Abstract: Woollyleaf bursage (Ambrosia grayi) is a noxious, rhizomatous perennial with an extensive creeping root system. It is found in the central and southern Great Plains of the U.S. Clopyralid alone or fluroxypyr, picloram, or glyphosate with either 2,4-D or dicamba were applied to woollyleaf bursage at anthesis and 30 d later in three field experiments. With the exception of treatments containing picloram, the effect of application timing was inconsistent. All treatments containing picloram consistently controlled woollyleaf bursage 93% or greater for 9 mo and 74% or greater for 11 mo. Control was poor or inconsistent with all other treatments. Although a rate response was seen with clopyralid, a level higher than 0.28 kg/ha may be necessary to control woollyleaf bursage. After 11 mo, control was less than 60% with treatments containing 1.7 kg/ha of glyphosate in 10 of 12 herbicide treatments and timing combinations over 3 yr.
Nomenclature: Clopyralid, 3,6-dichloro-2-pyridinecarboxylic acid; dicamba, 3,6-dichloro-2-methoxybenzoic acid; fluroxypyr, [(4-amino-3,5-dichloro-6-fluoro-2-pyridinyl)oxy]acetic acid; glyphosate, N-(phosphonomethyl)glycine; picloram, 4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid; 2,4-D, (2,4-dichlorophenoxy)acetic acid; woollyleaf bursage, Ambrosia grayi (A. Nels.) shinners #3AMBGR.
Additional index words: Woollyleaf franseria, bur ragweed, perennial weed control.