ASHS 2025 Conference

Minnesota (MN) in the US ranks 2nd for corn silage production and 3rd for soybean production. Studies have shown that off-target movement (drift) from an unshielded sprayer, in row crops, can range from 1 to 16% of the target dose; however, boom height and wind can double or triple these doses. Pre-study 37 pesticide residue tests were conducted in five Minnesota nurseries in 2018. Nine different herbicides were detected in the 37 tests. Glyphosate (sold as Roundup Power Max 32% glyphosate) became the focus of 2019 studies, conducted at concentrations of 0, 6, 18 and 54% for glyphosate as drift events. Relating ppm’s disclosed in laboratory analyte foliar residue samples collected by various state agriculture departments after drift events to injury symptom, development, and growth reduction or time after application have ever been conducted for nursery crops. From personal communication with nursery owners experiencing neighbor’s drift event (MD; KY, OH, MI, FL) the average time from the drift event to recognizing problems, and sample collection is six weeks (6 weeks after drift event) (6 WADE). Slower metabolism of glyphosate at higher residues, explained our finding that the largest residues were detected in all, but one species/ treatments used with glyphosate, versus three other herbicides examined or in combinations. Higher residues meaning higher injury, does not occur with glyphosate. Potentially high residues mean slow metabolism for glyphosate, and thus less injury. In five cases glyphosate applied alone had no impact on growth measured as caliper over the season, even though glyphosate had the highest residue in three of the five. In only one case was caliper growth reduction highest with glyphosate (Syringa 18%) over the entire study. In terms of phytotoxicity rating, glyphosate caused no commercially unacceptable injury in four cases. Residues increased with all rates on the three species evaluated between 3 DADE and 6 WADE. With Acer freemanii and Tilia cordata residue levels declined slightly between 6 WADE to 14 WADE with all rates. Only, Syringa reticulata showed an increase in foliar residues between 6 WADE to 14 WADE. Since carry over injury the next year (or three) is a concern with all glyphosate applications to perennial plants, all trees were observed in 2020. None were commercially acceptable and in severe decline (regardless of rate) in spring 2020, only 6% glyphosate on Syringa survived to the fall , and by spring 2021 were dead.