Dicamba-resistant crops are being rapidly embraced by growers in the United States to manage glyphosate-resistant and other difficult-to-control broadleaf weeds. However, dicamba resistance in kochia, one of the troublesome weeds of the North American Great Plains, is already widespread. Hence, POST application of dicamba may not adequately control kochia. In recent years in the High Plains Region of Colorado, Kansas, and Nebraska, dicamba has been widely applied, often in combination with atrazine or metribuzin, in early spring for PRE control of kochia. However, there is concern this use pattern may increase the selection for dicamba-resistant (DR) kochia. Hence, there is need to understand the efficacy of dicamba applied PRE versus POST for managing DR kochia. A greenhouse study was conducted to test the efficacy of PRE-applied dicamba compared with POST application using both DR and dicamba-susceptible (DS) kochia. Efficacies of PRE-applied dicamba were compared at seeding densities of 300, 600, 900 and 1200 viable seed m^-2. At eight weeks after PRE and four weeks after POST treatment, control of DR kochia seeded at 300 viable seed m^-2 was improved from 10% with 560 g ae ha^-1 dicamba applied POST to 94 and 97% with 350 and 420 g ha^-1 dicamba applied PRE, respectively. However, the efficacy of PRE-applied dicamba was negatively correlated with seed density. When kochia seeding density was increased from 300 to 1200 seed m^-2, the ED₅₀ of PRE-applied dicamba increased from 237 to 705 g ae ha^-1 for DR kochia, and from 129 to 361 g ae ha^-1 for DS kochia, respectively. Thus, PRE-applied dicamba was effective in controlling the population of DR kochia tested, suggesting that PRE-applied dicamba may still provide substantial control of some DR kochia populations. However, it is not advisable to apply dicamba alone for PRE kochia control.

Nomenclature: Dicamba, 3,6-dichloro-2-methoxybenzoic acid; kochia, Kochia scoparia (L.) Schrad.