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Issue No. 11, Article 3/June 17, 2010

A Few Weed Control Reminders

Plentiful and frequent precipitation has caused some delays in the application of postemergence herbicides. Weed size (both broadleaf and grass weeds) increases rapidly with warm air temperatures and adequate soil moisture, so be sure to scout before spraying to determine if the herbicide application rate should be adjusted for optimal control of larger weeds. The following are a few simple reminders with respect to postemergence herbicides.

  • The maximum amount of glyphosate that can be applied in any single application varies between corn and soybean. Applications to corn cannot exceed a maximum of 1.1 lb ae glyphosate, while 1.5 lb ae is the maximum rate for soybean. Table 1 lists the product rates (fl oz/acre) of various glyphosate formulations (lb ae/gallon) that are equivalent to these maximum single application rates.
  • Glyphosate can be applied to soybean from cracking through flowering (R2 growth stage). Glyphosate can be applied broadcast to corn through the V8 stage or up to 30 inches tall. Applications to corn between 30 and 48 inches tall must be made using drop nozzles. The risk of malformed ears later in the growing season is increased if these growth stage restrictions are not followed.
  • Several herbicides can be tank-mixed with glyphosate to improve control of certain weed species (such as glyphosate-resistant biotypes and species more tolerant of glyphosate) or to provide residual weed control. If you are tank-mixing to improve control of weeds present at the time of application, be sure to consult the product label for application rates and additive recommendations. In soybean, contact herbicides are sometimes tank-mixed to try to improve control of species such as morningglory. Keep in mind that these tank-mix partners can increase soybean injury/leaf burn and can antagonize glyphosate's activity on normally susceptible species.
  • Glufosinate may be applied to glufosinate-resistant soybean varieties from emergence up to the bloom stage. Applications allowed by label include sequential in-crop applications (each at 22 fluid ounces of Ignite) at least 10 to 14 days apart or a single application of 29 to 36 fluid ounces. Do not apply more than 22 fluid ounces to soybean beyond V3 to V4 growth stages.

--Aaron Hager

Aaron Hager

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