Spring Burndown Herbicide Tidbits

With the warm temperatures across much of the state, folks, particularly in the southern parts of PA, would like to burndown some fields and plant corn. It's a good time to remember that environmental conditions and the size of the weeds or cover crops can greatly affect the activity of burndown herbicides and weed control. Below are some tidbits to consider when applying burndown herbicides.

• Do not apply either glyphosate or Gramoxone unless daytime temperatures exceed 55-60°F. (Liberty can be used for burndown but it is a much better herbicide to use in-crop (on LibertyLink varieties) when the temperatures are warmer; plus it is one of the few herbicides that is effective on marestail and Palmer amaranth postemergence.)
• The higher the temperature the better.
• Cold nights (<40°F) will reduce activity, particularly for glyphosate; herbicides work slower under cool conditions.
• 2,4-D is generally more active than glyphosate in cool weather (<40°F); thus, tank mix them when possible.
• After a cold spell, wait until a few days of warm, sunny weather have occurred before applying herbicides.
• In general, burndown herbicides provide the best control when annual weeds are actively growing, are 6 inches tall or less, and still in the vegetative stage of growth.
• Winter annuals that are flowering may require higher rates or different combinations of products.
• Increasing the rate of the burndown herbicide may be necessary if weeds are stressed by cold conditions or are larger in size.
• Perennials should be at least 6 to 8 inches tall and preferable more when a systemic herbicide is applied.
• With the spread in glyphosate-resistant marestail across the state it is necessary to tank-mix herbicides to increase the spectrum of activity for successful control plus use multiple modes of action.
• Sunny weather is important for Gramoxone as sun causes greater plant activity.
• Be sure to include AMS at 8.5 to 17 lb/100 gal or equivalent with glyphosate and add a good quality NIS at a 0.25% v/v to Gramoxone. Of course, if the glyphosate doesn't come fully loaded, add good quality NIS as well.
• Nitrogen solutions (UAN) may be used as the carrier with either herbicide. UAN can increase Gramoxone activity and reduce glyphosate activity (especially for perennials and difficult to control weeds). Adding AMS also reduces the antagonism.
• Tank mixing a triazine such as atrazine with Gramoxone increases the activity; tank mixing any other herbicide and especially a clay-based herbicide (DF, F, WP) reduces glyphosate activity. You can overcome the antagonism by increasing the glyphosate rate by 20 to 25%.
• Another good burndown combination that does not contain glyphosate is: Gramoxone + atrazine (or metribuzin) + 2,4-D LVE + NIS/MSO + AMS; this is a viable option to control marestail if it is smaller than 4 inches tall.
• Use flat fan tips with both (not floods) and apply Gramoxone at a minimum of 20 GPA. Tip selection is less important with glyphosate than Gramoxone.
• Spraying in late afternoon is not as effective as mid-morning.
• Finally, scout fields prior to planting and spraying to ensure you use the correct herbicide program for the problem.

Orchardgrass spring burndown hints.  As always, fall applications of glyphosate are much better than spring applications for orchardgrass sod burndown. For those who want to control orchardgrass (or quackgrass) in the spring here are a few considerations:

• Wait until orchardgrass is 6 to 10 inches tall and actively growing
• Use at least 1.13 lbs ae/acre or higher glyphosate (this would equal 32 fl oz/A Roundup PowerMax or 48 fl oz/A of a "generic, 3L" glyphosate product)
• In some cases, up to 3 qt/A glyphosate may be necessary to provide effective control, especially if tank-mixing with atrazine or other clay-based herbicides due to antagonism
• Many pre-residual herbicides can be included in the spray mixture as well; products listed as "preplant" can be applied at this point; Prowl or pendimethalin products should not be applied if corn has not yet been planted