Leafy Spurge Control and Herbicide Residue from Annual Picloram and 2,4-D Application

Abstract

Annual application of picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) and picloram plus 2,4-D [(2,4-dichlorophenoxy)acetic acid] and biannual application of 2,4-D for 5 consecutive years was evaluated for leafy spurge (Euphorbia esula L.) control. The picloram treatments were evaluated for soil residue. The experiment was located at 2 sites in eastern North Dakota and 1 site in western North Dakota on various soil types. Picloram at 0.28, 0.42, and 0.56 kg/ha provided 48, 75, and 90% leafy spurge control after 4 annual treatments, respectively. Control increased to 85 and 91% when 2,4-D at 1.1 kg/ha was added to the annual treatment of picloram at 0.28 and 0.42 kg/ha, respectively. However, 2,4-D with picloram at 0.56 kg/ha did not increase leafy spurge control compared to picloram alone. Picloram did not accumulate in the upper 15 cm of the soil profile and generally was not detected above the 2 ppbw level 12 months following each annual application. Greater picloram residue was found deeper in sandy than clay soil and in soil with high compared to low organic matter. Picloram at 500 and 250 ppbw was required to reduce leafy spurge seedling emergence and subsequent survival by 50%, respectively. However, picloram at 125 ppbw reduced leafy spurge regrowth from root segments of 4 lengths to near zero. Picloram at 8 to 32 ppbw stimulated leafy spurge seedling emergence compared to the control. Annual application of picloram at low rates gradually controlled leafy spurge, but picloram soil residues were not high enough to control subsequent seed germination and shoot regrowth from roots.