June 10, 2010

Early postemergence herbicide applications in soybeans have begun and we are receiving calls on postemergence marestail control.

Please note that we are past the optimal time to control marestail. Marestail is most easily controlled with an effective burndown herbicide application prior to crop planting, when marestail is small. These burndown applications should include a growth regulator herbicide. Detailed information about marestail may be viewed in the Glyphosate, Weeds, Crops publication, Biology and Management of Horseweed.

Glyphosate-resistant marestail/horseweed (Conyza canadensis) was confirmed in Nebraska in 2006. We believe resistant marestail biotypes continue to spread throughout the state. The first two resistant biotypes (2006) had up to a 6x level of resistance compared to a biotype that had not been exposed to glyphosate.

The level of glyphosate resistance typically varies within a field where there is a resistant population. Some biotypes will display no resistance, other biotypes can be controlled with the use of high rates of glyphosate, and finally the most resistant biotypes will only show a temporary chlorosis and stunting after the maximum labeled rate of glyphosate is applied.

Since the level of resistance cannot be determined by any visual inspection of the plant prior to a glyphosate application, the best management approach is to tank-mix an effective second mode of action with glyphosate. If the population is resistant, the second mode of action will provide control. If the population is not resistant, you will have reduced the selection pressure from glyphosate and lowered the chance of developing a resistant population in that field.

Control in Soybeans

If you suspect glyphosate resistant marestail is present in your field(s), tank-mixing FirstRate (0.3 oz/ac) with glyphosate is recommended. As a secondary option, contact herbicides such as Flexstar or Cobra also may be used, but producers should only expect 75% to 80% control and re-growth from lower nodes would be possible.

If Liberty Link soybeans were planted, tank-mixing Ignite and FirstRate may be beneficial to control large marestail (greater than 4 inches). Ignite alone will typically control 80%-90% of the marestail.

In 2009 we collected seed from two locations in eastern Nebraska where FirstRate (an ALS inhibiting herbicide) did not control marestail. After one round of field and greenhouse trials, we believe at least one population is resistant to FirstRate. Research in Indiana, Ohio, and Michigan also has found marestail resistant to ALS herbicides, and in some cases ALS and glyphosate resistance were stacked within a biotype.

Producers should intently monitor FirstRate performance on marestail, as ALS resistance or stacked glyphosate + ALS resistance is possible. To slow the development of ALS resistance, the use of non-ALS and glyphosate herbicides to control marestail should be used the following year.

Control in Corn

Marestail control in corn has not been as difficult as in soybeans. However, this year we have also received calls for recommendations on postemergence control of marestail in corn. Currently, we are aware of little information about the efficacy of common postemergence corn herbicides in combination with glyphosate on marestail.

Based on preemergence burndown data, the growth regulator herbicides 2,4-D and dicamba will be effective. However, most corn is past the label limits (8 inch corn) for broadcast applications of these herbicides. Status is a product containing dicamba that may be broadcast on corn up to 24 inches, or applied with drop nozzles in corn up to 36 inches. Combinations of atrazine and Callisto are also effective on marestail for burndown applications. Atrazine should not be applied after corn reaches 12 inches, but Callisto, and the other HPPD inhibiting herbicides such as Impact and Laudis, may be tank-mixed with glyphosate and applied up to at least V8 corn. Fortunately, marestail is not very competitive with corn, so yield loss from marestail will be minimal.

If Liberty Link corn was planted, tank-mixing Callisto, Impact, or Laudis with Ignite will likely improve control of marestail.

Summary Recommendations

Please note these are emergency recommendations. In future years, if you have marestail, use burndown herbicides plus a residual to control marestail before planting. This will reduce glyphosate selection pressure and provide better control of marestail.

Lowell Sandell
Extension Educator, Weed Science, Lincoln
Mark Bernards
Extension Weeds Specialist, Lincoln
Greg Kruger
Cropping Systems Specialist, West Central REC, North Platte