Herbicide-resistant Corn

Herbicide-resistant field corn is now commonly grown in the Pacific Northwest. There are four major types of herbicide resistant corn: including some that are genetically engineered; and some that are developed by genetic selection of tolerant corn individuals and/or varieties. Failure to apply the correct herbicide to the correct crop will result in serious injury to the corn. Refer to the appropriate herbicide label for guidelines on how to use each product on herbicide-resistant corn.

IMI (IR/IT) or Clearfield (CL) Corn was developed by genetic selection to tolerate imidazolinone herbicides. Lightning (imazethapyr + imazapyr) controls nearly all annual grass and broadleaf weeds (except ALS-resistant weeds) and suppresses quackgrass and Canada thistle. Optill (imazethapyr + safluenfacil), Pursuit (imazethapyr), and Pursuit Plus (imazethapyr + pendimethalin) also can be applied to Clearfield corn. Some IMI varieties (IR) are tolerant to several sulfonylurea (e.g., Accent, Exceed) and sulfonamide (Broadstrike products, Python) herbicides and are sometimes used to reduce the injury potential of these products when applied alone or in combination with organophosphate (OP) insecticides.

LibertyLink Corn is tolerant to over-the-top applications of products containing glufosinate including Liberty, Ignite, Sinate, and Rely 280 herbicides. These products provide broad-spectrum control of annual broadleaf and grass weeds at low to moderate weed densities. Apply to LibertyLink corn up to 24 inches with seven or fewer collars. Apply when weeds are small because translocation is limited. Glufosinate does not control large or well-tillered grasses such as yellow foxtail, wild oat, or volunteer cereals. Glufosinate is non-residual; controlling multiple weed flushes may require multiple applications or applying with a residual herbicide. Apply with AMS fertilizer. Refer to label for weeds controlled, application information and timing, tank-mix options, and restrictions. Ignite can be used to control weeds resistant to other herbicides.

Roundup Ready Corn is tolerant to glyphosate at labeled rates up to 30 inches tall with eight or fewer collars, and will control most annual and perennial weeds. Roundup Ready Corn II has elevated tolerance to glyphosate. Apply glyphosate to corn only if it is confirmed to be tolerant to glyphosate, or the corn crop may be killed. Certain formulations do not require additional nonionic surfactant, while others require either partial or full nonionic surfactant rates. Add ammonium sulfate to all glyphosate formulations at 2 to 6 lb/100 gal water or at 1 lb/A if applied at less than 12 gal/A. In-crop application timing may not be appropriate for effective perennial weed control. Glyphosate is a nonselective, non-residual, translocated herbicide that controls grasses and broadleaf weeds. However, glyphosate may not control some broadleaved weeds such as kochia, nightshades, wild buckwheat, horseweed (marestail), dandelion, and common lambsquarters if applied alone or after only one application. Roundup resistant kochia is known to exist in eastern Oregon and parts of southwestern Idaho.

Poast Protected Corn hybrids were developed with traditional breeding and selection techniques and are tolerant to broadcast over-the-top applications of sethoxydim containing products such as Poast, Poast Plus, Rezult, and Sethoxydim SPC herbicides. These herbicides will control annual grasses when applied with other postemergence herbicides or help to manage escaped grasses.