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Giant ragweed is a troublesome, early emerging, summer annual weed found throughout the eastern and midwestern corn and soybean growing regions of the United States. Since the emergence of giant ragweed varies at different locations, our first objective was to determine the emergence pattern of giant ragweed in Nebraska and how spring tillage influences emergence. Results of a two-year study suggested that giant ragweed emerged from late March until mid-June, with the majority of emergence ceasing by early May. Spring tillage could be used as an alternative method for managing glyphosate resistant giant ragweed.
Water stress can affect the growth and development of both crop plants and weeds. Thus, in our second objective, we hypothesized that drought conditions can result in a water deficit that can hinder giant ragweed growth and reproduction. Results suggested that the degree of water stress had more effect on plant growth and fecundity compared to the duration of water stress. Plants watered at a 10-day interval with 100% field capacity were still able to produce seeds, whereas only a few plants survived at 12.5% soil moisture content when irrigated at a 2-day interval.
Early emergence and a rapid growth rate make giant ragweed a competitive weed early in the season and reduce crop yields; therefore, in our third objective, we determined the early spring control of giant ragweed using a preplant herbicide. Several herbicide programs were investigated with preplant followed by pre-emergence (PRE) and post-emergence (POST) herbicides for controlling glyphosate-resistant giant ragweed
in glufosinate-resistant soybean. Results suggested that herbicide programs containing 2,4-D in preplant followed by an in-crop application of glufosinate provided 99% control of glyphosate-resistant giant ragweed and increased soybean yields.
Finally, since fall and early spring application of herbicides may influence giant ragweed emergence, our fourth objective was to determine the effect of fall and/or early spring application of a prepackaged mixture of iodosulfuron and thiencarbazone-methyl applied alone or tank-mixed with 2,4-D, dicamba, or metribuzin on glyphosate-resistant giant ragweed in no-till corn. Results suggested that the premix of iodosulfuron and thiencarbazone-methyl tank-mixed with 2,4-D, dicamba, or metribuzin followed by PRE and POST herbicide applications provided > 95% control of glyphosate-resistant giant ragweed in no-till corn.
Advisor: Amit J. Jhala