Poor control of common lambsquarters with glyphosate: El Passo Extension IPM Agent

Poor control of common lambsquarters with glyphosate: On June 16, I inspected a cotton field southwest of Fabens with poor weed control after 2 applications of Roundup PowerMAX®. Both applications were made with a spray mixture of 15 gal/acre (using a 300-gallon tank), a spray width covering 18 cotton rows, and adding the non-ionic oil concentrate Penetrator®Plus at 1%. The first application was made on June 1, 2015 using 32 oz of Roundup PowerMAX®/acre. After observing poor weed control, a second application was made on June 9 using 48 oz of Roundup PowerMAX®/acre. Now (over two weeks after the first application), most weed species are dead, but approximately 10-15% of common lambsquarters plants, also known as Goosefoot in the plant family Chenopodiaceae, are doing well. Following suggestions by Dr. Charles Allen, I used RTU RoundUp to spray two herbicide rates in two rows. The first row received a “light” application and the second row received a “heavy” spray (soaking the plants well to the point of runoff). I returned to the field one and two days after treatment and I noticed herbicide-damaged cotton plants adjacent to the handsprayed lambsquarters. These weeds appeared undisturbed while the cotton leaves showed herbicide damage. It is too early to make any conclusions, but I will continue visiting this field and evaluate alternatives. Dr. Peter Dotray, Professor of Weed Science with Joint Appointment at Texas A&M Agrilife Research & Extension Service and Texas Tech University, indicated that broadleaf weed control in cotton is difficult, but it is possible to use Staple herbicides when weeds are small and being mindful of crop rotation restrictions. Also, the Liberty herbicide may be used in Liberty Link cotton varieties. Another possibility is the use of hooded sprayers with herbicides such as: Aim, ET, Liberty, or Gramoxone. Ultimately, in some cases, cultivation and hand hoeing may be the best options. I observed a mixture of dead, dying, and live common lambsquarter plants; which to me would suggest the possibility of a glyphosate resistance problem. However, there has not been a documented case of lambsquarter resistance to glyphosate anywhere in the world. We may be dealing with naturally reduced glyphosate susceptibility in these lambsquarter plants. Later in the season, I would like to obtain lambsquarters seeds, from this field, and send to Dr. Dotray for glyphosate-resistance studies. The extension publication “4-step Program for Managing Glyphosate Resistant Pigweeds in Texas Cotton” offers great advice on glyphosate resistance management (click here).