GM Free Cymru

Glyphosate-resistant weeds can wipe out a GM cotton crop after 3 years

Date Added to website 16th May 2014

This is a scary paper, which is based on experiments with GM (RR) cotton in Arkansas, USA. It shows that Palmer amaranth, a weed that infests cotton plantations, can very quickly develop glyphosate resistance, and from that point onwards, there is no stopping it. Even with 4 glyphosate applications per year on the cotton crop, from the date of original infestation with glyphosate-resistant Palmer amaranth, it only took 3 harvesting seasons for the whole cotton crop to be so heavily infested that it could not be harvested. The only way that GM cotton can continue to be grown in certain areas is to employ "multiple means of weed control." Er, wasn't the whole idea of GM cotton that "multiple means of weed control" could be avoided...........?


"..........In only 2 yr after introduction, GR Palmer amaranth had colonized each field, spreading from field edge to field edge. Although yields were not affected as a direct result of Palmer amaranth the first year after introduction, the implications of resistance evolution going ''unnoticed'' in the first year can have a devastating impact in the subsequent years. The amount of seed produced by GR Palmer amaranth allows it to rapidly spread throughout a field or entire farm. By the third cropping season after introduction (Year 2010), complete crop failure had occurred. The competition from high densities of Palmer amaranth resulted in little to no cotton at harvest. Moreover, the high densities in 2010 made harvest impossible due to potential equipment failure."


"............multiple means of control will be needed over an extended growing season due to the season-long emergence of Palmer amaranth..."


".........the spatial approach we implemented in this study was extremely valuable in understanding the pattern of withinfield dispersal of Palmer amaranth and demonstrating that a single escape is way too many to allow for this species, justifying the need for a zero-tolerance approach in managing this weed."

The Paper:

Jason K. Norsworthy, Griff Griffith, Terry Griffin, Muthukumar Bagavathiannan, and Edward E. Gbur (2014)

In-Field Movement of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) and Its Impact on Cotton Lint Yield: Evidence Supporting a Zero-Threshold Strategy.

Weed Science: April-June 2014, Vol. 62, No. 2, pp. 237-249.

Abstract This research was aimed at understanding how far and how fast glyphosate-resistant (GR) Palmer amaranth will spread in cotton and the consequences associated with allowing a single plant to escape control. Specifically, research was conducted to determine the collective impact of seed dispersal agents on the in-field expansion of GR Palmer amaranth, and any resulting yield reductions in an enhanced GR cotton system where glyphosate was solely used for weed control. Introduction of 20,000 GR Palmer amaranth seed into a 1-m2 circle in February 2008 was used to represent survival through maturity of a single GR female Palmer amaranth escape from the 2007 growing season. The experiment was conducted in four different cotton fields (0.53 to 0.77 ha in size) with no history of Palmer amaranth infestation. In the subsequent year, Palmer amaranth was located as far as 114 m downslope, creating a separate patch. It is believed that rainwater dispersed the seeds from the original area of introduction. In less than 2 yr after introduction, GR Palmer amaranth expanded to the boundaries of all fields, infesting over 20% of the total field area. Spatial regression estimates indicated that no yield penalty was associated with Palmer amaranth density the first year after introduction, which is not surprising since only 0.56% of the field area was infested with GR Palmer amaranth in 2008. Lint yield reductions as high as 17 kg ha−1 were observed 2 yr after the introduction (in 2009). Three years after the introduction (2010), Palmer amaranth infested 95 to 100% of the area in all fields, resulting in complete crop loss since it was impossible to harvest the crop. These results indicate that resistance management options such as a "zero-tolerance threshold" should be used in managing or mitigating the spread of GR Palmer amaranth. This research demonstrates the need for proactive resistance management.