One of the most difficult weeds to control in corn is morningglory. The backbone of any morningglory control program in corn is atrazine. The maximum use of Atrazine 4L in corn is 2.5 quarts per acre.
An excellent approach is to split the atrazine applications with one quart per acre applied preemergence and 1.5 quarts per acre applied over-the-top as a postemergence before the corn is twelve inches tall and the weeds are small (less than 1.5 inches). A benefit of atrazine in a weed control program is that it gives growers a different mode of action in combating glyphosate and ALS-inhibitors resistance pigweed.
Herbicide Resistance Info
These atrazine applications are usually applied with a broad-spectrum contact herbicide (in Roundup Ready corn varieties this would be glyphosate). If the glyphosate formulation already contains surfactants then additional surfactants (NIS or COC) are not needed and may increase injury to the corn. Ammonium sulfate (AMS) should always be added to glyphosate where hard water is a concern. The AMS must be added to the spray mixture before the glyphosate.
A problem with atrazine is that it may break down late in the season and morningglories emerge especially where there is not adequate corn growth to shade the ground. Growers may be tempted to apply atrazine later than the label restrictions but yield reductions are associated with applications past when labeled to apply atrazine.
Research by University of Georgia Extension weed scientist Dr. Eric Proskto shows there are no yield reductions from applying atrazine according to the label but when applying after the label states there can be a yield reduction. His research showed over a 12% yield decrease when applying atrazine at V7-8 compared to no yield reduction when applying at V2-3, V4 or V6-7.
Dr. Joyce Tredaway, Alabama Extension weed scientist, advises another postemergence herbicide application before the corn reaches 30 inches tall to improve morningglory control with the two above mentioned atrazine applications. I am going to break them down into bronze, silver and gold plans.
Bronze plan- Status at 5-10 ounces per acre. A NIS at quart per 100 gallons of spray mixture plus 1.25% UAN or 5-17 pounds of AMS should be used with Status. Status can be applied postemergence from 4 to 36 inches tall or V10, or within 15 days before tassel emergence, whichever comes first.
Silver plan- Status + Callisto at 3 ounces per acre. Callisto can be applied up to 30 inches tall.
Growers may want to add glyphosate with either of these two options (Bronze and Silver plans) if they see a need for additional contact weed control on Roundup Ready varieties. Glyphosate can be applied from emergence to 30 inches in height or V8 growth stage on Roundup Ready corn varieties.
Gold plan- Status + Halex GT (only on Roundup Ready varieties). Halex GT can be applied up to 30 inches tall or V8.
Please read the label for additional tank-mix adjuvants or restrictions.
Growers may balk at the time or additional costs of an additional herbicide application but may want to think about getting off the combine numerous times to clean out the header in 90 degree heat or the cost of skipping over corn covered in morningglories or combine inefficiency in these weed infested spots.
Another option, that would save time and money, would be to just spray the trouble spots or field edges where morningglories are most troublesome.