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 a
