Effect of Temperature and Moisture on Quinclorac Soil Half-life and Resulting Native Grass and Forb Establishment

Quinclorac will control leafy spurge and not injure many established native grasses and forbs. Seeding of desirable species is often required to reestablish native vegetation after an invasive weed-management program, but quinclorac residue may inhibit the reestablishment of native species. Greenhouse studies were conducted to estimate quinclorac dissipation rates in Northern Great Plains soils and the effect of residue on establishment of some native grass and broadleaf plants. Quinclorac 50% dissipation time (DT₅₀) ranged from > 21 to 112 d in four soils from the Northern Great Plains. The quinclorac DT₅₀ was dependent on several factors including soil type, moisture content, temperature, and especially organic matter (OM). Across four different soil textures, quinclorac dissipation generally increased as soil moisture content increased, but moisture had less of an impact in low OM soils. Quinclorac dissipation also increased as temperature increased in the four soils. The most rapid dissipation occurred in soils with higher OM (> 6%), with an average DT₅₀ of < 38 d, at 45% moisture content, held at 16 C. Wild bergamot, purple coneflower, blanketflower, and stiff goldenrod seedling growth were all reduced by quinclorac residue at 6 μg kg⁻¹, the lowest concentration evaluated in the study. The native grass species big bluestem, intermediate wheatgrass, and switchgrass generally were tolerant of quinclorac, but green needlegrass was sensitive, and seedling growth declined as quinclorac residue increased from 6 to 375 μg kg⁻¹. Based on a quinclorac application of 840 kg ha⁻¹ and 150 frost-free d, seeding of sensitive forbs and grasses should be delayed at least 12 mo after herbicide application. Nomenclature: Quinclorac; leafy spurge, Euphorbia esula L.; wild bergamot, Monarda fistulosa L.; big bluestem, Andropogon gerardii Vitman; blanketflower, Gaillardia aristata Pursh; green needlegrass, Nassella viridula (Trin.) Barkworth; intermediate wheatgrass, Thinopyrum intermedium (Host) Barkworth & D.R. Dewey; purple coneflower, Echinacea purpurea (L.) Moench; stiff goldenrod, Oligoneuron rigidum (L.) Small var. rigidum; switchgrass, Panicum virgatum L. Management Implications: Quinclorac can be used to control leafy spurge in pasture, rangeland, and wildland, with the added benefit of having little effect on most established native forbs and grasses. Seeding of native species is often desirable in a long-term invasive weed-management program, but these programs can fail if a herbicide is not used to control the target weed or other nondesirable species before seeded species establishment. However, herbicide residue can also result in reduction of desired species establishment and density. Quinclorac soil half-life (DT₅₀) ranged from > 112 to 21 d in four soils from the Northern Great Plains. The quinclorac DT₅₀ was dependent on several factors, including soil type, moisture content, temperature, and especially organic matter. Across four different soil textures, quinclorac dissipation generally increased as soil moisture content increased, but moisture had less of an effect in low organic matter soils. Based on the results of this study, between 140 and 190 g ha⁻¹ quinclorac could remain 12 mo after application at 840 g ha⁻¹ and a growing season of 150 frost-free d. Wild bergamot, purple coneflower, blanketflower, and stiff goldenrod seedling growth were all reduced by quinclorac residue at 6 μg kg⁻¹, the lowest concentration in the study. The native grass species big bluestem, intermediate wheatgrass, and switchgrass generally were tolerant of quinclorac but green needlegrass was sensitive, and seedling growth declined as quinclorac residue increased from 6 to 375 μg kg⁻¹. Landmangers that apply quinclorac for leafy spurge control should not plan to reseed the treated area until at least 12 mo after application or longer in low organic matter soils or cold conditions.