Common lambsquarters is one of the most common and problematic weeds that we deal with in Pennsylvania.

Triazine-resistant lambsquarters is widespread in the Northeast and ALS-resistant biotypes have been identified in Ohio and Michigan. Lambsquarters that are more tolerant to glyphosate have become more common in a number of states including Pennsylvania in response to the repeated use of glyphosate in Roundup Ready crops.

There are a number of effective soil-applied and POST products for lambsquarters control in corn, but the story is different for soybeans. Although, there are a number of effective PRE soybean herbicides for lambsquarters control, there are only a few effective POST products. For a more complete discussion on lambsquarters biology and control, see the Glyphosate Stewardship Series publication titled Biology and Management of Common Lambsquarters.

Here are some “slightly modified” key points that our colleagues at Ohio State and Purdue suggest for controlling lambsquarters in soybeans and corn

• Do not plant into existing stands of lambsquarters. Start weed-free at the time of planting by using tillage or a preplant herbicide treatment of 2,4-D ester plus either glyphosate or Gramoxone. In corn, a combination of 2,4-D plus atrazine will also control existing plants.
• Apply PRE herbicide(s) for control of lambsquarters, which can provide season-long control. PRE herbicides reduce early-season weed density and the rate of weed growth, creating more flexibility in the POST application window.
• Apply POST treatments when lambsquarters are less than 6 inches tall and have less than 6 nodes.
• In Roundup Ready crops, use a rate of 1.1 to 1.5 lb ae/A (equivalent to 32 – 44 fl. oz Roundup PowerMax) glyphosate in POST applications when lambsquarters are more than 6” tall and/or where control problems have previously occurred, or when weeds appear stressed by hot and dry weather. Better yet, tank-mix glyphosate with a second effective mode of action – Harmony GT for soybeans and numerous options for corn. In fields with a history of poor lambsquarters control with glyphosate, make a second POST glyphosate application 3 weeks after the first application.
• Control of lambsquarters in Roundup Ready soybeans is much more consistent with a combination of PRE and POST applications compared to just POST glyphosate alone.