In 2011, work by University of Nebraska, Lincoln (UNL) weed scientists confirmed a population of glyphosate-resistant kochia (Kochia scoparia) in Keith County, Neb. This population has resulted in numerous weed control failures in the past two years. Based on other field reports in 2011, it is believed that there may be many populations in western Nebraska that are resistant to glyphosate.

This development was not unexpected. Glyphosate-resistant kochia was first reported in the U.S. in Kansas in 2009 and since then has developed into a serious problem in western Kansas. The vast amount of seed produced by each kochia plant and its ability to disseminate over large distances makes it a major threat to crop production in the High Plains region of the U.S.

UNL Research Confirming Resistance

Greenhouse studies were conducted in 2011 to determine if a population from Keith County was glyphosate-resistant, and if so, to what level. Dose response studies were conducted with 10 treatments of a 5 lb ae/gal glyphosate formulation. Treatments were 0 to 12x the recommended rate. A known glyphosate-susceptible population from Kansas was used with suspected susceptible and resistant populations from Nebraska. Plants were sprayed when the average height reached three inches.

Results suggest a 10 to 15 fold level of resistance in the Keith County population relative to the Nebraska susceptible population. These results also indicate that labeled glyphosate use rates are not adequate for desired control. To achieve acceptable control of glyphosate-resistant kochia use an effective burndown program that does not rely solely on glyphosate and effective tank mixtures. Because glyphosate is relatively cheap, producers may be tempted to manage problem populations with higher rates of glyphosate. Our results indicate this is not a viable management option and will result in 1) greater selection pressure for higher levels of resistance in resistant populations and 2) a greater percentage of resistant plants within the total population.

Reports of herbicide resistance in kochia is not a new phenomenon in the U.S. Kochia populations resistant to triazine (1976), ALS (1987), and growth regulator (1997) herbicides have been reported on the International Survey of Herbicide Resistant Weeds website (source: http://www.weedscience.org/).

Kochia Biology

Kochia is an early germinating summer annual broadleaf weed species. It can be found in crop fields throughout the Midwest. It is especially common in corn, soybean, wheat, pasture, and right-of-way areas in central and western Nebraska. Kochia is capable of self- or cross-pollination, making it likely that glyphosate resistance could spread via gene flow. It also can spread via the plant’s unique seed dispersal mechanism. After the plant has matured, it will break off at the ground and roll in the direction of a slope or wind. As it rolls, seed is dislodged and deposited in new areas, disseminating the genetics from a single plant over great distances.

Integrated Weed Management is Necessary to Manage Glyphosate Resistance in Kochia

In a multi-state university study conducted by Andrew Kniss (Wyoming), Phil Stahlman (Kansas), Robert Wilson (Nebraska), Phil Westra (Colorado) and Mike Moechnig (South Dakota), crop rotation and herbicide programs were evaluated for kochia control. Their findings showed that the crop was a significant contributor to biomass reduction in kochia. Both corn and soybean were able to significantly suppress biomass, regardless of herbicide management. Fallow areas and wheat were not as effective as either corn or soybean, and sugarbeet crops were completely ineffective at suppressing kochia.

In both corn and soybean, applying a PRE herbicide treatment with residual activity plus a POST treatment including something other than glyphosate was effective at suppressing kochia. POST applications of Clarity, Sharpen and Rage D-Tech all suppressed kochia greater than 80% in fallow, while Huskie, Starane NXT, and Agility SG all did the same in wheat.

In sugarbeet the three PRE treatments followed by POST treatments only provided 40-50% control of kochia. All PRE followed by POST treatments provided better kochia control than POST treatments of glyphosate alone. In sugarbeet the glyphosate alone treatment was most effective. This indicates that if sugarbeet is in the crop rotation, growers should not over-rely on glyphosate in other crops in order to make sure that glyphosate is as effective as possible on kochia in sugarbeet.

This study provides further evidence of the need for diversified weed control programs that include crop rotation, herbicide mode-of-action rotation, and the use of tank mixtures with multiple herbicides. Weed control failures are less likely to occur when multiple herbicide applications are made with PRE herbicides with residual activity. Whenever herbicides are applied, thoroughly scout prior to and following applications to assess herbicide performance in-season and help ensure satisfactory performance in the future.

Herbicide Control Options

There are numerous options for effective burndown, preemergence, and postemergence control of glyphosate-resistant kochia in corn (Table 1), soybean (Table 2), and other crops. Glyphosate-resistant kochia confirmed in Neb.

Glyphosate-resistant kochia confirmed in Neb.Refer to the 2012 Guide for Weed Management in Nebraska for detailed information. In soybeans, preemergence herbicides are most effective on kochia. Much like glyphosate-resistant marestail, postemergence options in soybeans are limited and would be unlikely to provide greater than 85% control of glyphosate-resistant kochia.

Why Does Herbicide Resistance Evolve?

Herbicide resistance usually results from repeated use of the same herbicide. An over-reliance on glyphosate facilitated by widespread adoption of glyphosate-tolerant crops in the Midwest (primarily corn and soybean) has resulted in the evolution of glyphosate-resistant weed populations. The selection pressure exerted on weed populations by increased glyphosate use over the last 10 to 15 years is unprecedented in the era of herbicide weed control.

Only a few weed species worldwide were resistant to glyphosate prior to the introduction of glyphosate-tolerant crops. Currently, 12 weed species in the U.S. have evolved glyphosate-resistance due to repeated glyphosate use over a large land area (more than 300 million acres just in the U.S.). They are common waterhemp, giant ragweed, common ragweed, kochia, palmer amaranth, marestail (horseweed), hairy fleabane, junglerice, goosegrass, Johnsongrass, Italian ryegrass, and annual bluegrass (source: http://www.weedscience.org/).

Over-reliance on a single weed management tool will likely cause inconsistent and eventually ineffective control. While weed management is not impossible in the presence of resistant species such as glyphosate-resistant kochia, it is certainly more difficult. As new herbicides and traits reach the market, it is important to note they are in known herbicide mode-of-action classes. It has been more than 20 years since a new herbicide mode-of-action was introduced for row crops. Thus, it is imperative that we are good stewards of the current herbicides to delay the evolution of resistance as long as possible.