Managing Glyphosate-Resistant Palmer Amaranth
by Ted Webster and Stanley Culpepper
SITUATION: Palmer amaranth is the number one problem weed in Georgia. Approximately 84% of the population is ALS-resistant. Add resistance to glyphosate, and the problem becomes more critical.
DISTRIBUTION: The primary areas of resistant Palmer amaranth in Georgia are the Coastal Plain and the Piedmont areas, traditionally areas of high cotton production. Fields in Central Georgia (136 fields) were sampled in 2005 and 2006 to determine the extent of glyphosate-resistant Palmer amaranth; in 2005 and 2006, 49 and 62% of the sampled fields, respectively, had glyphosate-resistant populations. Sampling in 2007 expanded beyond counties in close proximity to the initial field location of glyphosate-resistant Palmer amaranth, and it appears that resistance is spreading. Palmer amaranth has been observed growing in a multitude of ecosystems, including roadsides, forest edges, and in plasti-culture vegetable production systems.
IMPACTS: The occurrence of glyphosate-resistant Palmer amaranth doubles the cost of effective weed control. The herbicides that are the basis for weed control of glyphosate-resistant Palmer amaranth rely on timely rainfall or irrigation for activation to be effective for weed control. In some fields in summer of 2007, growers could not harvest their crop due to their inability to control Palmer amaranth.
RESEARCH: Pollen dispersal: Pollen movement is likely to be a key factor in the spread of glyphosate within and across various Amaranthus species. Information concerning the physical characteristics of the pollen grains, which is needed to predict the potential pollen dispersal distance over the landscape, is as follows:
- Palmer amaranth pollen diameter ranged from 28 to 32 µm.
- The settling velocity of Palmer amaranth pollen is being evaluated.
- Pollen longevity is a critical component that needs to be accounted for when discussing potential dispersal distances, as only pollen capable of transferring the resistance characteristic is of interest.
- In pollen transmission studies, the glyphosate-resistance trait was transferred from males to glyphosate-susceptible females at a distance of 250 m, the farthest distance tested.
- Interference of Palmer amaranth with cotton: One Palmer amaranth plant per 20 row feet, which is a very low density in areas with naturalized Palmer amaranth populations, reduced cotton yields 7%.
- Glyphosate absorption and translocation: No differences could be detected in susceptible and resistant plants at 48 hours after application. Research has been initiated to evaluate possible glyphosate sequestration.
- Comparative growth of resistant and susceptible Palmer amaranth: Susceptible plants were taller than resistant plants and had a higher gas exchange curve, although it was suggested that plant size may simply be a result of variability in populations of Palmer amaranth.