University of Illinois

No. 7/May 9, 1997

Postemergence Broadleaf Weed Control in Corn

With most corn planting completed across much of Illinois, attention soon will turn to postemergence herbicide applications for control of broadleaf and grass species. Last week's edition of this Bulletin contained a brief summary of available options for postemergence control of grass species. This week, we turn our attention to postemergence options for broadleaved weed control.

Contact herbicides are usually applied and are more effective when weeds are small. Thorough spray coverage is essential to achieve good results.

Atrazine can be used as a postemergence treatment, before corn reaches 12 inches in height, to control certain annual broadleaves. Use 2.2 pounds of atrazine 9ODF, or 2 quarts of atrazine 4L plus 1 quart of COC per acre.

Buctril or Moxy 2EC (bromoxynil) is applied at 1 to 2 pints per acre. Applications of greater than 1 pint must be made between the 4-leaf stage and prior to tassel emergence. Additives may cause excessive crop leaf burn but are recommended with certain tank mixes.

Buctril + atrazine or Moxy + atrazine 3L (bromoxynil + atrazine) is applied at 1.5 to 3 pints per acre. Do not apply the higher rates until the 4-leaf stage of corn. Regardless of rate, applications must be made before corn reaches 12 inches in height. Adjuvants are allowed if the grower accepts the risk of crop injury.

Laddok S-12 5L (bentazon + atrazine) is applied at 1.33 to 2.33 pints per acre until corn is 12 inches tall. Add either COC, UAN, or Dash HC.

Tough 3.75EC (pyridate) is sometimes used at 1 to 2 pints per acre to control triazine-resistant kochia or pigweed. A tank mix with atrazine is often recommended to broaden the spectrum of control.

Resource 0.86EC (flumiclorac) may be applied alone at 4 to 6 ounces per acre when corn is between the 2- and 10-leaf stages. Resource at 4 ounces can be tank-mixed with atrazine, 2,4-D, or dicamba to improve velvetleaf control.

Sencor (metribuzin) may be included in tank mixes with several postemergence corn herbicides. The tank-mix partner determines the maximum corn height for application, type of application, and additive. Do not use COC with any tank mix. Use 3 to 4.5 fluid ounces of 4L, or 2 to 3 ounces of 75DF.

Liberty 1.67E (glufosinate) is applied postemergence only to corn hybrids with resistance to glufosinate (Liberty Link or Dekalb GR), at 16 to 28 fluid ounces per acre. Applications may be made from emergence until corn is 24 inches tall or V7, whichever comes first. Do not include any surfactants.

Translocated herbicides are classified as either PGR (plant growth regulator) or ALS-inhibiting in action. PGR herbicides include Banvel or Clarity, 2,4-D, and Stinger.

Banvel or Clarity (dicamba) and Marksman (dicamba + atrazine) may be applied from spike up to the 5-leaf or 8-inch stage of corn. Use 0.5 to 1 pint of Banvel or Clarity and 2 to 3.5 pints of Marksman per acre. Banvel may be applied at 0.5 pint per acre up to the 36-inch stage of corn, however do not apply to corn over 24 inches tall if soybeans in the area are over 20 inches tall or have begun to bloom.

2,4-D amine or ester may be applied to corn up to the tassel stage, but use drop nozzles if corn is over 8 inches tall. The application rate varies with the brand and formulation.

Stinger 3S (clopyralid), Hornet 85.6WG (flumetsulam + clopyralid) and Scorpion III 84.3WG (flumetsulam + clopyralid + 2,4-D) may be applied to corn up to 24 inches tall. Scorpion III requires drop nozzles if corn is over 8 inches tall because it contains 2,4-D. Rates per acre are 1.6 to 4 ounces of Hornet plus NIS or COC and UAN or 0.25 pound of Scorpion III plus NIS and UAN. Stinger is applied at 0.33 to 0.67 pint per acre for Canada thistle control.

ALS-inhibiting herbicides include Beacon, Exceed, Permit, Pursuit, Lightning, and Resolve on IMI-corn hybrids.

Beacon 75WG (primisulfuron) is effective on johnsongrass and shattercane but is often used at half rate with 2,4-D or Banvel for control of certain broadleaves. Do not use COC when tank-mixing Beacon with broadleaf herbicides other than atrazine.

Exceed 57WG (primisulfuron + prosulfuron) may be applied broadcast to corn between 4 to 20 inches tall or with drop nozzles from 20 to 48 inches. Use 1 ounce per acre and include COC or NIS plus UAN to improve velvetleaf control.

Permit 75WG (halosulfuron) is applied at 0.33 to 1.33 ounces per acre from spike to the lay-by stage of corn. Always add NIS or COC plus UAN to improve velvetleaf control.

If IMI corn hybrids (IR or IT) are planted, Resolve 74.9SG (imazethapyr + dicamba) or Lightning 70WG (imazethapyr + imazapyr) may be used postemergence to control 1- to 3- inch broadleaf weeds plus some grasses. Use 5.3 ounces of Resolve or 1.28 ounces of Lightning per acre plus NIS (Resolve) or COC (Lightning) and UAN.

Aaron Hager, and Marshal McGlamery, Department of Crop Sciences, (217)333-4424