Controlling annual weeds with fall-applied herbicides ahead of corn and sorghum
With row crop harvest underway, it’s time to start planning your fall herbicide applications to control winter annual broadleaf weeds and grasses ahead of grain sorghum or corn. Fall applications during late October and through November can greatly assist control of difficult winter annuals and should be considered when performance of spring-applied preplant weed control has not been adequate. Henbit and marestail frequently are some of the most troublesome weeds.
There are several options for fall application. If residual weed control is desired, atrazine is among the lowest-priced herbicides. However, if atrazine is used, that will lock the grower into planting corn or sorghum the following spring.
Atrazine is labeled in Kansas for fall application over wheat stubble or after fall row crop harvest anytime before December 31, as long as the ground isn’t frozen. Consult the atrazine label to comply with maximum rate limits and precautionary statements when applying near wells or surface water. No more than 2.5 lbs of atrazine can be applied per acre in a calendar year on cropland.
One half to two pounds (maximum) per acre of atrazine in the fall, usually with 1 to 2 pints/acre of 2,4-D LV4 or LV6 added, can give good burndown of winter annual broadleaf weeds -- such as henbit, dandelion, prickly lettuce, Virginia pepperweed, field pansy, evening primrose, and marestail -- and small, non-tillered winter annual grasses. Atrazine’s foliar activity is enhanced with crop oil concentrate, which should be included in the tankmix.
Atrazine residual should control germinating winter annual broadleaves and grasses. When higher rates of atrazine are used, there should be enough residual effect from the fall application to control early spring-germinating summer annual broadleaf weeds such as kochia, common lambsquarters, wild buckwheat, and Pennsylvania smartweed. While it is always important to manage herbicide drift, herbicide applications made after fall frost have less potential for drift problems onto sensitive targets.
Marestail is an increasing problem in Kansas that merits special attention. Where corn or grain sorghum will be planted next spring, fall-applied atrazine plus 2,4-D has been very effective on marestail rosettes, and should have enough residual activity to kill marestail as it germinates in the spring. Atrazine alone will not be nearly as effective postemergence on marestail as the combination of atrazine plus 2,4-D. Sharpen is very good on marestail, and should be tankmixed with 2,4-D, atrazine, or glyphosate.
If the spring crop will be corn, other residual herbicide options include an ALS herbicide such as Autumn or Basis Blend. ALS-resistant marestail will survive an Autumn or Basis Blend treatment if applied alone. For burndown, producers should mix in 2,4-D and/or glyphosate. Aim and Rage D-Tech are other non-residual, contact herbicide options for fall application.
Winter annual grasses can also be difficult to control with atrazine and success is dependent on the stage of brome growth. For downy brome control, 2 lbs/acre of atrazine plus crop oil concentrate (COC) has given excellent control, whereas 1 lb/acre has given only fair control. Volunteer wheat and brome species that have tillered and have a secondary root system developing will likely not be controlled even with a 2-lb rate. Adding glyphosate to atrazine will ensure control of volunteer wheat, annual bromegrasses, and other grassy weeds. Atrazine antagonizes glyphosate, so if the two are used together, a full rate of glyphosate (0.75 lb ae) is essential for good control. The tankmix should include AMS as an adjuvant.
If fall treatments control volunteer wheat, winter annuals, and early-emerging summer annuals right up to planting corn or sorghum, then at planting time a preemerge grass-and-broadleaf herbicide application with glyphosate or paraquat will be needed to control newly emerged weeds. Soils will be warmer and easier to plant where winter weeds were controlled in fall.
Curtis Thompson, Extension Agronomy State Leader and Weed Management Specialist