Source: Michelle Wiesbrook/University of Illinois at Urbana-Champaign

Ever finished filling your spray tank with pesticides, adjuvants, and carrier, only to realize that a thunderstorm is about to hit your intended application site? Do you go ahead and spray and risk the application being washed off? Or do you wait it out? Your chemicals, if allowed to sit in the tank for an extended period can settle out, degrade, or bind to the tank. Which is the lesser of the two evils?

In December, I attended the North Central Weed Science Society of America meetings, where I learned of some recent research focused on determining the length of time corn herbicides can remain in the spray tank prior to application in the field without impacting efficacy (control).

Studies were conducted at two locations in southwestern Ontario by Robert E. Nurse, research scientist with Agriculture and Agri-Food Canada, and Peter H. Sikkema, associate professor, University of Guelph. Between 2006 and 2008, ten field trials were conducted.

Preemergent treatments included (Canadian registered products noted in parentheses; similar U.S. products follow.)

  • Isoxaflutole + atrazine (Converge PRO), BALANCE + ATRAZINE
  • Dimethenamid + dicamba/atrazine (Frontier + Marksman), OUTLOOK + MARKSMAN
  • S-metolachlor/atrazine + mesotrione (Lumax), LUMAX
  • Rimsulfuron + s-metolachlor + dicamba (Battalion)

Postemergent treatments included

  • Nicosulfuron/rimsulfuron (Ultim), STEADFAST
  • Dicamba/diflufenzopyr (Distinct), DISTINCT
  • Mesotrione + atrazine (Callisto), CALLISTO + ATRAZINE
  • Glyphosate (Roundup WM), ROUNDUP WEATHERMAX
  • Glufosinate (Liberty), LIBERTY, IGNITE

The research abstract states that four preemergence and five post-emergence herbicides were mixed at their labeled rates and then applied in field corn following label specifications. Herbicides were either applied immediately, or after being left for one, three or seven days in the spray tank. The most common weed species in the trials were velvetleaf (Abutilon theophrasti), redroot pigweed (Amaranthus retroflexus), common ragweed (Ambrosia artemisiifolia), and common lambsquarters (Chenopodium album). Delaying herbicide application did not affect the efficacy of postemergence herbicides in this study.

Similarly, control of redroot pigweed and common lambsquarters was not affected by a delay in the application of preemergence herbicides. However, control of velvetleaf was decreased when isoxaflutole + atrazine, dimethenamid + dicamba/atrazine, or rimsulfuron + s-metolachlor + dicamba applications were delayed by more than 1 day. Nonetheless, there were no decreases in yield for any treatment combinations.

The results of this study suggest that for most herbicides and weed species it is better to postpone application (even if it's a week later) rather than make applications under nonideal conditions. It is also better to delay an application, rather than to dispose of the mixture. If an application must be delayed, it is recommended that the tank mixture be agitated at least once per day for best results. The spray tank should also be stored so that it is protected from direct sunlight, which can further degrade chemicals. Be sure to monitor treated fields closely. Of course, it is best to wait for inclement weather to pass before mixing your chemicals.

Nurse tells me that the data are being submitted for publication with the journal Crop Protection.