With the warm temperatures across much of the state, folks would like to burndown some fields and plant corn or soybeans, particularly in the southern parts of Pennsylvania. Begin spraying when possible; fields with a high density of winter annuals may benefit from the burndown now to conserve soil moisture which is almost always a problem later. This will also reduce the amount of vegetation present at planting time. 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.
Temperature Issues
- 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.
- Sunny weather is important for Gramoxone as sun causes greater plant activity.
Weed and Cover Crop Size and Growth Stage
- There is often a tradeoff in getting good control of the winter annual weed populations like chickweed and deadnettle (where smaller is better) and trying to suppress perennials like Canada thistle and quackgrass (where bigger is better). It's difficult to do a good job controlling both of these types of weeds with a single application.
- 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 preferably more when a systemic herbicide is applied.
Herbicide and Spray Application Issues
- 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 reduces antagonism from hard water (but add to the spray tank first).
- When using Sharpen, always include a good quality MSO and AMS in the spray solution.
- 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.
- In general, use flat fan tips (not flood-type nozzles) that allow for adequate spray coverage on the leaf surface. Nozzle selection is less important with glyphosate than Gramoxone and other contact herbicides such as Sharpen. And it is best to apply Gramoxone and Sharpen at 15-20 GPA for best coverage.
- Spraying in late afternoon is usually 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.
Cover Crop Termination Suggestions
Pre-plant termination of cereal rye. Glyphosate is the most effective burndown herbicide for cereal rye and other winter cereals (wheat, triticale). Under optimal conditions for application, a 0.75 - 1.0 lb ae/ac rate with appropriate adjuvants (surfactant + AMS) should provide good control. Optimal conditions for termination include several sunny and warm days (> 55°F) where night temperatures exceed 40°F. Avoiding common tank mix antagonism issues will be critical if attempting to reduce glyphosate rates when terminating cereal rye stands. Common antagonism issues include tank-mixing glyphosate with liquid N fertilizer (i.e., 28 or 32% UAN) or clay-based herbicide formulations (WG, DF, DG, F), which include atrazine and metribuzin.
Paraquat-based burndown programs can be used as an alternative for termination of winter cereal cover crops but may be less consistent across plant growth stages. Tank-mixing paraquat with a triazine herbicide (atrazine, metribuzin) will improve burndown activity for cereal rye, and we strongly recommend their inclusion in this scenario. Using a high carrier volume (20 gpa) and flat fan nozzle tips to improve coverage will also be critical for achieving adequate coverage, particularly in thick cover crop stands.
Annual ryegrass termination. Careful consideration of herbicide management for annual ryegrass is needed, particularly given concerns about its potential to become an established and persistent volunteer weed problem. Glyphosate has significantly better activity on annual ryegrass than paraquat (Gramoxone) and should be used as the base product. In order to optimize glyphosate applications for annual ryegrass control:
- Time applications to sunny, warm days (> 55° F) and actively growing plants.
- Consider terminating prior to rapid growth stages (< 8-inch) for improved control.
- Use high glyphosate rates (1.25 to 1.5 lb ae/ac) with appropriate adjuvants.
- Avoid the use of 28 or 32% UAN and high rates (> 0.25 lb ai) of clay-based herbicides (WDG, WG, DF, DG, F) like atrazine, simazine, and metribuzin to reduce tank-mix antagonism.
Penn State field trials conducted a few years ago demonstrated the utility of tank-mix partners to improve annual ryegrass control under challenging weather conditions. Roundup PowerMax (36 fl oz/ac; 1.25 lb ae) applied on April 20th to 6–8-inch annual ryegrass during a week of cooler weather conditions (~50°F day-time temperatures) provided only partial control, which resulted in ryegrass regrowing and going to seed. In comparison, tank mixing glyphosate with Select Max (8 oz/ac + MSO; clethodim) or Resolve (1 oz/ac; rimsulfuron) resulted in >95% control 2 months after treatment.
Consult herbicide labels if considering tank-mixing glyphosate with clethodim- or rimsulfuron-containing products. Select Max (clethodim) can be applied at a rate of 6-fl-oz/ac, and Clethodim 2EC can be applied at a 3-fl-oz/ac rate up to 6 days prior to corn planting and have no soybean planting restrictions. Resolve (rimsulfuron) at a 1-oz rate or Basis Blend (rimsulfuron + thifensulfuron) at a 1.5-oz rate should provide adequate weed control, increase cereal rye termination efficacy, and reduce the potential for corn injury. Application of rimsulfuron can result in corn injury under cool, wet conditions and under coarse soils. Rimsulfuron products are not labeled for soybean production.
