By Dwight Lingenfelter, Extension Associate, Weed Science

A weedy field before burndown. Photo credit: Penn State Weed Science

With the wet cool spring it has been difficult to yet get into the field in much of the state. Folks particularly in the Southern half of PA are starting to burndown some of these 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. In addition, daytime temperatures above 55°F and nights above 32°F with sunny days will help improve herbicide activity and cool cloudy days will reduce their effectiveness.

In general, burndown herbicides provide the best control when annual weeds 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. Perennials should be at least 6 to 8 inches tall and preferably taller when a systemic herbicide is applied. With the increase in glyphosate resistant marestail across the state, it is increasingly important to tank-mix herbicides to increase the spectrum of activity for successful control plus use multiple modes of action. Also, in some parts of the state corn has already been planted prior to the killing any weeds and/or cover crops. Make sure to control these within a couple weeks of planting otherwise they can cause problems with crop establishment and growth.

Some reminders include; after a cold spell, wait until a few days of warm, sunny weather have occurred before applying herbicides. Herbicides work slower under cool conditions. Increasing the rate of the burndown herbicide may be necessary if weeds are stressed by cold conditions or are larger in size. And scout fields prior to planting and spraying to ensure you use the correct herbicide program for the problem. For corn, some common burndown herbicides include the following:

Broad-spectrum products:

Glyphosate — Generally applied at 0.75 to 1.13 lb ai per acre. Use the 1.13 lb rate or higher if tank-mixing with residual herbicides and nitrogen carriers. Do not use with suspension type liquid fertilizers. Add 2-4 lb/acre ammonium sulfate (AMS) or a similar product for improved performance. Typically glyphosate products are applied in 10 to 20 gallons of liquid carrier per acre. Glyphosate may be applied in clear liquid nitrogen fertilizer or clear liquid complete analysis fertilizers, but this mixture may be less effective on certain annual and perennial weeds.

Gramoxone SL 2.0 — This product contains paraquat and is applied at 2-4 pt 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. 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.

Liberty 280 (glufosinate) — This active ingredient has been labeled postemergence in Liberty Link corn and soybeans for a number of years, but is also labeled for burndown in conventional corn and soybean. We have not really considered this much of a burndown herbicide simply because its strength is small annual weeds and it is not very effective on larger winter annuals, perennials, or burndown of cover crops. However, it is labeled for burndown application up to 43 fl. oz per acre, and can now be used sequentially in both Liberty Link corn and soybean (burndown followed by post). Most of the fit for this is for soybean and not corn. It has gained some traction in the Mid-Atlantic for burndown control of marestail in particular. Remember, Liberty is most effective on small actively growing weeds under warm temperatures, with bright sunlight, include AMS and use sufficient carrier volume and spray tips to get thorough coverage of the weeds.

Broadleaf only products:

2,4-D ester — A PGR herbicide that can be applied at 1 pint per acre, 7 to 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. Therefore, plant corn at least 1.5 inches deep and apply 7 to 14 days ahead of planting or delay application until corn is emerging. Dicamba is often necessary for successful alfalfa and/or clover burndown.

Control of small emerged broadleaves (and maybe grasses depending on the product):

Acuron/Lumax EZ/Lexar EZ — All of these contain Dual II Magnum, atrazine, and mesotrione (Callisto) while Acuron also contains bicyclopyrone (another HPPD). The atrazine and mesotrione components can provide substantial burndown and control of small annual broadleaf weeds. Mesotrione is also quite effective for suppression of dandelion. Control of emerged grasses can be inconsistent.

Atrazine — apply 1 to 2 lb per acre preplant or preemergence to corn (rate may depend on tillage system). Controls small emerged annual broadleaf weeds and will suppress annual grasses (as well as provide residual control). Can be applied in UAN for improved burndown activity.

Balance Flexx and Corvus — Balance Flexx and Corvus contain the herbicide isoxaflutole. Corvus also includes an ALS-herbicide called thiencarbazone. Both products include a corn safener. Isoxaflutole in combination with atrazine can provide control of small emerged annual weeds. The thiencarbazone component will improve foliar and residual grass control. Tank mix with other herbicides including 2,4-D, Gramoxone, or glyphosate for broader spectrum activity. For greater crop safety, plant corn at least 1.5 inches deep and make sure the seed is well covered.

Basis Blend 30DF — is a mixture of rimsulfuron (Resolve) and thifensulfuron (Harmony) and can be applied preplant or premergence to corn. The typical rate is 0.825 to 1.5 oz per acre. Plant corn 1.5 inches deep and apply 7 days before corn planting for greater crop safely. Typically tank-mixed with atrazine and/or 2,4-D LVE to increase the spectrum of activity. Resolve (rimsulfuron) and Resolve Q (rimsulfuron plus thifensulfuron plus safener) have similar use labels.

Clopyralid (Component of Hornet, Resicore, SureStart II or TripleFlex II) — A PGR herbicide that can provide burndown and residual control of selected annual broadleaf weeds. It is particularly effective on the ragweeds, can help control clover and alfalfa, and will suppress emerged Canada thistle. For best crop safety when using these products, make sure to plant corn at least 1.5 inches deep.

Harmony 50SG (thifensulfuron) — Applied at 0.45 to 0.90 oz per acre preplant or at planting to corn (or soybean). Thifensulfuron will help control wild garlic and provide some suppression of certain winter annual weed species. Include an appropriate surfactant. It is not as broad spectrum at Harmony Extra and is typically tank-mixed with glyphosate and/or other herbicides.

Harmony Extra 50SG (thifensulfuron plus tribenuron) — Applied at 0.45 to 0.90 oz per acre. Allow at least 14 days between application and planting of corn. (7 days for soybeans). For improved control of wild garlic and some winter annual weeds include an appropriate surfactant. Typically Harmony Extra is tank-mixed with glyphosate and/or other herbicides.

Instigate 46WG — is a mixture of rimsulfuron and mesotrione that is typically applied at 6 oz per acre. Instigate can provide some initial control of small emerged annual weeds and can be tank-mixed with 2,4-D, atrazine and other herbicides to increase the spectrum of activity. For greater crop safety, plant corn at least 1.5 inches deep and make sure the seed is well covered.

Prequel 45WG — is a mixture of isoxaflutole plus rimsulfuron that is generally applied at 1.66 oz per acre (up to 2.5 oz). Prequel can provide some control of small emerged annual weeds and can be tank-mixed with 2,4-D, atrazine and other herbicides to increase the spectrum of activity. For greater crop safety, plant corn at least 1.5 inches deep and make sure the seed is well covered.

Pyroxasulfone-containing products, Anthem Maxx, Fierce, and Zidua — can provide annual grass activity as well as good control of several annual broadleaves. These can be mixed with typical burndown herbicides to broaden spectrum.

Sharpen 2.85SC (saflufenacil) — The product is a contact type herbicide and can be applied at 1 to 3 oz per acre up to 14 days ahead of corn planting. Include MSO in the tank to maximize burndown activity on emerged broadleaf weeds including glyphosate resistant marestail (horseweed). The active ingredient saflufenacil is also found in the premix Verdict which contains saflufenacil and the grass herbicide found in Outlook. At the higher rate, Sharpen also provides residual control on annual broadleaf weeds.

Valor 51WDG (fumioxazin) (not Valor XLT) — is a contact type herbicide and may be included in the corn burndown program to enhance the speed of burndown and provide residual control of annual broadleaves. Corn may be planted 7 days after application at the 2 oz per acre rate (requires minimum of 25% surface residue and 0.25 inch of rainfall between application and planting). Valor may be applied at up to 3 oz per acre, 14 to 30 days prior to corn planting.

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