results support field observations of high innate glyphosate tolerance in C. virgata …”

“…Feathertop Rhodes grass ( Chloris virgata Swartz) is a warm‐season, C 4 , annual grass that has been considered a major weed in cotton ( Gossypium hirsutum L.) and grain crops in the subtropical region of Australia for many years, and has become an emerging weed in southern Australia over the last 5 years. In a 2008 and 2010 survey of the northern subtropical grain region of Australia, C. virgata was ranked as one of the 20 most important weed species . C. virgata was the third most common weed species identified in 50% of paddocks in a summer fallow survey in 2012 .…”

“…In Australia, C. virgata is considered hard to kill with glyphosate . The dominance of reduced‐tillage systems combined with the heavy reliance on glyphosate for weed control has made it difficult to control this weed species effectively .…”

Genetically modified foods: A critical review of their promise and problems

“…"Starlink" maize provides an example of a food hazard caused directly by the expression of the inserted gene [29,35,[37][38][39]. The modified plant was engineered with genetic information from Bacillus thuringinesis in order to endow the plant with resistance to certain insects.…”