By William Curran, Weed Scientist; Dwight Lingenfelter, Program Develop Specialist

Prior to last week’s cool weather, a number of farmers and applicators are thinking about or even starting to apply burndown herbicides. Remember that all vegetation should be actively growing and capable of intercepting the herbicide spray (e.g., not covered with crop residue). 

Air temperature before, during and after application can influence control with burndown. Cold nights (under 40 F) will reduce activity, particularly for glyphosate, and especially when followed by cool (<55 F) cloudy days. With the cold nights we are experiencing, wait 2 or 3 days if possible after the nights have warmed before making foliar-applied applications. Here are some reminders to consider for common foliar-applied burndown products.


Remember to use a sufficient rate, which generally ranges from 0.75-1.5 lb ae per acre. The 22-fluid-ounce rate of Roundup, 24-fluid-ounce rate of Durango or Touchdown Total, or 32-fluid-ounce rate of Credit, Gly Star, Makaze, Rascal, etc. = 0.75 pounds. 

In general, application alone along with appropriate adjuvants (surfactant + AMS) is best, and reducing the carrier volume to 10 gallons per acre can increase activity. The AMS or another water softener helps alleviate hard water problems and also can reduce antagonism if tank-mixing with other herbicides such as 2,4-D. Be sure to add the AMS first to the spray tank and agitate before adding the glyphosate. Do not add 28% or 32% UAN or other fluid fertilizers to the spray tank. 

If the water source has a high pH (8 or greater), consider adding an acidifying agent to the spray solution. Avoid tank mixing with higher-rate (greater than 0.25 lb) clay-based herbicides (WDG, WG, DF, DG, F) like atrazine, simazine and metribuzin. Other herbicides such as 2,4-D, dicamba, clopyralid, Balance or Corvus, Resolve or Basis Blend are OK. Use a clean water source that does not contain soil or other sediment that can reduce glyphosate activity.

Gramoxone SL (Syngenta)

This product contains paraquat and is applied at 2-4 pints per acre for control of emerged weeds. Paraquat is the main alternative to glyphosate for nonselective burndown. Since it is a contact herbicide, apply in 20 gallons of liquid carrier or more and do not use flood jet tips. For best results, use flat fan nozzle tips that produce a uniform spray pattern and thorough coverage. Be sure to include an appropriate nonionic surfactant. 

Add a triazine herbicide (atrazine, metribuzin, etc.) to Gramoxone to increase burndown activity. Using UAN as a partial carrier will also increase the activity. Use a clean water source that does not contain soil or other sediment that can reduce Gramoxone activity.

Broadleaf-Only Products

2,4-D ester

A PGR herbicide that can be applied at 1 pint 7-14 days before planting or 3-5 days after planting for greater crop safety. Plant corn at least 1.5 inches deep. Use the ester formulations of 2,4–D, instead of the amine. Esters (LV4) are usually more effective under cool conditions, are less water soluble and better on perennial weeds. Also, esters usually penetrate the waxy leaf surface (cuticle) better than amines. 2,4-D will help control a number of emerged winter annuals, including marestail.

Dicamba (Clarity/Banvel, others)

A PGR herbicide that can be applied at up to 1 pint per acre on medium and fine textured soils with at least 2.5% organic matter. It can be tank-mixed with 2,4-D, and like 2,4-D, direct contact with corn seeds must be avoided. So plant corn at least 1.5 inches deep and apply 7-14 days ahead of planting or delay application until corn is emerging. Dicamba is often necessary for successful alfalfa and/or clover burndown. Not for use on soybeans in Pennsylvania.