Fall Herbicide Applications for Winter Annual Weed Control

Fall Herbicide Applications for Winter Annual Weed Control

September 20, 2008

 

Reminders

  • Fall application of atrazine, or any product that contains atrazine, is prohibited in an annual corn/soybean, sorghum/soybean, continuous corn, or continuous sorghum rotation in Nebraska. Fall atrazine applications are permitted in fallowed ground following wheat harvest.
  • If marestail/horseweed is present, do not rely solely on glyphosate for control.
  • Glyphosate-resistant marestail was confirmed in Nebraska in 2006. Adding another mode of action (2,4-D, dicamba) will help control glyphosate-resistant marestail, and will also reduce the probability of developing other glyphosate-resistant winter annual weeds.

Winter annual weeds are a growing issue in Nebraska row crops. The increase in winter annual prevalence is likely, in part, due to:

  • Reduced use of residual pre-emergence herbicides in glyphosate based cropping systems.
  • A shift to total post-emergence herbicide programs, primarily based on glyphosate.
  • An increase in the adoption of no-till practices.

Numerous winter annual weed species are present in Nebraska fields (Table 1). An excellent North Central Region publication on winter annual identification is available online at: (http://extension.missouri.edu/explorepdf/regpubs/ncr614.pdf (3.1 MB)). Winter annuals begin germination in early fall as temperatures cool, commonly from mid-September through November. They then overwinter and complete their life cycle by mid summer. Most of the broadleaf species have a rosette growth habit when young. Because the seedlings are small and grow close to the ground, they are easily overlooked with a casual scouting. An excellent time to scout is during harvest. When you get out of the combine, take a moment and look at the soil for small germinating plants.

Since most producers have not been accustomed to controlling winter annuals, populations have flourished in recent years. If winter annuals have been allowed to go to seed for a number of years, the weed seedbank for those species have likely built to a point where scouting and herbicide control may be necessary each year.

Table 1.  Common winter annual species of concern in Nebraska row crops.

Broadleaf Species

Grass Species

Catchweed bedstraw (Galium aparine)
Common chickweed (Stellaria media)
Corn speedwell (Veronica arvensis)
Dandelion* (Taraxacum officinale)
Field pennycress (Thlaspi arvense)
Field pansy (Viola rafinesquii)
Henbit (Lamium amplexicaule)
Horseweed/Marestail (Conyza canadensis)
Prickly lettuce (Lactuca serriola)
Purslane speedwell (Veronica peregrin)
Shepherdspurse (Capsella bursa-pastoris)
Pinnate tansymustard (Descurainia pinnata)
Virginia pepperweed (Lepidium virginicum)

Annual bluegrass (Poa annua)
Carolina foxtail (Alopecurus carolinianus)
Downy brome (Bromus tectorum)
Foxtail barley* (Hordeum jubatum)
Little barley (Hordeum pusillium)
Ryegrass, annual or Italian (Lolium multiflorum)


* Species is actually a perennial, however its time of most robust growth often coincides with winter annual growth and development.

There are several advantages to controlling winter annual weeds in the fall:

  • Herbicides are highly effective at controlling younger, smaller weeds. Fall applications target the most vulnerable growth stage of winter annuals. Most of these weeds — particularly dandelion, marestail, field pennycress, and henbit —are much harder to control after over-wintering.
  • If soil moisture is in short supply, stopping winter annual growth in the fall and reducing subsequent populations in the spring can reduce water loss due to weeds.
  • Fall applications, especially those with long lasting residual activity, can potentially reduce the spring work load and allow producers to start timely planting.
  • Fall in Nebraska is typically warmer then spring, allowing herbicide to perform better.

Table 2 lists a number of effective herbicide options labeled for fall applications. In general, these herbicide options are highly effective on a broad spectrum of broadleaf winter annuals. For example, according to the current Guide for Weed Management in Nebraska, every herbicide or herbicide combination listed in Table 2 will control henbit at 90% or better when fall applied. If there is a significant population of winter annual grass species such as downy brome or Carolina foxtail in a field, adding glyphosate will likely enhance grass control. For specific herbicide efficacy ratings on individual weed species, refer to the current Guide for Weed Management in Nebraska or the interactive online herbicide tables on the UNL Weed Science Web site (http://weedscience.unl.edu).

Table 2.  Common herbicides and tankmixes to consider for fall applications to control winter annual weeds.

Herbicide (Rate(s)/acre)

Estimated Cost ($/acre)

2009 crop options

2,4-D Ester (1 qt)

$4.75

Corn or Soybean

Authority First + 2,4-D (3.2 oz + 1 pt)

$19.00

Soybean

Authority MTZ + 2,4-D (14 oz + 1 pt)

$19.00

Soybean

Autumn1 (0.3 oz)

$6.90

Corn or Soybean

Banvel (0.5 pt)

$3.75

Corn or Soybean

Basis + 2,4-D (0.5 oz + 1 pt)

$11.88

Corn

Canopy Ex (2 oz)

$13.00

Soybean

Classic1 (1.5 oz)

$25.00

Soybean

Extreme (3 pt)

$18.00

Soybean

Generic Glyphosate (32 oz)

$10.50

Corn or Soybean

Glyphosate + 2,4-D (24 oz + 1 pt)

$10.30

Corn or Soybean

Gramoxone Inteon (1 qt)

$9.50

Corn or Soybean

Princep + 2,4-D (1 qt + 1 qt)

$12.75

Corn

Rage D-Tech (16 oz)

$7.75

Corn or Soybean

Roundup PowerMAX (22 oz)

$14.27

Corn or Soybean

Valor XLT1 (3 oz)

$11.44

Soybean

Valor SX1 (3 oz)

$15.00

Corn or Soybean


1Adding 2,4-D or glyphosate can enhance and broaden the spectrum of weeds controlled.

 

Some residual herbicides will control summer annuals until mid-May or early June. This can eliminate the need for a preemergence herbicide application in the spring, and protect yields until a timely postemergence application can be made. Canopy EX+Classic and Valor XLT performed particularly well in our soybean trials this past spring where we had henbit, pennycress, tansymustard and marestail in the fall, and sunflower, velvetleaf, lambsquarters and Palmer amaranth in the spring. Authority First, Authority MTZ, Basis, Extreme, Princep, Python, and Valor SX controlled some, but not all of the summer annual species. Depending on the weed pressure and species present in your field, they may also be effective options.

Making a fall herbicide application can be an effective way to add another herbicide mode of action to your weed control program and reduce the risk of developing glyphosate resistance in some species, particularly marestail. With the increased cost of input prices for 2009, this is an excellent time to consider using generic herbicides or alternatives to glyphosate to reduce herbicide costs.

Lowell Sandell
Extension Educator, Weed Science
Mark Bernards
Stevan Knezevic
Extension Weed Specialists