Ready to Tackle Lambsquarters?

Chris Boerboom, Extension Weed Scientist

The common lambsquarters season is almost here. Are you ready? It may seem odd to discuss the growing season as a lambsquarters season, but most if not all fields in Wisconsin have common lambsquarters. So, everyone will deal with lambsquarters, one way or another. 

 Common lambsquarters control in Roundup Ready soybeans seems to be a common concern for many growers, agronomists, and consultants. Control has been good in some years and inconsistent in other years. Some reasons suggested for inconsistent lambsquarters control include spraying larger plants (which happens in WI) or using low glyphosate rates, stem boring insects, rain after glyphosate applications, and even dusty plants. Lambsquarters with variable sensitivity to glyphosate have also been noted by many researchers although there are no reported cases of lambsquarters with higher levels of glyphosate-resistance. 

 When planning your management of lambsquarters for this season, a review of a recent research study from Indiana and Ohio might be useful. This study was conducted at four farms with histories of lambsquarters with reduced glyphosate control. Three of the five on-farm trials were conducted under no-till and the other two were managed with chisel-till. The management options tested were 1) skipping a burndown, 2) using glyphosate + 2,4-D as a burndown, and 3) using glyphosate + 2,4-D plus Gangster as the burndown. The glyphosate rate was 22 oz/a of WeatherMax and the Gangster rate was at 2.4 oz/a. These treatments were followed with one or two postemergence passes of 22 or 44 oz/a of WeatherMax. 

 Key results

1. A residual herbicide added to the burndown treatment reduced the number and size of the lambsquarters when the first postemergence glyphosate treatment was sprayed (Figure 1). The lambsquarters density was reduced by over 85% and the remaining seedlings were all less than 3 inches tall when they were sprayed postemergence. These smaller lambsquarters should be easier to control when sprayed postemergence with glyphosate.

 2. Any burndown treatment increased the level of lambsquarters control provided by an early postemergence treatment of glyphosate compared to when the burndown treatment was skipped (Figure 2). When the burndown treatment was skipped, lambsquarters control was only 66% after the early postemergence glyphosate treatment (22 oz/a), but control increased to 81% after the glyphosate + 2,4-D burndown treatment and 99% after the burndown treatment that included the residual herbicide Gangster. 

3. Following the residual herbicide, the researchers were able to delay the postemergence glyphosate treatments by 10 to 14 days while maintaining this high level of control.  In Figure 2, this is highlighted as a mid-post timing. The other postemergence treatments started with an early postemergence timing. This added time for application has implications for protecting soybean yield against early season weed competition.

4. These tough lambsquarters (fields with previous histories of reduced control) were 4 to 8 inches tall when they were sprayed without a burndown treatment or after the glyphosate + 2,4-D treatment. In these fields and with lambsquarters this tall, lambsquarters were controlled better when the higher rate of glyphosate was used compared to the standard rate (83 vs 61% with no burndown and 90 vs 81% when following glyphosate + 2,4-D burndown; Figure 2).  However, a higher rate of glyphosate was not needed when following the residual herbicide. Control was 99 and 98% with either the standard or higher glyphosate rate.

5. Two applications of a standard glyphosate rate (22 + 22 oz/a) provided similar control to a single higher rate (44 oz/a) except when a residual herbicide was used (Figure 2). Following the residual herbicide, a single glyphosate application was as effective as two glyphosate applications (99 vs 100%). An early 44 oz/a glyphosate application followed by a later 22 oz/a glyphosate application was needed to obtain control equal to the control when the residual herbicide was followed with a single postemergence glyphosate application (95 and 97% control with 44 oz/a followed by 22 oz/a with no burndown or after the glyphosate + 2,4-D burndown, respectively; data not shown in figure).   

Hopefully, these results highlight a couple important considerations when managing common lambsquarters, especially the need to use a burndown treatment and the value of a preemergence residual herbicide in the system. Remember, this study was conducted on “tough” lambsquarters, but we may be facing similar lambsquarters in some of our Wisconsin fields too.

Source: Westhoven and others. 2008. Management of glyphosate-tolerant common lambsquarters (Chenopodium album) in glyphosate-resistant soybean. Weed Technology 22:628-634.