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Windy conditions present challenges for early-season weed control

Agriculture.com Staff 06/05/2007 @ 8:37am

Have wind, good corn growing conditions and weed control issues created the perfect storm for you this spring?

Although the window for glyphosate applications has arrived, you might be able to buy time based on your weed height, according to University of Minnesota (U of M) research.

According to a university report, a U of M study conducted in 2004-2006 demonstrated that corn is sensitive to early-season weed competition. For example, when no pre-emergence herbicide was applied, a delay of four to seven days after weeds exceed four to five inches in height resulted in a yield loss of 12 bushels per acre. Delaying glyphosate application another seven days, when weeds were eight to nine inches tall, resulted in an average yield loss of 27 bushels per acre.

Timeliness of weed removal is dependent on weed density. In fields with a low weed seed bank, the timeliness issue is not as important. 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.

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 two to 10 miles per hour.

Many herbicide labels do not provide specific wind speed restrictions. According to Minnesota Department of Agriculture regulations, "a person may not direct a pesticide onto property beyond the boundaries of the target site," and "a person may not apply a pesticide resulting in damage to adjacent property."

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. In addition, spray volume of glyphosate applications can lead to an increase in 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 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. 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.

You might suffer some yield loss this year from a delay in weed control, but learn from the experience and develop an effective pre-emergence/post-emergence strategy for next year.

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