Fall Herbicide Applications for Winter Annual Weed Control

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. An excellent North Central Region publication on winter annual identification is available online at: http://extension.missouri.edu/explorepdf/regpubs/ncr614.pdf. 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.

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 workload and allow producers to start timely planting.
• Fall in Nebraska is typically warmer then spring, allowing herbicide to perform better.

There are 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. 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.

Some residual herbicides will control summer annuals until mid-May or early June. This can eliminate the need for a pre-emergence 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.

Reminders:

• Fall application of atrazine, or any product that contains atrazine, is prohibited in Nebraska.
• 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.