Kochia is a troublesome annual weed in the Great Plain’s dryland cropping systems. It was introduced to North America from Europe and Asia in the 1890s as an ornamental plant, and now is present in every county in Montana. It heavily infests 200,000 acres of the eastern Golden Triangle. Kochia is very difficult to control because it can emerge at any time during the winter or growing season with later seedlings emerging after post-emergence herbicides have been applied. Mature kochia plants are very competitive as they are exceptionally drought tolerant and can grow under a wide range of soil pH and salt concentration. Kochia plants produce many thousands of seeds that, because of its tumbling behavior, are dispersed though long distances. Luckily, these do not stay viable in the seed-bank for long with more than 90 percent of them dying within a year.
Many kochia populations have developed herbicide resistance. In 1989, and after several years of continuous use of ALS inhibitor herbicides, herbicide resistant kochia biotypes were detected infesting Montana croplands. These biotypes can be found in most Montana cropland, are highly resistant to Glean (chlorsulfuron) and Ally (metsulfuron-methyl) and may be cross-resistant to other ALS inhibitors herbicides. In 1984, kochia populations that had evolved resistance to Photosystem II inhibitors such as atrazine were detected in Montana croplands. Research has shown that these resistant biotypes may be cross-resistant to other herbicides with the same mode of action. In 1995 kochia biotypes with moderate resistant to synthetic auxins herbicides including Banvel, Clarity (dicamba) and Starane (fluroxypyr) were detected in Montana croplands (Source: International Survey of Herbicide Resistant Weeds. http://www.weedscience.org.)
Montana’s producers have recently added Spartan (sulfentrazone) to their farming toolbox as a way to manage problematic weeds including kochia, Russian thistle, buckwheat, common lambsquarter and pigweed. Because Spartan is a PPO-inhibiting herbicide, it can be used to control resistant kochia plants. Specifically it inhibits the synthesis of a precursor of chlorophyll, causing rapid destruction of contacted tissue.
Though Spartan has foliar activity, its main use is as a selective soil applied herbicide that can be applied as a preplant incorporated or pre-emergence (to weed and crop) treatment. Soil pH, organic matter content and soil type play a very important role in determining the availability of Spartan in the soil solution. As a general rule, as soil pH increases, Spartan availability increases. Also, because Spartan is adsorbed to the clay and organic matter fraction of the soil, higher rates may be needed on soils with high organic matter and fine texture. Finally, irrigation with alkaline water (pH greater than 7.5) can increase the availability of Spartan in the soil.
Spartan is stable in soil with a half-life of 18 months. Because of its relatively long persistence, it is very important to consider the rotational restrictions following Spartan application. For example, the minimum interval after Spartan application is four months for wheat, barley, rye and triticale, 12 months for alfalfa and cereal grains such as buckwheat, oat, and proso millet, 24 months for canola, and 36 months for sugar beets. It is recommended to conduct a bioassay prior to planting to determine crop sensitivity to Spartan.
Recent studies done at Montana State University showed that Spartan does an excellent job on kochia and many broadleaf weeds and it can be tank mixed with Roundup (glyphosate). Moreover, post harvest applications of Spartan have sufficient residual activity to control kochia seedlings the following spring. More information on specific rates and tank mixtures can be found in Post-harvest Kochia Management in Small Grains (MontGuide fact sheet #200213/Agriculture from the Montana State University Extension Service, available through the Montana State University Extension Service or at http://www.montana.edu/wwwpb/pubs/mt200213.html.)
Unfortunately, farmers at several locations have reported severe wheat injury due to Spartan carryover. Researchers at MSU have also detected barley and dry pea injury after Spartan applications. We do not know why Spartan injures crops in some locations but not in others but we suspect it is due to a combination of soil factors, climate conditions, and seeding depth. Specifically, the risk of carryover injury appears to be higher in areas where soils have high pH (7.8 – 8.1) and low organic matter (less than 1.5 percent).
Research conducted at Iowa State University indicated that crop injury with Spartan can happen when heavy rains occur as crops are breaking through the soil surface splashing high concentrations of herbicide onto hypocotyls, cotyledons and growing points. While there will always be a potential for crop injury with herbicides, good management can minimize this risk. For example crop injury due to Spartan splashing on emerging crops can be reduced by a few management strategies. For example, you could check with your seed supplier to determine the sensitivity of varieties to Spartan prior to purchasing seed. Also, you could increase the likelihood that the herbicide will be moved into the soil profile by rain by spraying one to two weeks prior to planting. Finally, the potential for injury is greater on poorly drained soils where puddling and splashing may occur with heavy rain.
Producers know that losing a crop due to lack of rain or hail is an unfortunate but inevitable experience that could have dramatic financial consequences. Losing a crop due to herbicide injury could be as bad as a weather calamity. However, if precautions are taken, herbicide damage can be avoided. To achieve this goal, producers should shift emphasis from considering herbicide benefits on the current season to understanding the mid- and long-term consequences of their management practices. In doing so, farmers should consider the integration of multiple control practices aimed at reducing weeds abundance and avoiding herbicide carryover, crop injury and development of herbicide resistance.
Disclosure. Common chemical and trade names are used in this publication for clarity by the reader. Inclusion of a common chemical or trade name does not imply endorsement of that particular product or brand of herbicide and exclusion
Categories: Weed, Kochia, Preemergence Herbicide, Herbicide Injury
Date: 04/28/2005