Kochia plants resistant (R) to field rates of dicamba were characterized for their frequency of occurrence and levels of resistance and for the physiological fate of applied 14C-dicamba. Of 167 randomly sampled fields and seven fields identified by producers to contain R kochia, 19 contained plants that produced 1% or more R progeny. The maximum percentage of R progeny produced by parental plants from any field was 13%. An inbred R line derived from a field collection was 4.6-fold more resistant to dicamba than an inbred susceptible (S) line. Rates of 14C-dicamba uptake and translocation were similar in R and susceptible (S) plants up to 168 h after treatment (HAT). Concentrations of the primary metabolite, 5-hydroxy dicamba, were similar in R and S tissues up to 60 HAT, although amounts were significantly greater in R treated leaves by 96 and 168 HAT. However, because there were negligible levels of dicamba metabolites in R shoots and because the rate of metabolism was relatively slow, the observed changes were inadequate to account for observed resistance levels. Thus, dicamba resistance in kochia cannot be attributed to differential herbicide absorption, translocation, or metabolism. These findings, together with our field observations on the unusually slow spread of resistance within and among fields may indicate that dicamba resistance is a quantitative trait.
Nomenclature: Dicamba; kochia, Kochia scoparia (L.) Schrad KCHSC.