As mentioned several weeks ago, this may be a year where the benefits of including residual herbicides in POST corn treatments are substantial, even where herbicides were applied at planting.
Corn planted several weeks ago still has a long way to go until crop canopy development, and we are really just starting into the period when most summer annual weeds emerge. The addition of residual herbicides can be especially appropriate when the corn is less than 12 to 14 inches tall at the time of the POST treatment. Residual herbicides are largely not needed in POST applications to corn that is getting close to canopy closure (20 inches or so), with the exception of burcucumber. Scouting and making the decision before corn is 12 inches tall allows for the most options where residual herbicide is needed, since this is the cutoff for atrazine use.
A list of POST corn herbicide options, with a brief summary of their residual activity follows. All of these herbicides have activity on emerged weeds as well (Table 4 in OH/IN Weed Control Guide), so that they can improve control and provide some resistance prevention when mixed with glyphosate or Ignite, or can replace these in some instances.
Atrazine – residual control of broadleaf weeds, and more limited control/suppression of grasses. Also available as premixes with dicamba, which improves control of emerged broadleaf weeds. Apply when corn is less than 12 inches tall.
Atrazine/acetamide premixes – residual control of grass and broadleaf weeds. Where an atrazine product was applied at planting, keep in mind that the total amount of atrazine for the year cannot exceed 2.5 lbs ai/A. Guardsman Max can be applied up to 12-inch corn; acetochlor/atrazine premixes can be applied up to 11-inch corn.
Callisto – residual control of broadleaf weeds. Up to 30-inch or 8-leaf corn. Callisto Xtra is a premix of Callisto and atrazine (up to 12-inch corn).
Capreno – residual control of grasses primarily. Broadcast up to V6 corn.
Halex GT – residual control of grass and broadleaf weeds. Up to 30-inch or 8-leaf corn.
Hornet – residual control of broadleaf weeds. Broadcast up to 20-inch corn or 6 collars.
Resolve Q – residual control of grasses, lambsquarters, pigweed, and smartweed. Broadcast up to 20-inch corn or 6 collars.
Spirit – residual control of some broadleaf weeds. Broadcast up to 20-inch corn or 6 collars.
Herbicides that have to be applied before corn is about V2 or 5 inches have been omitted here since that is really a delayed PRE application, where a comprehensive residual program is needed. There are some fields that remain untreated with herbicide where corn has emerged but is still small. Most of the PRE corn herbicide and herbicide premixes can be applied POST to small corn. Check labels and the OH/IN Weed Control Guide for information on maximum corn size and use of adjuvants and tank-mix partners.
More on Sharpen tank-mixtures:
We heard from several manufacturers after last week’s article about Sharpen labels. Here’s what we know at this point. Sharpen cannot be mixed with any product containing flumioxazin, which includes Valor, Valor XLT, Envive, Enlite, and Gangster. FMC indicates that labels allowing mixtures of Sharpen with FMC Authority products are still valid, and we assume that Sonic is included here. Contact your dealer or manufacturer rep for more information.