Graham Charles

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.

Tolerance of cotton expressing a 2,4-D detoxification gene to 2,4-D applied in the field

The tolerance to 2,4-dichlorophenoxy acetic acid (2,4-D) of a genetically modified (transgenic) cotton (Gossypium hirsutum) expressing a 2,4-D detoxification gene was compared with conventional (non-transgenic) cotton over 2 seasons. The 2,4-D was applied over-the-top of cotton in the field at 7–17 nodes of crop growth at rates of 0.004–1.12 kg a.i./ha. The transgenic cotton displayed better tolerance to 2,4-D than conventional cotton at all growth stages and herbicide rates. Some damage was apparent on both types of cotton at 2,4-D rates of 0.07 kg/ha and above, with damage most pronounced when the plants were exposed at 7 nodes. The transgenic cotton also had some tolerance to MCPA. Commercial use of transgenic, 2,4-D-tolerant cotton has the potential to greatly reduce problems of 2,4-D damage in cotton from accidental spray drift and herbicide residues in spraying equipment, where plants are predominantly exposed to low rates of 2,4-D.