By Dallas Peterson, Extension weed management specialist
Now that row crop harvest is underway and fall moisture has been received, it is 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 pre-plant weed control has not been adequate. Henbit and marestail frequently are some of the most troublesome weeds we try to manage with these fall herbicide applications.
Fall applications have another side-benefit. While it is always important to manage herbicide drift, herbicide applications made after fall frost have less potential for drift problems onto sensitive targets.
There are several herbicide 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, or leave the land fallow during the summer and come back to winter wheat in the fall. If atrazine is applied too early, warm weather and moisture will reduce the length of residual. November is often the best time for atrazine applications.
Atrazine is labeled in Kansas for fall application over wheat stubble or after row crop harvest any time before December 31, as long as the ground is not 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 per acre of atrazine can be applied in a calendar year on cropland.
One half to two pounds (maximum) per acre of atrazine in the fall, tank-mixed with 1 to 2 pints per acre of 2,4-D LV4 or 0.67 to 1.33 pints LV6, 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 tank-mix. Winter annual grass control with atrazine is discussed below.
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 — unless the weed population is triazine-resistant. These two graphs (Figure 1 and 2) show the residual control effects of December herbicide applications on kochia ahead of corn and sorghum planting.
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 or dicamba have effectively controlled 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 or dicamba. Sharpen can be very good on marestail, but should be tank-mixed with 2,4-D, dicamba, atrazine, or glyphosate to prevent regrowth.
If the spring crop will be corn, other residual herbicide options include ALS herbicides such as Autumn Super or Basis Blend. ALS-resistant marestail will survive an Autumn Super or Basis Blend treatment if applied alone. For burndown, producers should mix in 2,4-D, dicamba, and/or glyphosate. Winter annual grasses can also be difficult to control with atrazine alone.
Success depends on the stage of brome growth. For downy brome control, 2 lbs per acre of atrazine plus crop oil concentrate (COC) has given excellent control, whereas 1 lb per 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 winter annual grassy weeds. Atrazine antagonizes glyphosate, so if the two are used together, a full rate of glyphosate (0.75 lb ae) is recommended for good control. The tank-mix should include AMS as an adjuvant.
Where fall treatments control volunteer wheat, winter annuals, and early-emerging summer annuals, producers should then apply a preemerge grass-and-broadleaf herbicide with glyphosate or paraquat at corn or sorghum planting time to control newly emerged weeds. Soils will be warmer and easier to plant where winter weeds were controlled in fall.