The Sheyenne National Grassland (SNG) is a native tall grass and mixed grass prairie located in southeastern North Dakota. Approximately half of the SNG has been invaded by leafy spurge (Euphorbia esula L.) and control methods have been limited. Many herbicides cannot be utilized at the SNG due to sandy soils (> 80%) and shallow groundwater, nor can they be applied near the western prairie fringed orchid (Platanthera praeclara Sheviak and Bowles), a federally listed threatened plant found on the SNG. Quinclorac and aminocyclopyrachlor were considered for use on the SNG but potential to enter the groundwater was unclear. Movement of quinclorac and aminocyclopyrachlor was evaluated in 70 cm soil columns that simulated field conditions from five ecological sites. Quinclorac leached further following the heavy rainfall event of 15 cm in 48 h compared to the annual precipitation of 51 cm applied over 9 wk. Quinclorac leached approximately 45 cm into the soil profile averaged over all soil types and both precipitation events but never exceeded 65 cm regardless of soil type. Aminocyclopyrachlor leaching was greater than quinclorac and moved through all soil types into the leachate following both watering regimes. Desorption of both herbicides was incomplete. Approximately 10 and 32% of applied aminocyclopyrachlor and quinclorac, respectively, remained in the top 5 cm of soil regardless of soil type or watering regime. Quinclorac but not aminocyclopyrachlor was considered suitable for use at the SNG to control leafy spurge and has the added benefit of not harming the western prairie fringed orchid.
Nomenclature: Aminocyclopyrachlor; quinclorac; leafy spurge, Euphorbia esula L.; western prairie fringed orchid (Platanthera praeclara Sheviak and Bowles).
Management Implications: The Sheyenne National Grassland (SNG) is a native tall grass and mixed grass prairie located in southeastern North Dakota that has been invaded by leafy spurge (Euphorbia esula L.). Most herbicides commonly used for leafy spurge control cannot be used on the SNG due to very sandy soils (> 80%) and shallow groundwater. Previous control programs had utilized picloram which subsequently was found in a stock well on the grassland. The widely successful biological control agent Aphthona spp. has not established in a high enough population to noticeably reduce leafy spurge. At the time of the study, 2,4-D was the single herbicide option that could be used on the SNG, but only provided short-term (< 3 mo) top-growth control. A new noxious weed management plan was adopted which included aminocyclopyrachlor and quinclorac as possible options for leafy spurge control. However, the potential for leaching into the groundwater was a concern. Movement of quinclorac and aminocyclopyrachlor was evaluated in soil columns that replicated field conditions from five ecological sites on the SNG. Precipitation was applied to the 70 cm long soil columns as either the average annual precipitation of 51 cm applied over 9 wk or the largest recorded heavy rain event of 15 cm water over 48 h. Quinclorac leached to a depth of approximately 45 cm following the 48 h heavy rain event while movement did not exceed 65 cm following the average annual precipitation regardless of soil type. Aminocyclopyrachlor leaching was greater than quinclorac and moved through the column into the leachate in all soil types following both precipitation events. Approximately 10 and 32% of applied aminocyclopyrachlor and quinclorac, respectively, remained in the top 5 cm of soil regardless of soil type or watering regime. Only quinclorac was considered suitable for use on the SNG to control leafy spurge and has the added benefit of not harming the western prairie fringed orchid (Platanthera praeclara Sheviak and Bowles), a federally listed threatened plant found throughout the SN
