Repeated Sublethal Rates of Glyphosate Lead to Decreased Sensitivity in Palmer Amaranth

Abstract

Glyphosate-resistant Palmer amaranth is the result of over-reliance on glyphosate for weed control in glyhosate-resistant crops. The use of lower-than-recommended rates of glyphosate is associated with the development of glyphosate resistance; however, the impact of low rates of glyphosate on development of glyphosate-resistant Palmer amaranth is lacking, and hence, an experiment was conducted to determine if glyphosate-resistant Palmer amaranth could be selected through treatment of plants with sublethal glyphosate rates. Three hundred seedlings of a known susceptible Palmer amaranth biotype were treated with glyphosate at 0.094 lb ae/acre at the eight-leaf stage. Plants that survived the glyphosate application (14%) were allowed to produce seeds. The sequence of selection to higher glyphosate rates was continued until the fourth generation (i.e., F0 to F3), after which rate-mortality response was determined. Following four generations of selection, the F4 population showed a 2.15-fold less sensitivity to glyphosate than the initial population based on an LD95. Survival of the F4 generation did not occur when glyphosate was applied at the recommended rate of 0.75 lb/acre. This research provides evidence that repeated use of sublethal rates of glyphosate can contribute to reduced sensitivity of Palmer amaranth to glyphosate, which may play a partial role in the development of glyphosate resistance.