Kochia is a problematic weed species across the Great Plains states, including Kansas. Kochia emerges early in the spring (February – March) and continues in flushes through late spring (late May to early June), then slows down with occasional plant emergence into summer months. Kochia produces enormous amount of seeds (>100,000 seeds/plant) and spread those seeds through wind-mediated tumbling over landscapes. Season-long competition of kochia at higher densities is known to cause significant crop yield reductions (up to 58% yield losses in wheat).

Since the discovery of glyphosate-resistant (GR) kochia in western Kansas in 2007, the utility of auxinic herbicides (Banvel/2,4-D/Starane Ultra) alone or in tank-mixtures with other burndown herbicides has increased dramatically for kochia control. In general, Kansas fields receive 3 to 4 applications of a tank-mixture of glyphosate with Banvel and/or 2,4-D for weed control during summer fallow. In addition, Kansas growers also rely on PRE/POST applications of Clarity, Diflexx Duo, Starane Ultra, and Status for kochia control in wheat and corn.

In late summer of 2017, seeds of kochia plants surviving preemergence applications of Banvel (dicamba) at 16 fl oz/a, followed by two separate applications of a tank mixture of Roundup PowerMax at 32 fl oz/a and Starane Ultra (fluroxypyr) at 6.4 fl oz/a, were collected from two different corn fields (designated as KS-4 and KS-10) near Garden City. The sampled fields were under wheat–fallow–wheat rotation for > 6 years followed by corn (for KS-4 field) or a wheat–corn–fallow rotation (for KS-10 field) with frequent use of auxinic herbicides. Progeny seeds collected from individual kochia plants were kept separately and were designated as KS-4A, KS-4D, KS-4H, and KS-10A, KS-10G, KS-10H accessions.

Greenhouse studies on kochia resistance to dicamba and fluroxypyr

Dose-response experiments were conducted in a greenhouse at the K-State Agricultural Research Center (KSU-ARC) near Hays to characterize the levels of resistance to dicamba (Banvel) and fluroxypyr (Starane Ultra) in selected kochia accessions. Doses of Banvel herbicide tested were 0, 4, 8, 16, 32, 48, and 64 fl oz/a. Similarly, doses of Starane Ultra herbicide used were 0, 2.4, 4.8, 9.6, 19.2, 28.8, and 38.4 fl oz/a. In these studies, seeds of an herbicide-susceptible kochia accession (KS- SUS) that were originally collected from pastureland at KSU-ARC research field near Hays were used. Seeds of another susceptible kochia accession (MT-SUS) from Huntley, Montana were also included. Data on shoot dry weights were determined at 28 days after treatment (DAT) and were analyzed using a 3-parameter log-logistic model in R software.

Results indicated that selected kochia accessions had 2.7- to 7.2-fold resistance to Banvel herbicide, relative to the averaged GR50 values (dose of Banvel herbicide causing 50% reduction in shoot dry weights) of the MT-SUS and KS-SUS accessions on the basis of shoot dry weights (Figure 1). All suspected kochia accessions had survivors with Banvel at highest dose (64 fl oz/a) at 28 DAT and eventually produced seeds when they were transplanted in 10-L plastic pots containing commercial potting mixture under greenhouse conditions (Figure 2).

Figure 1. Shoot dry weight response of six suspected auxinic herbicide-resistant (Aux-HR) and two susceptible kochia accessions in a dose-response experiment with Banvel herbicide at 28 DAT.

Figure 2. Visual response of KS-SUS and KS-10H kochia accessions to Banvel doses at 28 DAT. Photo by Vipan Kumar, K-State Research and Extension.