This greenhouse experiment examined the response of homozygous susceptible and acetolactate synthase (ALS) inhibitor–resistant plants from six Canadian kochia accessions with the Pro197 or Trp574 mutation to six alternative herbicides of different sites of action. The null hypothesis was ALS-inhibitor–resistant and –susceptible plants from within and across accessions would respond similarly to herbicides of different sites of action. This hypothesis was accepted for all accessions except that of MBK2 with the Trp574 mutation. Resistant plants of that accession were 80, 60, and 50% more sensitive than susceptible plants to pyrasulfotole, mesotrione (hydroxyphenylpyruvate dioxygenase [HPPD] inhibitors), and carfentrazone (protoporphyrinogen oxidase [PPO] inhibitor), respectively. However, no differential dose response between resistant and susceptible plants of this kochia accession to bromoxynil, fluroxypyr, or glyphosate was observed. A previous study had found marked differences in growth and development between resistant and susceptible plants of this accession, but not of the other accessions examined in this experiment. Negative cross-resistance exhibited by resistant plants of accession MBK2 to PPO and HPPD inhibitors in this experiment may be a pleiotropic effect related to the Trp574 mutation.

Nomenclature: Bromoxynil; carfentrazone; fluroxypyr; glyphosate; mesotrione; pyrasulfotole; kochia, Kochia scoparia (L.) Schrad. KCHSC, synonym: Bassia scoparia (L.) A.J. Scott.