2010 North Central Weed Science Society Conference Proceedings 65:100
SENSITIVITY OF GLYPHOSATE-RESISTANT AMARANTHUS TO GLYPHOSATE IS ALTERED BY SOIL APPLIED NITROGEN. Jon R. Kohrt*, Bryan G. Young, Joesph L. Matthews, Julie M. Young; Southern Illinois University, Carbondale, IL (100)
ABSTRACT
The phenotypic expression of glyphosate resistance in Amaranthus populations may be influenced by herbicide application variables such as field and environmental conditions. Previous field observations of improved control of glyphosate-resistant (GR) common waterhemp populations with glyphosate in corn production compared with soybean suggest that the presence of nitrogen fertilizer may influence the phenotypic expression of glyphosate resistance. Field and greenhouse experiments were conducted to determine the influence of soil-applied nitrogen fertilizer on the growth rate and sensitivity of glyphosate-resistant Palmer amaranth and common waterhemp to glyphosate. Field experiments with GR waterhemp were conducted in De Soto, IL in 2011 and 2012 while experiments with GR Palmer amaranth were conducted in Collinsville and Valmeyer, IL in 2012. In the greenhouse the addition of supplemental nitrogen fertilizer increased the relative growth rate, as a function of shoot height and volume, of glyphosate-susceptible and –resistant common waterhemp and Palmer amaranth. The glyphosate-susceptible populations of both common waterhemp and Palmer amaranth were more sensitive to glyphosate under high nitrogen rate compared to no nitrogen. However, only the GR common waterhemp was sensitive to glyphosate applications under high nitrogen and the same was not observed for Palmer amaranth. Ultimately, the higher application rate of nitrogen reduced the magnitude of resistance in common waterhemp, but not Palmer amaranth as that species exhibited exceptionally high magnitudes of resistance in the greenhouse which likely masked any influence of nitrogen. Field experiments confirmed the possibility that nitrogen fertilizer can influence the response of glyphosate-resistant amaranth to glyphosate, but the results were variable, which suggests the importance of soil moisture and other environmental variables in the field. In summary, the application of nitrogen fertilizer, or lack thereof, may influence the survival of common waterhemp and Palmer amaranth plants in fields infested with glyphosate-resistant populations.