Evaluation of herbicide programs for the management of glyphosate-resistant waterhemp (Amaranthus rudis) in Maize

Field experiments were conducted in corn to evaluate the influence of herbicide treatments on glyphosate resistant waterhemp control in Platte County, Missouri during 2006 and 2007. Pre-emergence (PRE), pre-emergence followed by post-emergence (PRE fb POST), and post-emergence-only (POST-only) herbicide programs were evaluated for use in conventional, glyphosate-resistant, or glufosinate-resistant corn hybrids. All programs containing a PRE herbicide, conventional POST herbicide, or glufosinate resulted in greater than 98% control of glyphosate-resistant waterhemp and reduced seed production by at least 99%. Waterhemp densities were also reduced by programs that contained a PRE herbicide, conventional POST herbicide, or glufosinate. The highest waterhemp density and poorest control occurred with a sequential glyphosate program. Corn yields did not differ between herbicide treatments in 2006 however certain conventional PRE and PRE fb POST programs increased yields compared to the sequential glyphosate program in 2007. Results from this research indicate that several herbicide options are available for the control of glyphosate-resistant common waterhemp in corn. However, the lack of crop rotation at this field site for a period of at least 15 consecutive years and lack of residual herbicide applications typically utilized in corn likely contributed to the increased sensitivity of this waterhemp population to PRE-only corn herbicide programs.