Editor’s note: The following was written by Prashant Jha, professor and Extension weed specialist at Iowa State University, for the Integrated Crop Management Blog April 10.
With the onset of planting season, it is necessary to watch for winter annuals and early emerging summer annuals in corn and soybean fields.
Consider applying burndown herbicides to control those weeds when temperatures are above 55 degrees to improve the effectiveness of herbicide application.
Effective burndown herbicides to control marestail would be an important consideration. This is because a majority of marestail populations in Iowa have developed resistance to glyphosate (HG 9) and ALS-inhibitor (HG 2) herbicides. Therefore, glyphosate is not recommended alone in burndown programs.
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Adding 0.5 lb. acid equivalent (ae) 2,4-D LVE, 0.25 lb ae dicamba, or saflufenacil (Sharpen at 1 oz./acre) to glyphosate will increase the consistency of marestail control as well as control of other winter annuals.
Based on the results from ISU field trials, dicamba and 2,4-D both can provide a greater control of marestail when applied at the rosette stage compared to the bolting stage. Glufosinate (Liberty at 32 oz./acre) along with 2,4-D or dicamba is an effective spring burndown option to control marestail if plants have already bolted.
Most 2,4-D labels have a 7-14 day planting restriction for corn or soybean, and dicamba has a longer replant restriction than 2,4-D when used preplant in soybean (14 days at 0.25 lb. ae and 28 days for 0.50 lb. ae). Dicamba-tolerant (Xtend/Xtendflex) and 2,4-D choline-tolerant (Enlist E3) soybean varieties allow use of approved formulations of dicamba (Xtendimax) and 2,4-D choline (Enlist One/Enlist Duo), respectively, for preplant burndown without any planting delays.
Use the full-labeled rate along with a full load of recommended adjuvant(s) to prevent any control failures, especially when weeds are stressed by cold conditions or are larger in size at the time of application.
Adding atrazine (in corn) or metribuzin (in soybean) plus 2,4-D would also be an effective spring burndown program to control glyphosate-resistant marestail in the field. Residual herbicides (flumioxazin, metribuzin) should be a valuable component of spring burndown programs for consistent control of other early-emerging weed species such as common lambsquarters and giant ragweed prior to soybean planting.
Take all needed steps to maximize herbicide activity on weeds such as proper use of adjuvants and sprayer set up (nozzles, volume, pressure, speed) as directed by the herbicide label.