Greg A. Elmore, Glen P. Murphy and William B. Parker, Monsanto Company

May 2006

While excellent weed control has commonly been achieved in soybean fields with the use of glyphosate alone, common lambsquarters and common waterhemp have occasionally escaped single or multiple glyphosate applications. Experiments were conducted by Monsanto Company and university researchers in 2005 to determine how residual herbicides affect the density of these weeds at post glyphosate application timing. The tests compared the effects of residual herbicides followed by glyphosate to glyphosate-alone treatments. Waterhemp control tests were conducted in South Dakota, Illinois, Indiana, Kansas, Missouri and Tennessee. Lambsquarters control tests were conducted in Minnesota, South Dakota, Nebraska, Iowa, Ohio, Delaware and Kentucky.

All trials were arranged in a randomized complete block design and replicated 3 or 4 times. Plots receiving just a burndown application of 22 oz/A of Roundup Original MAX™ (glyphosate) plus 1 pt/A 2,4-D or tillage to remove all weeds present at planting were compared to plots that received this burndown treatment plus a residual herbicide at planting. The residuals used included 2 quarts/A of INTRRO® (alachlor) or 2 oz/A of Valor® (flumioxazin) in waterhemp plots and 2 pts/A Prowl® (pendimethalin) or 2 oz/A Valor in lambsquarters plots.

Each plot was evaluated at the first post-emergence Roundup Original MAX application (3” weeds) timing. Weed counts were taken using a 0.5 m2 quadrant between the two center soybean rows in three locations per plot. Weed counts from plots that received a burndown and a preemergence residual application were compared to plots that received only a burndown treatment. The reduction in weeds present at the post emergence timing (3” weeds in the burndown only plots) when a residual herbicide was used is shown in the figures below.

Reduction of Waterhemp by Residuals at Post-emergence Glyphosate Timing.6 locations, 16 to 43 days after Pre-emergence treatment

Reduction of Lambsquarters by Residuals at Post-emergence Glyphosate Timing.6 locations, 16 to 43 days after Pre-emergence treatment

Summary:

INTRRO and Valor reduced waterhemp density at the post-emergence Roundup Original MAX application timing by 82% and 94%, respectively. Waterhemp control was near complete 2 weeks after the post-emergence Roundup Original MAX application. Prowl and Valor reduced lambsquarters present at the post Roundup Original MAX application timing by 74% and 80%, respectively. Lambsquarters control was also nearly complete 2 weeks after the post glyphosate application.

Conclusion:

Control of waterhemp and lambsquarters ranged from 96 to 100% 9 to 28 days after the post-emergence application of Roundup Original MAX regardless of the presence of a residual herbicide in the treatment. Even though weed control may be the same with or without the use of a residual herbicide, the use of a residual can reduce selection pressure on glyphosate by reducing the number of weeds present at the post-emergence glyphosate application. Timing of the post-emergence glyphosate application is important even after a pre-emergence residual. It may be more important for waterhemp, a continuous emerger, than for lambsquarters, an early emerger.

The authors would like to thank Dr. Charles H. Slack, University of Kentucky, Dr. Lawrence E. Steckel, University of Tennessee and Dr. Larry D. Maddux, Kansas State University for their cooperation in these experiments.

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Always read and follow pesticide label directions. Roundup Ready^® crops contain genes that confer tolerance to glyphosate, the active ingredient in Roundup^® agricultural herbicides. Roundup agricultural herbicides will kill crops that are not glyphosate tolerant. Roundup, Roundup Ready, Roundup Original MAX and INTRRO are trademarks of Monsanto Technology LLC. All other trademarks are the property of their respective owners. ^© 2006 Monsanto Company.