Preplant Control of Horseweed for No-Till Cotton in Oklahoma

Horseweed continues to be a challenge to control in many no-till cotton fields in Oklahoma.  Due to its competitive nature a failure to control horseweed prior to planting results in reduced stands, harvest complications and reduced lint yields.  Controlling horseweed prior to planting is essential since in-season chemical options are either in-effective or not economical.  In addition, the discovery of glyphosate resistant horseweed in numerous cotton production areas of Oklahoma establishes the need for reliance on alternative chemistries.  Sulfonylurea herbicides offer a different site-of-action compared to glyphosate and have proven to be effective on many broadleaf weed species.  Traditionally hormone type herbicides have been the basis of many preplant horseweed control programs for cotton but many cotton producers prefer not to use these chemistries due to the likelihood of a potential tank and/or boom contamination scenario later in the year.  This project focused on the use of three sulfonylurea herbicides: Firstshot, Panoflex and Amathon.  Each of these herbicides were applied alone, with 2,4-D or with both 2,4-D and Glyphosate.  Applications were made in early April of 2013 to horseweed ranging in size from 1-4 inches (rosettes).  Evaluations were taken at 14, 30 and 45 days after treatment.  Analysis of variance was performed on the data and conclusions will be provided