Dicamba-resistant kochia confirmed in Nebraska

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Kochia is a troublesome weed in crop production in the semiarid region of the Great Plains, made more troublesome by its growing resistance to common herbicides such as triazine, ALS-inhibitors, glyphosate, and now dicamba.

Kochia is usually found in western and southwest Nebraska, but also can be found in central and eastern Nebraska along railroads and highways.

It has a high tolerance to drought and can be very competitive with crops. It typically emerges early in the season and begins maturing in mid-August. Kochia has an ovoid shape and when senesced, a strong wind can break the stem at the soil surface, allowing the plant to roll (tumble) across the landscape and spread seed over long distances.

Kochia flowers are wind-pollinated by flowers on the same plant or cross-pollinated by neighboring plants. Its seed is short-lived in the soil and remains viable for about 12 months. When the same herbicide mode of action is used widely and resistance develops, these characteristics (tumble dispersal, wind pollination, and short seed life) can result in the rapid spread of resistant populations.

In Nebraska, populations of kochia resistant to PSII-inhibiting (triazine) herbicides and ALS-inhibiting (sulfonylurea and imidazilinone) herbicides occur in many fields and glyphosate-resistant kochia has recently been confirmed in western Nebraska. In addition, there have been periodic reports of poor control of kochia by dicamba since the late 1990s.

Testing for Resistance to Dicamba

In 2009, UNL researchers surveyed experts to evaluate the potential for various weeds to evolve resistance to dicamba after commercialization of dicamba-resistant soybean. More than 80% of surveyed experts rated kochia as having a medium or high risk of evolving resistance to dicamba. In fall 2009 and again in 2010 kochia seed was collected from fields and roadsides in 59 Nebraska counties.

Greenhouse experiments were conducted beginning in 2011 to determine the most and least susceptible populations to a dicamba dose of 0.5 lb ae/ac (16 fl oz/ac Clarity). From this initial screening four kochia populations representing the extremes in sensitivity to dicamba were selected for additional characterization in a dose-response study. The populations were treated with 12 dicamba doses (0, 0.03, 0.06, 0.13, 0.25, 0.5, 1, 2, 4, 8, 16, and 32 lb/ac) and allowed to grow for 28 days after treatment. They were then evaluated for control.

Results from the dose-response study indicated a 19-fold difference between a population collected from Box Butte County and the three more susceptible populations (collected in Morrill and Kimball counties). The resistant population required 22 lb/ac of dicamba (22 qt/ac Clarity) to achieve 80% control. Although dicamba-resistant kochia has been suspected in areas of western Nebraska for many years, the magnitude of resistance we observed was surprising. In fact, this level of resistance had previously only been reported in a kochia population that was developed using selective inbreeding in greenhouse studies at Colorado State University.

The owner of the Nebraska field exhibiting high dicamba resistance reported that the field had been in irrigated continuous corn for the previous 10 years and that dicamba had been frequently applied in tank-mixtures with glyphosate or atrazine to 1-inch tall kochia. In addition, glyphosate plus dicamba or 2,4-D had been applied on wheat stubble in the pivot corners to suppress weed growth during the fallow period.

Recommendations

Soybean with resistance to dicamba has been developed and its commercialization is expected in 2015 or 2016, pending USDA approval. This new technology will permit dicamba use for postemergence weed control in soybean. Farmers or advisors who plan to use dicamba to manage kochia populations that are already resistant to glyphosate or to triazine- and ALS-inhibiting herbicides should exercise extreme caution to avoid selecting populations that are resistant to dicamba.

Dicamba should only be used in tank-mixtures or in sequence with a herbicide(s) with a different mechanism(s) of action that is effective at controlling kochia. Non-chemical weed management practices such as tillage and crop rotation also should be incorporated into the weed management program to help prevent kochia seed production.

Finally, producers should monitor fields for plants that escape dicamba applications and remove them before they produce seed.


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