Evaluation of Weed Scouting Methods and the Effects of Glyphosate Drift in North Carolina Peanut (Arachis hypogaea) Production.

Abstract

Research was conducted to evaluate weed scouting methods in peanut, and the effects of glyphosate drift in peanut. Weeds were scouted on-farm in eight North Carolina counties in 2003 and 2004. A total of 16 unique fields were scouted, and weed species and density data were collected using four different scouting methods. HADSS was used to determine expected net return, yield loss ($ and %), and total treatment costs for each field. Net returns were averaged for optimal treatments in each field to determine the whole-field expected net return. Values of theoretical net return for the optimal treatment in each field ranged from $244 to $1,444 per ha, and averaged $867 per ha for all sixteen fields. The windshield method, while quickest, was also the least accurate at making herbicide recommendations. The range method was more accurate at estimating broadleaf weed densities than grass or sedge weed densities. Herbicide recommendations using the count method were more. The count method required the most time for completion (30 min). The maximum theoretical loss for fields scouted with the windshield method was as high as $528/ha. The windshield method resulted in accurate (top 10%) herbicide recommendations at least 82 and 77% of the time in 2003 and 2004, respectively. To evaluate peanut injury and pod yield when exposed to glyphosate, five experiments were conducted during 2001 and 2002 in North Carolina. Glyphosate was applied to 10 to15 cm diameter peanut plants at rates ranging from 9 to 1,120 g ai/ha. Visual injury was noted 7 DAT when glyphosate was applied at 70 g/ha and higher. Glyphosate at 280 g/ha or higher significantly injured the peanut plant and reduced pod yield. Shikimic acid accumulation was negatively correlated with visual injury and pod yield. Shikimic acid presence can be detected using plant samples, and accumulation can be an effective diagnostic tool for determining exposure to glyphosate in peanut 7 DAT, but not at 14, 21 and 31 DAT.