Iowa State University

Are weeds getting harder to kill with glyphosate?
by Bob Hartzler

December 20, 2004 - There is little doubt that the average use rate of glyphosate in Roundup Ready soybeans has increased significantly since the introduction of the herbicide resistant varieties.  There are several possible explanations for this rate increase:  1) an increase in glyphosate tolerance within a species (e.g. waterhemp), 2) an increase in the prevalence of species naturally tolerant of glyphosate (e.g. Asiatic dayflower, wild buckwheat), or 3) the lower cost of glyphosate has resulted in increased rates for 'insurance' purposes.   Scientists in Australia (Eyres et al. 2004) and Illinois (Volenberg et al. 2004) investigated this phenomenon in rigid ryegrass and waterhemp, respectively, in efforts to determine whether repeated exposure to glyphosate has caused a gradual increase in the weeds' tolerance to glyphosate.

The Australian project evaluated populations of rigid ryegrass with known histories of glyphosate exposure (low = 1-5 applications; medium = 6-14 applications; high = >14 applications).   A dose response study was conducted with these populations to determine if glyphosate tolerance was correlated with glyphosate history.  While significant differences were found in the LD₉₀ (rate required to kill 90% of the population) among the populations, differences were not related to glyphosate use history.

The Illinois project evaluated the response of 100 waterhemp populations collected in different years.  Ten populations were collected prior to 1996, 45 in 1998-99, and 45 in 2003.  Populations collected in 2003 would be expected to have been exposed to glyphosate more frequently than earlier collected populations.  As with ryegrass, differences in glyphosate sensitivity were observed among the waterhemp populations, but tolerance was not associated with collection date. 

Both the Australians and Americans concluded that there was no evidence of an increase in glyphosate tolerance in ryegrass and waterhemp populations associated with exposure to glyphosate.  Monsanto's glyphosate resistance management recommendations (use of full rates of glyphosate rather than rotation of different strategies) are based on the premise that development of resistance will be due to gradual increases in tolerance/resistance within a population following exposure to sub-lethal doses of glyphosate.  These two studies found no evidence of rate creep in two species with a documented history of adapting to management strategies.  While glyphosate resistance has not been found in waterhemp, rigid ryegrass was the first species with confirmed resistance to glyphosate.  At this time 39 resistant rigid ryegrass populations have been identified.  I believe the lack of evidence for rate creep suggests that simply using full rates of glyphosate will not effectively reduce the risk of selecting resistant weeds.

Eyres, J., P. Neve, S. Powles.  2004.  Ryegrass and glyphosate rate creep.  Western Australian Herbicide Resistance Initiative.  http://wahri.agric.uwa.edu.au/News%20&%20Views%20Articles/Summer200405/JEratecreep.htm

Volenberg, D.S., W.L. Patzoldt, A.G. Hager, and P.J. Tranel.  2004.  Has waterhemp become harder to control with glyphosate?  North Central Weed Science Society Proceedings 59:84.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University