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Biology and Control of Glyphosate-Resistant Giant Ragweed.

Giant ragweed is a troublesome, early emerging summer annual weed found throughout the Midwest and eastern Corn Belt. Knowledge about the emergence pattern of giant ragweed and how it responds to tillage can help in developing effective integrated management strategies. The objectives of this study were to explore the emergence pattern of a glyphosate-resistant giant ragweed population from Nebraska and to investigate the effect of spring tillage on timing of giant ragweed emergence. Field experiments were conducted in a grower’s field in 2012 and 2013 near David City, NE infested with a glyphosate-resistant giant ragweed population confirmed in 2010 in the same field. Treatments included four tillage timings at a bi-weekly interval starting when the first giant ragweed seedlings were observed in the field early in the spring. Results suggested that tillage timing had no effect on total giant ragweed seedling emergence and days required to 50% emergence (T50); however, results for these response variables differed between years. More giant ragweed seedlings emerged in 2012 compared to 2013, probably due to higher temperature during early season in 2012. The weibull function that included TT (Thermal time) and HTT (Hydrothermal time) as the explanatory variables was fit to describe the emergence data. The base temperature of 4 C was selected from a set of 16 temperatures ranging from 2 to 17 C using Akaike’s information criteria (AIC). Results from the TT model suggested that 50% giant ragweed emergence occurred after accumulation of 106 growing degree days (GDD). Giant ragweed emerged from late March until mid-June with the majority of emergence ceasing by the beginning of May. There was no giant ragweed emergence observed after tillage