Kochia (Kochia scoparia (L.) Roth) continues to be a serious weed problem in western Kansas, especially with the development of resistance to atrazine, ALS-inhibitor, and glyphosate herbicides. Controlling this weed can be challenging, but it helps to understand the biology of kochia. In particular, it helps to know when kochia emerges from the soil and for how long it can emerge during the season.
Previous studies have documented that kochia can emerge early in the spring and those seedlings can tolerate early frosts. It has been reported to survive temperatures as low as 9 degrees F. It thrives in hot weather, too, and can continue to germinate throughout the summer growing season. Lab experiments have shown kochia to germinate at temperatures from 39 to 106 degrees F.
In the spring of 2010, we followed the emergence of kochia from naturally occurring populations at several locations across western Kansas. Locations included kochia in fence row and non-cropped areas at Stockton, Hays, and Ness City, along with populations occurring in previously cropped fields at Hays and Garden City. Beginning March 15, any kochia seedlings that had emerged were counted and removed. These observations continued weekly until no new kochia plants were observed.
In order to make a uniform comparison of environmental conditions at emergence across these different locations from south to north, we used growing degree days (GDD in degrees C) accumulated since January 1, 2010 based on maximum and minimum air temperatures and a base temperature of 0 C (32 degrees F). For each plot, we described kochia emergence as a percentage of the total number of seedlings emerged through the entire season.
Populations of kochia seedlings observed at these locations ranged from as few as 4 seedlings per square meter to as many as 379,000 seedlings per square meter over the spring. To highlight the start, end, and duration of emergence, we determined the number of GDD that accumulated for 10% emergence (start) and 90% emergence (end), and the difference between the two (duration of emergence).
Cumulative GDD at start (10% emergence), end (90% emergence), and duration of emergence
Comparing emergence at the non-cropland and cropland sites at Hays, there is a 2-week delay in kochia emergence in the cropped fields (Figure 1). This is also observed at the Garden City location, with similar start of emergence on March 31. Kochia in the non-cropland locations emerge earlier with March 11 at Ness City, March 18 at Hays, and April 3 at Stockton. Most seedlings emerged in six days or less at Hays and Stockton, but kochia emergence extended for five weeks or more at Garden City.
Origin and growth habit
Kochia is native to southern and eastern Russia. It was actually introduced to North America as an ornamental and then escaped into areas where it was adapted. It is not just a Kansas problem. Kochia has been reported in 42 of the lower 48 states and in the seven Canadian Provinces neighboring the USA border.
In the absence of competition, kochia can be very bushy in appearance and achieve heights of greater than 7 feet. Kochia can have an extensive root system. Kochia roots grew to a depth of 16 feet in a sorghum field during a drought in Kansas. A single plant has been reported to have a root system 22 feet wide. Kochia is daylength sensitive and begins to flower sometime in mid-July to early August in Kansas. A critical light period triggering flowering ranges from 13 to 15 hours among kochia accessions.
Kochia is self-fertile. Initially, female flower parts are receptive to pollen prior to pollen actually being released from that same flower. This mechanism facilitates outcrossing. Kochia seed are brown, oval and flattened with a star shaped hull enclosing the seed. It has been reported that a single plant can produce as much as 14,600 seed. In a seed burial experiment in Nebraska, kochia seed viability was 5% after 1 year and zero after 2 years. However, seed burial experiments in Colorado indicate that a low percentage of both a dormant and a non-dormant kochia seed remained viable even after 3 years. Seed viability declined more rapidly when seed was buried 4 inches or less below the soil surface.
As the kochia plant matures, an abscission layer develops in the stem near the soil surface. In the presence of wind, this weakened area allows the dried plant to sever from the root system and tumble across the landscape spreading viable seed where ever it rolls. Thus some people call kochia tumbleweed.
Kochia in wheat stubble, showing the pattern of emergence after these "tumbleweeds" have blown across the field in waves. Photos by Curtis Thompson, K-State Research and Extension.
Kochia seedlings are quite pubescent. Kochia is susceptible to several broadleaf herbicides when young.
-- Anita Dille, Weed Ecology
-- Curtis Thompson, Weed Management Specialist
-- Dallas Peterson, Weed Management Specialist
How Widespread is the Problem? Herbicide Resistance in Kochia
Kochia has long been a serious weed problem in western Kansas, but getting good control of kochia has become even more of a challenge in recent years as populations resistant to atrazine and ALS-inhibitor herbicides have developed. Now there are confirmed populations of glyphosate-resistant kochia sampled during 2007 from several counties in western Kansas. Many more such populations are suspected.
The Weed Science Society of America has defined herbicide resistance as: "The inherited ability of a weed or crop biotype to survive a herbicide dose normally lethal to the original 'wild' population." In testing populations that are suspected of having developed herbicide resistance, a range of herbicide doses are used – normally ranging from a fraction of the recommended rate to more than 4 times the recommended rate. The resulting dose response curve allows a comparison between suspect and known susceptible populations.
In testing kochia populations for glyphosate resistance using this method, the charts below show the results of four populations. The population from Jerome County, Idaho is susceptible. At the normal, 1X rate of glyphosate, there was almost complete control of this population of kochia. In comparison, there was very little control of three Kansas populations at the 1X rate, and only about 40 percent control or less at the 2X rate. This is one method used to confirm herbicide resistance. A fourth population in Thomas Co. also was confirmed resistant, but is not shown in the charts below.
The GR50 (the dose required to cause a 50 percent growth reduction) shown in these figures represents the fraction of the recommended glyphosate use rate (0.75 lb ae/a) required to reduce the greenhouse-grown kochia population plant growth by 50 percent. In performing such tests, K-State Research and Extension scientists have confirmed glyphosate-resistant populations of kochia in four western Kansas counties (Gray, Norton, Stevens, and Thomas). Reports of lack of control of kochia with glyphosate escalated in 2010. Field inspections confirmed these reports. The following map shows locations of both confirmed and suspected glyphosate-resistant populations -- but this map only shows counties where seed was collected for resistance testing. In some cases, resistant populations in the different areas have developed independently. But any give resistant population can also spread to adjacent fields or even adjacent counties.
Kochia seed is spread by plants that become detached after their growing season is over, and roll across fields with the wind. This "tumbleweed" effect is illustrated in the photo below from Greeley County, which shows how a single glyphosate-resistant plant can roll through a field, spreading its seed. At least some of these seeds will also produce glyphosate-resistant plants.
If no herbicides other than glyphosate are used on fields with glyphosate-resistant populations, these populations will survive and can quickly spread to large areas of the field, as illustrated below, in this soybean field in Lane County.
Glyphosate-resistant populations of kochia may also be resistant to atrazine and/or ALS-inhibitor herbicides, making control a challenge. It is difficult to state with certainty at this time whether such herbicide-resistant populations are naturally any more or less fit to survive environmental conditions than normal populations.
In newspaper articles in upcoming weeks, there will be a series of articles on kochia, including control strategies that can be used on glyphosate-resistant populations in various crops.
There are suspect fields in the Sunflower District. Seed was collected from one last fall, however germination on this seed has been a problem. Therefore resistance has not been confirmed in it. If you have fields with suspected resistance, please contact the extension offices to arrange further testing.
From K-State Agronomy eUpdate (3/4/2011)
--Phil Stahlman, Weed Scientist, KSU Agricultural Research Center-Hays
--Brian Olson, Northwest Area Crops and Soils Specialist
--Curtis Thompson, Weed Management Specialist
Kochia Control in Corn and Grain Sorghum
For fields that will be planted to corn this spring, a combination of glyphosate (using a minimum of 0.75 lb ae/acre) with herbicides that have PRE and POST activity on kochia are most valuable. Tank mixing 8 to 16 oz of dicamba and or 1 to 2 pints of atrazine, will control small kochia and other existing broadleaf and grass weeds. If producers wait until later so they can apply the burndown and preemergence herbicide in the same application, the kochia will be larger and most likely will not be controlled adequately. If that occurs, the surviving plants will go on to cause problems throughout the growing season.
Other herbicides that could be tankmixed with the glyphosate include Lexar or Lumax, or for CORN ONLY, 3 to 4 fl oz of Balance Flexx or Corvus. Be sure to include some atrazine.
Figure 1 shows the effectiveness of these herbicides for controlling kochia. Many of the chloracetamide/atrazine prepack mixtures could be used at this time to manage this early flush of kochia. However, keep in mind that atrazine is the primary work horse. If the kochia is triazine-resistant, these chloracetamide/atrazine mixtures will not be effective.
The results in all the figures below are research conducted at Tribune under irrigation, so the PRE residual herbicides were properly activated. Under dryland conditions, the results may be more variable, depending on the amount and timing of precipitation received.
Figure 1. Corvus and Balance Flexx gave long-lasting residual control of kochia ahead of corn. Herbicides were applied March 16, 2012 at Tribune under irrigation.
Figures 2, 3 and 4 show the value of atrazine, dicamba, and other herbicides for residual control of kochia. In Figure 2, the main difference between Harness Xtra (acetochlor + atrazine) and Warrant (acethchlor) is the atrazine. Verdict and Warrant, without atrazine, were not effective.
Figures 3 and 4 have 8 and 16 oz of Clarity compared. In 2011, similar control occurred with both rates, however, in 2012 there was a significant advantage to having a full pint of Clarity applied. The 1.0 lb of atrazine in Figure 3 did provide excellent kochia control into May. Keep in mind that the population of kochia in this trial was susceptible to triazines.
Figure 2. Early preplant treatments applied March 22, 2011 for kochia control ahead of corn at Tribune under irrigation. Balance Flexx and Harness Xtra gave longer residual control than Verdict or Warrant. Harness Xtra and Warrant can also be applied ahead of grain sorghum.
Figure 3. Shorter residual herbicide options for Early Preplant control of kochia. Treatments were applied March 22, 2011, at Tribune under irrigation.
Figure 4. Early Preplant treatments applied March 16, 2012 for kochia control at Tribune under irrigation. Atrazine and Clarity can be used ahead of corn or sorghum. Metribuzin can be used ahead of soybeans or wheat. Clarity used alone had less residual control than treatments that include atrazine or metribuzin. This population of kochia was susceptible to triazine herbicides.
Kochia Control in Sunflowers and Soybeans
The development of populations of kochia resistant to either ALS-inhibitor herbicides, glyphosate, and/or triazines in Kansas means that producers may have to put a little extra time and effort into controlling this tough summer annual broadleaf weed. Control can still be achieved in almost all cases in both sunflower and soybeans, especially if: (1) a combination of herbicides is used with different modes of action, and (2) control measures begin early in the spring.
Control in Sunflowers
It is important to plant sunflower into a weed-free seedbed. This will help in getting good season-long control of all broadleaf and grassy weeds. But it is especially important for getting good control of any weed populations, such as kochia, that may be resistant to glyphosate or ALS-inhibitor herbicides.
The best approach to kochia control in sunflower is to start with an early preplant burndown treatment in early April to control the early emerged plants. Although glyphosate-resistant kochia has been confirmed, it is a moderate level of resistance. Proper application timing, glyphosate rate, and the addition of ammonium sulfate are still important to provide as much kochia control and suppression as possible.
In the burndown treatment, use a minimum of 0.75 lb ae/a glyphosate in combination with ammonium sulfate when kochia is less than 2 inches tall. Combinations with other herbicides will also be beneficial, but labeled preplant options in sunflowers to control emerged kochia are limited. Aim is one herbicide that has good activity on small kochia and could be used as a tank-mix at the rate of 1 oz/A for enhanced early season control. 2,4-D could also be added to the tankmix to help with broadleaf control, but 2,4-D generally is not very effective for control of kochia. A waiting interval of 30 days is required between 2,4-D application and planting sunflowers. An early April application should allow for enough time for timely sunflower planting.
The other burndown option to control emerged glyphosate-resistant kochia ahead of sunflowers would be Gramoxone Inteon. Gramoxone Inteon is a contact herbicide, so adequate spray volumes and thorough coverage are essential for good weed control. Gramoxone Inteon should be applied in combination with 0.5% v/v nonionic and on a warm sunny day for best results.
A second preplant herbicide application of glyphosate or Gramoxone Inteon plus Spartan plus either Prowl H2O or Dual II Magnum should then be applied about two weeks prior to planting. Spartan herbicide can provide good residual control of germinating kochia as long as the herbicide is activated by moisture, but probably will not control emerged kochia. The Prowl H2O or Dual II Magnum are primarily for residual control of grasses but Prowl H2O in particular can enhance control of kochia compared to Spartan applied alone. Spartan Charge is a premix that contains Spartan and Aim for added burndown. Spartan Advance is a premix of Spartan plus glyphosate.
Postemergence control options are determined by the type of sunflower planted. There are no postemergence herbicide options labeled for kochia control in conventional sunflowers, although this should not be a problem if the two-pass preplant strategy above is used and the Spartan is activated by moisture.
If Clearfield sunflowers are used, producers can apply Beyond herbicide as a postemergence treatment. If Express sunflowers are used, producers can apply Express herbicide postemergence. Both of these herbicides are ALS-inhibitors. They will be effective in controlling kochia only if the kochia populations present are not resistant to ALS-inhibitor herbicides.
In most cases, the difficulty in getting good kochia control in sunflower comes when producers try to make just one preplant herbicide application instead of two. If producers try to do everything with one application about two weeks or so prior to planting, there can be some large kochia plants present by that time. It then becomes more difficult to get complete foliar control with the various preplant burndown options. If sunflower is not planted into a weed-free seedbed, weed problems are almost certain to cropup in-season and cause yield reductions or create a need for postemergence treatments, which may or may not be effective.
Control in Soybeans
The best management strategy for controlling kochia in soybeans is similar to sunflowers, but there are more herbicide options in soybeans than in sunflower. Start in March with a tankmix of glyphosate (using a minimum of 0.75 lb ae/are) and 8 to 16 oz/acre of Clarity. The use of Clarity requires a minimum accumulation of 1 inch of rain and then 28 days prior to planting soybeans. As indicated in the label, Clarity should not be used as a preplant treatment in soybeans in areas with less than 25 inches of annual precipitation.
Gramoxone Inteon tankmixed with metribuzin (Dimetric, Metribuzin, Sencor) will control the very small kochia, and metribuzin will provide extended residual control of kochia, as long as the population of kochia is susceptible to triazine herbicides.
Figure 4 (above) shows the effectiveness of a full pound of metribuzin. It is very likely that a lower rate of metribuzin would also be very effective to provide residual kochia control in western Kansas, if the population of kochia is susceptible to triazines. Metribuzin can injure soybeans depending on soil texture, organic matter and soil pH, so be sure to follow label guidelines regarding soil characteristics and rate guidelines regarding use on soybeans.
Authority-based herbicides that contain sulfentrazone could be considered for use in March to manage an early flush of kochia. It's important to note crop rotation restrictions on these products, however. The Valor-based products have provided less effective control of kochia (see Figures 5 and 6).
Figure 5. Spartan 4F at 6 oz/acre gave longer residual control of kochia than Tripleflex at 1 qt/acre or Valor SX at 3 oz/acre. Herbicides were applied March 22, 2011 at Tribune under irrigation.
From K-State Agronomy eUpdate (3/22/2013)
-- Curtis Thompson, Weed Management Specialist
-- Dallas Peterson, Weed Management Specialist
Kochia Control in Wheat
Fields going to wheat this fall
If kochia is emerging in row crop stubble intended to be planted to wheat this fall, herbicide options exist that provide residual kochia control. Atrazine should not be used in this situation, however. The following herbicides could provide effective residual control of kochia for fields to be planted to wheat this fall: Dicamba, Metribuzin or Dimetric (Dimetric label indicates ½ to 2/3 of a pound), Corvus, Balance Flexx, or Lumax. These products allow wheat to be planted 4 months following application.
Fields of standing wheat
If kochia is emerging in a field of standing wheat, the options for control depend on whether the population of kochia is susceptible or resistant to ALS-inhibitor herbicides and whether or not, wheat has reached the jointing stage. There are three big challenges to kochia control in wheat:
There are many populations of kochia with resistance to either ALS-inhibitor herbicides, or glyphosate. There may even be some populations resistant to dicamba.
A majority of kochia emerges early in the spring, but some emergence can extend over a period of weeks or months. A herbicide applied early in the spring will need to have residual activity to be effective on later-emerging kochia. Several ALS-inhibitor herbicides have good residual activity, but are ineffective on ALS-resistant kochia.
Dicamba, a non-ALS herbicide is one of the more effective products on most populations of kochia, but must be applied before the jointing stage of wheat.
Many populations of kochia present in wheat in western Kansas are resistant to ALS-inhibitor herbicides, however tank mixtures with dicamba and Starane can be very effective to control kochia. In general, 2,4-D, MCPA, Aim, and Cadet, are not very effective on the vast majority of kochia currently present in western Kansas.
Additional products containing dicamba, include Rave (Amber + dicamba) or Pulsar (Starane + dicamba). These products have to be applied before the jointing stage of wheat. Dicamba has some residual soil activity, but not as much as most sulfonylurea herbicides. Rave will have residual activity from the Amber, but since Amber is a sulfonylurea herbicide, it would not provide any residual control of kochia populations that are resistant to ALS-inhibitor herbicides. Both ingredients in Pulsar have limited residual activity.
Another option producers have for kochia control is Starane or other fluroxypyr products. Like dicamba, Starane is a growth regulator herbicide, but it can be applied up to the early boot stage of wheat. Starane also has limited residual activity, so good coverage is still important for control. Starane is weak on mustard control.
Huskie is also effective on kochia. It is a broad-spectrum herbicide effective on most broadleaf weeds in wheat, and can be applied up to the boot stage of wheat. Huskie also has limited residual activity, so producers will need to make sure kochia plants are thoroughly covered with Huskie to get the best control. Ideally, the Huskie should be timed for application after the majority of kochia has emerged, but before the wheat canopy has become thick.
Buctril can control kochia and can be applied at later stages of wheat development, but is a contact herbicide with no soil residual activity. Consequently, Buctril has similar challenges as Starane and Huskie in terms of getting good coverage. Getting thorough coverage is even more critical with Buctril since it is a true contact herbicide and not translocated in plants. Buctril is effective on very small kochia only.
From K-State Agronomy eUpdate (3/22/13)
-- Curtis Thompson, Weed Management Specialist
-- Dallas Peterson, Weed Management Specialist