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Better Long-Term Weed Management Demonstrated by the "PRE Challenge"

By Lizabeth Stahl, Extension Educator in Crops

Benefits of a preemergence (PRE) herbicide application in soybean were demonstrated through the "PRE Challenge" - a series of on-farm research and demonstration trials conducted across southern MN in 2012 and 2013. In these University of Minnesota Extension trials, made possible through financial support of the Minnesota Soybean Research and Promotion Council, cooperators compared a postemergence-only herbicide program to a program that included a PRE herbicide application.

One benefit observed with a PRE application was a significant reduction in weed densities early in the season.  Although the cooperators were able to make timely postemergence applications in these trials, this is not always possible in each field every year.  Crop yield can be reduced when weeds are left to compete with the crop for too long early in the season.  How long is "too long" depends on factors such as weed density, weed species, environmental conditions, and the existence and level of other crop stresses.

Weed escapes at the end of the season were also virtually eliminated where PRE herbicides were applied in these trials.  Weeds allowed to go to seed help replenish the weed seedbank, which in turn can lead to long-term weed control challenges.  When weed seedbanks are large, herbicide efficacy can be reduced and the risk of yield loss due to weed competition increases.  Preventing replenishment of the weed seedbank is also an important strategy in the management of resistant weeds. 

Although yields were not greater where PRE herbicides were used in these trials, yield advantages have been observed in previous University of MN research.  Long-term benefits, such as increasing herbicide diversity and limiting replenishment of the weed seedbank, are difficult to quantify but should not be ignored.  Use of a PRE herbicide with residual activity also helps control weeds, such as waterhemp, that emerge over an extended period of time.  Producers should also have a wider window to make a postemergence herbicide application when a PRE herbicide is used. 

Although the planting season has been off to slow start for many across the state this year, growers are encouraged to follow through with plans to apply a PRE herbicide.  This is particularly important in fields where herbicide-resistant weed populations are present or suspected.  An effective PRE application can go a long way in helping control a problem weed population, especially when postemergence herbicide options are limited or limited in effectiveness.  

To learn more about the "PRE Challenge" project, check out the report at:
For more information about weed management, visit
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