Broom and threadleaf snakeweed are major rangeland weeds in the western United States, and picloram is the major herbicide used for their management. Previous work has shown that these species are most susceptible to picloram applied in autumn or when precipitation is high and that differences in herbicide absorption and tissue sensitivity as measured by picloram-induced ethylene production do not fully explain variation in seasonal response. Therefore, the role of picloram metabolism in seasonal susceptibility to picloram was examined. Because snakeweed is characterized as highly genetically variable, picloram metabolism was evaluated monthly for 3 yr among populations from two species as well. Picloram metabolism was examined monthly for 3 yr among two populations of threadleaf and nine populations of broom snakeweed grown in a common garden. Metabolism ranged from 30 to 70% of picloram applied, and picloram was converted to two metabolites more polar than picloram regardless of species or population. Although metabolism was greatest in the year with the most precipitation, rate of metabolism was unrelated to precipitation received in the 7-d period before treatment. Application timing as defined by a given month or specific phenological stage was not related to the level of metabolism. We conclude that variation in picloram metabolism is not involved in differential susceptibility across season or population.

Nomenclature: Picloram; broom snakeweed, Gutierrezia sarothrae (Pursh) Britt. & Rusby GUESA; threadleaf snakeweed, Gutierrezia microcephala (DC.) Gray GUEMI.