Has wind, good corn growing conditions and postemergence weed control created the perfect storm?  This spring a combination of good growing conditions and a significant increase in glyphosate application on increasing acres of Roundup Ready™ corn has run into conflict with a very windy spring.  Research by the University of Minnesota has clearly demonstrated that corn is sensitive to early-season weed competition.  When no preemergence herbicide was applied, a delay of four to seven days after weeds exceeded 4 to 5 inches in height resulted in a yield loss of 12 bushels per acre. Delaying glyphosate application another seven days, when weeds were 8 to 9 inches tall, resulted in an average yield loss of 27 bushels per acre.  Timeliness of weed removal is, of course, dependent upon weed density so in fields with a low weed seed bank the timeliness issue will not be as acute. Therefore growers need to scout fields for weed height and density in order to assure timely weed control but avoid post-emergence herbicide applications when wind speeds exceed legal label guidelines.

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A simple remedy to this problem is to use a one-half rate of a preemergence herbicide and plan to follow up with a postemergence glyphosate application.  Results from the 2004-2006 study indicated that the best economic returns were achieved when a one-half rate of a preemergence herbicide was followed with a postemergence (POST) glyphosate application when corn was in the three- to four-leaf growth stage (V3-V4).  Consider the investment in the preemergence herbicide as insurance in case you cannot get into your Roundup Ready™ corn fields in a timely manner; in other words, the preemergence herbicide is buying you time.  In many cases the preemergence herbicide also improves the effectiveness of the glyphosate because weeds are more uniform in size at time of glyphosate application.

Time for a preemergence herbicide has obviously passed but the urgency of timely weed control in corn must be balanced by the stewardship of safely applying your herbicide to the intended target.  The Harness™ and Surpass™ labels prohibit application when wind speeds exceed 15 miles per hour.  The Roundup WeatherMax™ label states that drift potential is lowest between wind speeds of 2 to 10 miles per hour.  Many herbicide labels do not provide specific wind speed restrictions.  The Minnesota Dept. of Agriculture regulations state that “a person may not direct a pesticide onto property beyond the boundaries of the target site. A person may not apply a pesticide resulting in damage to adjacent property.”

Please consider the consequences of off-target movement of glyphosate to non-Roundup Ready™ crops and the long-term consequences of poor stewardship when applying glyphosate under windy conditions. Drift reduction nozzles and drift control agents can help reduce drift potential, but under windy conditions, drift can still occur. Glyphosate can be applied in a coarse spray droplet size without sacrificing effectiveness. With conventional nozzles, dropping boom pressure or moving to larger-sized nozzles will increase droplet size and decrease driftable droplets. In addition, spray volume of glyphosate applications can have an impact on increasing spray drift if carrier volume is decreased substantially. Relatively small droplets are required for uniform coverage at spray volumes less than 10 gallons per acre. Small droplets increase the likelihood of spray drift. For most agronomic situations, spray volumes of 10 to 15 gallons per acre allow for adequate coverage in spring applications.

When the winds subside and fields dry out keep in mind that the effectiveness of glyphosate is still top-notch and potential tank mix partners such as Hornet™, Status™, Callisto™, and Impact™ can still be applied to corn stages of V6, V10, V8, and 45 days before harvest, respectively. Perhaps you will suffer some yield loss this year from a delay in weed control but learn from the experience and develop an effective preemergence/postemergence strategy next year.

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