Occurrence and Management of Glyphosate Resistant Amaranthus Weed Species in Central Texas Cotton and Evaluation of Spray Droplet Size Spectra as Affected by Nozzle Design and Herbicide Formulation

With the recent confirmation of glyphosate-resistant Amaranthus species in many important agronomic regions of the United States, a study was initiated to identify and document the occurrence of glyphosate resistant common waterhemp in East-central Texas. Accessions of several suspected glyphosate-resistant biotypes of common waterhemp were grown in a greenhouse before receiving rates of glyphosate from 434 to 3468 g ae ha^-1. Dose-response analyses were conducted to provide LD50 values for each accession. LD50 values ranged from 387 to 4549 g ae ha^-1 glyphosate.

A study evaluating the efficacy of twelve different weed control programs for common waterhemp and Palmer amaranth control in cotton was conducted in Burleson County, TX in 2012 and 2013. The study was conducted in cotton possessing stacked glyphosate-, glufosinate-, and dicamba-tolerant technologies. Preplant and preemergence treatments included fomesafen, pendimethalin, prometryn, pyrithiobac, S-metolachlor, and trifluralin. These treatments were followed by a variety of early- and mid-postemergence treatments. Preplant and preemergence treatments resulted in 81 to 100% control of Palmer amaranth and common waterhemp with the exception of pyrithiobac, which provided only 29 to 60% control of these species. Following early- and mid-postemergence applications, 92 to 100% control of these species was obtained. Applications of pendimethalin PRE followed by pyrithiobac EPOST and glufosinate MPOST in 2013 provided lower control of both species (92 to 93 %) than all other treatments evaluated in the study 14 days after MPOST applications.

With the potential commercialization of synthetic auxin-tolerant crops, there is an increased need for understanding of the influence of spray nozzle design and herbicide formulation on physical spray drift reduction. A study was conducted in a low speed wind tunnel utilizing laser diffraction technology to analyze the droplet size spectra produced by different spray nozzles and herbicide formulations. Nozzles utilizing a pre-orifice design or a combination of pre-orifice and air-inclusion design were observed to produce significantly larger spray droplets than those without these features. Herbicide formulations were shown to have a significant influence on droplet size as well. Different herbicide formulations were observed to decrease the production of drift-prone fine droplets by as much as 64%.