Common Lambsquarters Control in Glyphosate-Resistant Crops

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Greensboro, North Carolina (November 6, 2008)--Reports of poor common lambsquarters control continue to surface in situations where “normal” rates of glyphosate have been applied alone in glyphosate-resistant crops. This problem is most often seen in soybeans because the surviving weeds are more obvious than in corn, and because glyphosate is less likely to be tank-mixed with other herbicides in soybeans than in corn. Although the results discussed here are from a corn experiment, the glyphosate only treatments are relevant to soybeans as well.

Trial Information

Grower/cooperators near Moravia, NY planted Pioneer Hybrid 38M60 April 24, 2008 following pre-plant incorporation of 1qt/Aof AAtrex 4L. Mid-postemergence (MPO) herbicide treatments were applied June 21 when lambsquarters averaged 7 inches tall and corn was in the V7 stage of development (~16 inches tall). Herbicides were applied in 20 gallons/A of spray solution and all glyphosate treatments included 1.7 lb/A of ammonium sulfate (AMS). Lambsquarters control ratings were made 3 and 6 weeks after treatment (WAT).

Results for Glyphosate-Resistant Hybrids

Perhaps of greatest interest are control ratings with MPO applications of glyphosate alone. Roundup PowerMAX at the “normal” rate of 22 fl oz/A controlled 78 and 83% of the lambsquarters 3 and 6 WAT respectively. Simply increasing the application rate of Roundup to 33 fl oz/A resulted in 91 and 96% lambsquarters control 3 and 6 WAT respectively. These results are somewhat similar to those of a 2007 soybean experiment where late postemergence (14-inch lambsquarters) applications of 22 and 44 fl oz/A of Roundup WeatherMAX controlled 75 and 97% of the lambsquarters respectively. In both cases, increasing rates improved control.

Tank mixes of several herbicides with 22 fl oz/A of Roundup also provided excellent lambsquarters control in 2008. Among tank-mix partners that improved control compared to the “normal” rate of Roundup alone were 4 fl oz/A of Clarity and 2.5 oz/A of Status. Control when Clarity or Status was tankmixed with Roundup was 98 and 100% respectively 6 WAT. The active ingredient in these tank-mix partners is dicamba. Require Q, a pre-mix of rimsulfuron and dicamba, also provided 100% control 6 WAT. Other herbicides that provided very good control when applied with 22 fl oz/A of Roundup were 1.25 oz/A of Resolve Q and 2 oz/A of Hornet WDG with an average control rating of 94% 6 WAT.

Halex GT, the new pre-mix of glyphosate with metolachlor (Dual II Magnum) and mesotrione (Callisto) at 3.6 pt/A and the tank-mix of 2 fl oz/A of Callisto with 22 fl oz/A of Roundup provided 96 and 100% control 6 WAT respectively (Table 1). Finally, the addition of 0.5 fl oz/A of Impact with Roundup controlled only 86% of the lambsquarters 6 WAT. This was no better than control with Roundup alone.

Results for Conventional Hybrids

Some treatments included in the 2008 experiment could be used for postemergence lambsquarters control with conventional or glyphosate-resistant corn hybrids. When applied MPO, 8 fl oz/A of Clarity or 5 oz/A of Status controlled 100% of the lambsquarters 6 WAT. A summary of four previous experiments indicated that Status might provide slightly better (99%) lambsquarters control than Clarity (92%). Two herbicides that inhibit pigment formation in sensitive weeds, Callisto and Impact, were each applied with 1 pt/A of AAtrex 4L plus appropriate spray additives. The addition of AAtrex to these treatments would not affect results since the lambsquarters in this field is dominated by triazine-resistant biotypes of this weed. The postemergence rate of 3 fl oz/A of Callisto controlled 100% of the lambsquarters while 0.73 fl oz/A of Impact controlled only 83%.

Summary

These results show that 33 fl oz/A of Roundup PowerMAX provided better control of 7-inch lambsquarters (96%) than the 22 fl oz/A rate (83%). This information is useful in both glyphosate-resistant corn and soybeans. The high rate of glyphosate seems to control lambsquarters that is more than a few inches tall. Tank-mixing herbicides that have different modes-of-action with 22 fl oz/A of Roundup or using pre-mixes such as Halex GT provide equally good lambsquarters control and may play an important role in preventing the development of glyphosate-resistant weed populations.