Characterization and management of kochia exhibiting variable responses to dicamba

Abstract

Kochia (Kochia scoparia (L.) Schrader), at isolated locations, was identified as having variable responses to dicamba. These responses included lower observed injury and increased plant survival compared to susceptible populations. The response of kochia at these locations varied spatially in relation to field perimeters, area within a field, and plant within an area. In addition, injury varied among progeny from an individual plant. Kochia injury increased as dicamba rate increased for all kochia samples. In an effort to further characterize kochia and develop an objective method of characterizing kochia responses to dicamba, experiments were conducted to examine ethylene production as an herbicidal response and the response of kochia to ethylene. Ethylene was produced in response to dicamba treatments, but ethylene did not cause any symptoms on young vegetative kochia plants. Dicamba-susceptible kochia (accession S2) produced more ethylene than non-susceptible kochia (the Henry accession). The rate of ethylene evolution increased over time and as dicamba rate increased. Accession S2 produced as much as 470% of the ethylene generated by the Henry accession. Many herbicides proved to be effective management tools in these experiments. Accessions varied in susceptibility to alternative herbicides, but there were no consistencies in the order of accessions when ranked from most to least injury. Smaller kochia was more injured for all herbicide treatments; however, systemic herbicides tended to retain more herbicidal activity as kochia size increased. Pre-emergence dicamba applications caused over 93% injury to kochia from the Henry accession compared to 84% injury for comparable post-emergence treatments. Accessions responded similarly to dicamba applications directed to the root zone or to the foliage. The difference in injury between kochia accessions was not affected by spray volume. Crop competition from winter wheat reduced kochia above ground biomass but had no effect on kochia population. Dicamba application reduced the number of kochia plants and caused lower kochia biomass production in some instances.