UNL CropWatch March 18, 2011: Q & A: Herbicide Considerations for Winter Annual Weed Control

UNL CropWatch March 18, 2011: Q & A: Herbicide Considerations for Winter Annual Weed Control

March 18, 2011

In a companion article we recommend controlling winter annual weeds at least two weeks before planting to protect corn and soybean yield. This article reviews questions to consider when selecting burndown herbicides for spring application.

Q:  What weeds are present in my field?

A:  The weeds that occur in the field should drive herbicide selection. The 2011 Guide for Weed Management in Nebraska with Insecticide and Fungicide Information includes tables that describe winter annual weed response to various corn (pages 56-57) and soybean (pages 100-101) burndown herbicides. Interactive tables containing the same information are also available at weedscience.unl.edu under the “UNL Herbicide Efficacy Tables” section. In the web tables, herbicides can be sorted from most to least effective for each species. Choose herbicides that will control all of the emerged weeds, adjusting your treatment in the two scenarios outlined below.

  • If marestail is present in your field, include a growth regulator (2,4-D or dicamba) as part of the burndown. Our observation is that glyphosate-resistant populations of marestail are becoming more widespread, and relying on glyphosate alone will select for resistant individuals and lead to increased marestail populations in future years.
     
  • If field pansy is present in your field, you should probably avoid using 2,4-D or dicamba. In research conducted in 2009-2010 in southeast Nebraska, neither herbicide controlled field pansy, and 2,4-D pus glyphosate tank-mixes were less effective than glyphosate alone.

Q:  Should I include a residual herbicide with the burndown?

A: If you haven’t decided what you’ll be planting in the field, the answer is probably “no.” Once a residual herbicide is applied, your ability to change crops is restricted.

Many residual herbicides have effective burndown properties, either alone or tank-mixed with glyphosate or 2,4-D:

  • For corn — atrazine, Balance Flexx, Corvus, Prequel, Resolve, Sharpen, Valor, and Verdict
  • For soybean — Authority products, Canopy, Pursuit (and other imazethapyr containing products), FirstRate, Flexstar, Optill, Sharpen, and Valor products

Apply a burndown plus residual preplant herbicide to:

  • control winter annual weeds,
  • provide a weed-free seed bed at planting, and
  • control summer annual weeds for a couple weeks after planting. Achieving these three goals will protect the crop against early season competition and yield loss if a timely postemergence application is made (before summer annual weeds are 4 inches or greater).

When selecting a burndown herbicide, also consider the weeds likely in that field's seedbank. Using an early preplant burndown without a residual herbicide or with one that's not effective against your weed group may require another burndown application at planting.

The length of residual activity depends on herbicide properties, activation by adequate rainfall or irrigation, and the herbicide rate. The length of residual activity of full rates of most combination herbicides (two or more active ingredients) is approximately four to six weeks. Reducing the rate of residual herbicides will shorten the length of residual activity proportionally. When an early preplant burndown (two weeks or more before planting) is made, applying the highest labeled rate for the soil type is recommended.

Applying appropriate residual herbicides is also a good practice to manage against herbicide-resistant weeds. Appropriate residual herbicides are effective on species that are at high risk for developing resistance. In Nebraska, weeds to watch (and treat with residual herbicides) are marestail, kochia, common waterhemp, Palmer amaranth, and giant ragweed. Populations of each of these species have been documented to be glyphosate-resistant within Nebraska (marestail) or in other states.

Q: What planting restrictions are associated with burndown applications?

A: In fields where 2,4-D is applied, the preplant interval for corn is seven days for rates less than 1 pt/ac and 14 days for rates more than 1 pt/ac. The preplant interval for soybean is seven days for rates less than1 pt/ac and 30 days for rates more than 1 pt/ac.

In fields where dicamba is applied, the preplant interval for corn is five days for use rates of 4 fl oz/ac and seven days for 8 fl oz/ac. It is not recommended that dicamba be applied before soybean. Flumioxazin (Valor) may be applied preplant in corn, but there is a 7 day interval between burndown and planting.

Q: What adjuvants should be included in burndown applications?

A: Adding a crop oil concentrate (COC) or methylated seed oil (MSO) plus a nitrogen fertilizer source (2-4 qt UAN/ac or 2-4 lb AMS/ac) will enhance burndown activity of most foliar and residual herbicides. Products containing saflufenacil (Sharpen, Verdict, Optill) require a methylated seed oil. If only glyphosate is used, add 8-17 lb AMS/100 gal. Adding COC or MSO is unlikely to enhance control. Specific adjuvant recommendations may be found in the 2011 Guide for Weed Management in Nebraska on pages 40-42.

Q: What are the economics of a burndown application?

A: Allowing moderate to dense infestations of winter annual weeds to grow until planting may cause yield losses of 5% or greater (see Control Winter Annual Weeds Early to Protect Crop Yield). At current prices ($6 corn and $13 soybean), the potential losses are $32-$45/ac for 150 bu corn and 50 bu soybean.

Burndown applications without residual herbicides will likely cost $4-$8/ac, plus application costs. Adding residual herbicides will increase that to $12-$40/ac, plus application costs. Adding residual herbicides will likely benefit in-crop weed control, so the return on investment may be much higher than that obtained by controlling only the winter annual weeds.

Mark Bernards
Extension Weeds Specialist
Lowell Sandell
Extension Educator - Weed Science