University of Illinois

No. 6/May 2, 1997

No Rain! What Now?

At the time of writing, many preplant and preemergence herbicides have been on the ground for 1 to 3 weeks without adequate precipitation to move them into the soil. Rain may soon be on the way, but not all areas of the state may receive precipitation.

How much rainfall is required to move the herbicide into the soil and how soon after application is precipitation needed? While there is no absolutely defined amount, surface-applied herbicides generally require between 0.5 to 1.0 inch of precipitation within 7 to 14 days after application for "activation." Factors such as soil condition, residue cover, and the chemical properties of the herbicide influence how much rain is needed and how soon after application it's necessary. If weeds have begun to emerge before the herbicide has had an opportunity to work, then it's time to think of alternate action. Rotary hoeing can control small emerging weeds and give surface-applied herbicides some incorporation. Postemergence herbicides for grass control in corn include Basis, Basis Gold, and Accent. The triazine herbicides atrazine and cyanazine can provide some control of small annual grasses.

Basis 75WDG (rimsulfuron + thifensulfuron) may be applied at 1/3 ounce (1/4 soluble packet) per acre up to the 4-leaf stage or two visible leaf collars of corn to control 1- to 2-inch foxtail, fall panicum, or barnyardgrass; and 1- to 3-inch smartweed, pigweed, and lambsquarters. Applications of Basis must include an adjuvant (nonionic surfactant or crop-oil concentrate) and an ammonium nitrogen fertilizer. To enhance weed control, cultivation is suggested 10 to 15 days after application.

Basis Gold 89.46WDG (rimsulfuron + nicosulfuron + atrazine) is applied at 14 ounces (1/4 soluble packet) per acre to control 1- to-3 inch grasses and 1- to 4-inch broadleaved weeds. Applications of Basis Gold are to include a COC; ammonium nitrogen fertilizer is recommended in addition to the COC. Basis Gold can be applied to corn up to 12 inches tall. Do not apply Basis Gold to corn taller than 12 inches or exhibiting six visible leaf collars, whichever is most restrictive.

Accent 75WDG (nicosulfuron) may be applied at 2/3 ounce per acre, over the top of corn up to 20 inches tall or with six visible leaf collars, to control grasses 2 to 4 inches tall. Applications must include either a COC or NIS, and the addition of an ammonium nitrogen fertilizer is recommended. Keep in mind the restrictions that appear on the Accent label regarding applications to corn previously treated with soil insecticides.

Atrazine may be used as a postemergence treatment before corn reaches 12 inches in height, to control annual grasses (not fall panicum) up to 1.5 inches tall. Use 2.2 pounds of atrazine 9ODF or 2 quarts of atrazine 4L plus 1 quart of COC per acre.

Cyanazine may be applied before the fifth-leaf stage of corn, but best grass control is achieved when corn is at the 2- to 3-leaf stage. Cyanazine is sold as Bladex or CyPro, while cyanazine plus atrazine is sold as Extrazine II or CyPro AT. Do not use adjuvants with cyanazine formulations. Check the label, as cyanazine rates vary according to soil type and formulation.

Resolve or Lightning may be used in IMI-corn hybrids, and Poast Plus may be used in SR-corn hybrids for grass control.

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