Herbicide Resistance Management for Common Lambsquarters and Horseweed

In the June 13th issue of the Wisconsin Crop Manager, I discussed herbicide resistance management for giant and common ragweed. This week’s featured herbicide resistance threats are common lambsquarters, Chenopodium album, and horseweed, Conyza canadensis. Weed scientists across the Midwest and Midsouth have identified eleven species of weeds that are of most concern for herbicide resistance because of their ability to compete with crops and to develop resistance to different herbicide sites of action. In 1979, University of Wisconsin weed scientists identified a population of common lambsquarters resistant to atrazine, a photosystem II inhibitor (www.weedscience.org). Lambsquarters populations in Michigan and Ohio have been found resistant to ALS inhibitors. In 2013, University of Wisconsin researchers identified horseweed plants resistant to glyphosate. Ohio and Delaware have horseweed populations resistant to both ALS inhibitors and glyphosate. Resistance to a single site of action has occurred in over twenty states.

Now is the time to start thinking about fall horseweed management. Emergence typically occurs in the early spring and again in the fall. Long-term no-till systems tend to harbor significant horseweed populations. Scouting in mid to late summer to locate any escapes from spring herbicide applications is important for herbicide resistance management and to decide whether to switch to a horseweed management program that includes both spring and fall control measures. Fall herbicide applications can help to reduce horseweed populations in problem fields. Also, if dandelion is an issue, then there are two reasons to consider fall herbicide applications. University of Wisconsin researchers found that fall dandelion control is best prior to next year’s corn crop. Their results are available as a slide presentation. Kevin Bradley at the University of Missouri has a video, less than five minutes, discussing the importance of application timing for horseweed control.

Ohio State and Purdue University have a great fact sheet on horseweed management. Also, the TakeAction website has a fact sheet with spring and fall herbicide recommendations.

Common lambsquarters can be difficult to manage because of an early and sustained emergence period, long seed persistence, and competitive ability. A 50 percent reduction of seed in the soil seedbank requires about 12 years and 78 years for a 99 percent reduction. Management goals should include: starting with a clean field, using a pre-emergence residual herbicide, scouting, and applying a post-emergence herbicide if necessary. For specific management recommendations, please consult the TakeAction fact sheet.

To-Do List: Fall Dandelion Management

post_dandelion

It’s time to start planning ahead for fall dandelion management. The Weed Science website at the University of Wisconsin Extension has a thorough description of the dandelion’s life cycle and some suggestions for control. A group of researchers at the University of Wisconsin evaluated fall and spring herbicide applications for control of dandelion in 2011. Their key findings were:

  1. Herbicide applications in the fall were most effective for early-season dandelion control compared to applications at normal corn planting in the spring.
  2. Applications during the spring were ideal for late-season control when soybean planting occurs.

For specific information on herbicides, please see their slide presentation.

The flowering stage of dandelion is easily identified by most. However, identifying the seedling stage is helpful because management is best before dandelions enter the adult stage. The following plant characteristics will help with identifying seedling dandelions:

  • Seed leaves (cotyledons) are oval or spoon-shaped
  • First true leaf is ovular and 0.4 inches (1 cm) long. Leaf margins have a few very small teeth that point back toward the base of the leaf. The midvein is prominent on the underside of the leaf.
  • Later leaves are more elongated and will have the toothed margins and rosette arrangement typical of the mature plant.

Managing dandelions can either be done through the use of herbicide or physical removal in either the spring or fall. However, dandelions are more susceptible to herbicides applied in the fall. If physical removal is the management mode of choice, it must be done in the spring as well as the fall.

Management options for corn and soybean are listed below.

Corn

Chemical control options:

  • Fall application of 2,4-D or dicamba at 1 pint per acre applied alone or as a tank mix prior to the first killing frost. Another option is glyphosate but for optimal activity applications need to be made when the air temperature is above 50 degrees F and the plant is still actively growing.
  • Spring applications of 2,4-D ester at 1 pint per acre prior to corn emergence or a tank-mix of 2,4-D with glyphosate will provide some dandelion control. Also, post-emergence applications of growth regulator herbicides typically provide acceptable control.

Mechanical control options:

  • Moldboard or chisel plowing will weaken the plant by disturbing the taproot and may make chemical applications more effective.

Soybean

Chemical control options:

  • 2,4-D ester may be applied prior to soybean planting if a seven-day interval is observed between application and planting.
  • There are no good post-emergence options for dandelion control in soybeans.
  • If possible, control heavy infestations of dandelions in the fall if the field is to be planted to no-till soybeans.

References:

  1. http://www.extension.psu.edu/pests/weeds/weed-id/common-dandelion, site accessed July 27, 2014.
  2. http://www.umanitoba.ca/outreach/naturalagriculture/articles/dandelion.html, site accessed July 27, 2014.
  3. http://fyi.uwex.edu/weedsci/2002/11/12/dandelion/, site accessed July 27, 2014.

Authors:

Madeline Fischer and Liz Bosak

Madeline Fischer is an undergraduate research assistant working for WCWS at the Arlington Agricultural Research Station. In Fall 2014, Madeline will be a sophmore pursuing a degree in Life Science Communcations and Environmental Studies.

From the field: dandelions

post_dandelionThis spring, dandelions carpet the fields. The Weed Science website at University of Wisconsin Extension has a thorough description of the dandelion’s life cycle and some suggestions for control. A group of researchers at the University of Wisconsin evaluated fall and spring herbicide applications for control of dandelion in 2011. Their key findings were:

  1. Herbicide applications in the fall were most effective at early-season dandelion control corresponding to normal corn planting.
  2. Applications during the spring were ideal for late-season control when soybean planting occurs.

For specific information on herbicides, please see their slide presentation.